JP2004109763A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus provided with a constitution for securing image reproducibility of high resolution at low costs in a device provided with a constitution for transferring images from a plurality of latent image carriers. <P>SOLUTION: Since a linear velocity difference (Vpc(t)-Vbelt(t)) on a transfer nip part (Nw) between a latent image carrier (3) and a transfer body (2) is regulated within a prescribed range, the extension of an image caused by the dragging of the image can be suppressed and the drop of resolution on minimum pixels can be prevented. <P>COPYRIGHT: (C)2004,JPO

Description

と表すことができる。
【００２７】
ここで、細線の伸び量は、要求される解像度を解像したまま転写するという命題に直結する。すなわち、例えば６００ｄｐｉの感光体画像をそのまま中間転写体上に転写するためには、細線にはわずかの伸びしか許容されないのである。
【００２８】
今回の検討で得られた、ニップと速度差の式１−１、１−２から、速度差ΔＶは
ΔＶ＝Ｖｐｃ＊ΔＩｗ／（Ｎｗ±Ｉｗ）…式２
と表すことができる。
これらの実験結果から得られた式によって、感光体の平均速度と、中間転写体の平均速度および一次転写ニップ幅の関係から、速度差が発生した場合の細線の伸び量が推定できるようになり、逆に言えば、伸び量を一定値以内に収めるためには、一次転写位置における感光体３と、中間転写体２との外周速度の差を有る値以内に収めなければならないことになる。
【００２９】
上述した結果は、感光体３と中間転写体２との外周速度が一定速度であることが前提であるが、実際には軸の偏心などにより、一次転写部の速度は一定になっていないことが多い。そのため、各々の平均速度を一致させた場合においても、速度に一定の変動があるために画像の伸びることが分かる。
【００３０】
ここで、図５に示すように、回転軸の偏心のみを考えた場合、時刻ｔにおける感光体と中間転写体の一次転写ニップにおける外周速度は次のように表すことができる。なお、図５において実線は感光体の幾何中心を基準とする円を示し、二点鎖線は感光体の駆動回転中心を基準とする円を示している。
Ｖｐｃ（ｔ）＝ωｐｃ＊（ｒｐｃ＋ｄｐｃ＊ｓｉｎ（Ｖｐｃ＊ｔ／ｒｐｃ＋Ａ））…式３
但し、
Ｖｐｃ：感光体の平均外周速度
ωｐｃ：感光体駆動軸の角速度
ｒｐｃ：感光体ドラム半径
ｄｐｃ：ドラムの回転中心と幾何中心との距離
Ａ　　：初期位相
Ｖｂｅｌｔ（ｔ）＝ωｂｅｌｔ＊（ｒｂｅｌｔ＋ｄｂｅｌｔ＊ｓｉｎ（Ｖｂｅｌｔ＊ｔ／ｒｂｅｌｔ＋Ｂ））…式４
但し、
Ｖｂｅｌｔ：中間転写体の平均外周速度
ωｂｅｌｔ：中間転写体（ここではベルト）の駆動ローラ軸の角速度
ｒｂｅｌｔ：中間転写ベルト駆動ローラ半径
ｄｐｃ　　：中間転写体ベルト駆動ローラの回転中心と幾何中心との距離
Ｂ　　　　：初期位相
従って、式２より、最も厳しい条件として、線速差が常に式５の様になっていれば、画像の伸びを防止することができることになる。
ΔＶ＝｜Ｖｐｃ（ｔ）−Ｖｂｅｌｔ（ｔ）｜＜　Ｖｐｃ＊ΔＩｗ／（Ｎｗ−Ｉｗ）　……式５
線速比で表現すれば、式５を変形して
ΔＶ／Ｖｐｃ　＜ΔＩｗ／（Ｎｗ−Ｉｗ）　……式５’
と表される。
以上が請求項１記載の発明において特定した数式を設定した理由である。
【００３１】
（請求項２記載の発明の実施形態）
最小画素を対象とした場合の伸び量はニップ幅とも相関があり、ニップ幅が広くなれば伸び量も大きくなり、逆に言えば感光体と中間転写体の線速差が有る一定値になっている場合には、細線の伸び量はニップ幅によって決定されるということが本発明者による実験により判明した。
【００３２】
感光体の振れのみを考えた場合、式３，４から感光体の速度変動の幅ΔＶｐｃは、
ΔＶｐｃ＝ωｐｃ＊ｄｐｃ＊ｓｉｎ（Ｖｐｃ＊ｔ／ｒｐｃ）…式６
同じく中間転写体の速度変動の幅は、
ΔＶｂｅｌｔ＝　ωｂｅｌｔ＊ｄｂｅｌｔ＊ｓｉｎ（Ｖｂｅｌｔ＊ｔ／ｒｂｅｌｔ）　…式７
と表すことができる。このような偏心による速度変動は、個々の部品精度で決定されるために全くなくすことは不可能であり、コストと機能との兼ね合いで精度が決定される。そのため、この値を一定値以内にするのは難しく、その際には式１からΔＩｗを小さくするためにはニップ幅Ｎｗを小さくすることが望ましいことが分かる。このことから、所望の画像の伸び量を、感光体、中間転写体の平均速度が一致し、速度変動を一定量許容した場合には、
Ｎｗ＜ΔＩｗ＊Ｖｐｃ／（ΔＶｐｃ±ΔＶｂｅｌｔ）±Ｉｗ…式８―１
となり、最も厳しい条件として、
Ｎｗ＜ΔＩｗ＊Ｖｐｃ／（ΔＶｐｃ＋ΔＶｂｅｌｔ）−Ｉｗ…式８−２
という設定とすることが好ましいことになる。
【００３３】
なお、上述の例では感光体３がドラム形状、中間転写体２はベルト形状で駆動ローラで内側から駆動される径を考えた。これは感光体３がベルト、中間転写体２がドラムや、両方ともベルト、両方ともドラムの場合でも同様に考えることができる。また、同じく上述の例では速度変動の要因として感光体３や中間転写体２に用いられるベルトの回転軸の偏心のみを考えているが、これ以外にも、感光体３の真円度やベルトの厚みムラ、ベルト駆動ローラの真円度、回転中心軸の角速度の揺らぎなども現実の径では考慮しなければならないが、周期的で、最も影響の大きなものが軸の偏心であり、その他の速度変動も同様に考えることができる。
【００３４】
（請求項３，４記載の発明の実施形態）
解像度を議論する場合、副走査方向での６００ｄｐｉの解像度は、図６に示すように、主走査方向に引かれた中心間隔８４．６μｍの二本の線が、分離して見えるか見えないかで判別されることが多い。
すなわち、理想的には４２．３μｍの太さである線が、４２．３μｍの間隔を持って理想的な線間隔で並んでいるのであり（図６（Ａ）参照）、感光体上の４２．３μｍの線が中間転写体上で４２．３μｍ伸びて８４．６μｍとなった場合、つまり画像部：非画像部＝１０：０の場合には、隣の線と接触してしまい、二本の線として認識されなくなる（図６（Ｃ）参照）。この場合、このシステムの一次転写部に許された細線の許容伸び量は４２．３μｍであり、一次転写部では細線がこれ以上伸びることのないように機械を設計する必要がある。実際の許容量、最終的に得られる紙上での解像度が基準となり、一次転写部での画像の伸びはさらに許容度が低くなり、図６（Ｂ）に示すように、画像部との非画像部の比率が８：２程度までのことが多い。すなわち、元の線の幅に対して、３０〜６０％程度の伸びまでしか許容されないということである。
【００３５】
なお、線速差の影響を受けるのは副走査方向（紙搬送方向に対して直交する方向）の線であり、最も細い最小画素長さが、最も影響を受けるので、最小画素長さを基準とすることが望ましい。
【００３６】
通常は上記の例のように感光体３上に形成される潜像の最小分解能に相当させていることが多い。
【００３７】
（請求項５記載の発明の実施形態）
ここで、式１−１と式１−２を見比べてみると、画像の伸びΔＩｗは中間転写体の速度が感光体の速度よりも遅い場合の式１−２の場合に、式１−１よりも伸び量が小さくなっていることが分かる。すなわち、上述の説明で、理想細線のニップ幅による引き延ばされ効果は感光体が早い場合も遅い場合もどちらも線速差の絶対値によって伸び量が決まっているが、全体のピッチは速度比によるので中間転写体が遅い場合には線が縮まり、有る値では縮む分と伸びる分が相殺されて画像の伸びが抑えられる。よって、速度差がどうしても付いてしまう場合や、つけなければならない場合（例えば転写率を上げたり、逆転写を抑えたりするために相対速度差をつける場合など）では感光体に対して中間転写体の速度が遅くなるようにするとよいことになる。
【００３８】
（請求項６記載の発明の実施形態）
ここで、前記した式３，４に注目する。請求項１記載の発明の実施形態において示した式５に着目した場合、ある時刻ｔにおける一次転写部での感光体３と中間転写体２の速度差が、主に感光体３及び中間転写体２の回転中心の偏心による場合、一定周期で各々の速度は変動していることが分かる。線速差さえ生じなければ画像が伸びることはないので各々の速度は変動しても構わないことがわかる。
ただし、お互いの変動周期が一致しなければ線速差を常に最小にすることは出来ない事が分かる。例えば感光体３がドラム形状であり、中間転写体２がベルト形状である場合には、周期が一致するためには感光体径と中間転写ベルトの駆動ローラ径が同一であり、位相が一致している必要がある。
【００３９】
その場合、式３，４から、ｒｐｃ＝ｒｂｅｌｔ＝ｒ、ωｐｃ＝ωｂｅｌｔ＝ω、Ｖｐｃ＝Ｖｂｅｌｔ＝Ｖとして、Ａ＝Ｂ＝０とした場合、
｜Ｖｐｃ（ｔ）−Ｖｂｅｌｔ（ｔ）｜
＝ωｐｃ＊（ｒｐｃ＋ｄｐｃ＊ｓｉｎ（Ｖｐｃ＊ｔ／ｒｐｃ＋Ａ））−ωｂｅｌ
ｔ＊（ｒｂｅｌｔ＋ｄｂｅｌｔ＊ｓｉｎ（Ｖｂｅｌｔ＊ｔ／ｒｂｅｌｔ＋Ｂ））
＝ω＊（ｒ＋ｄｐｃ＊ｓｉｎ（Ｖ＊ｔ／ｒ））−ω＊（ｒ＋ｄｂｅｌｔ＊ｓｉｎ
（Ｖ＊ｔ／ｒ））
＝ω＊（ｄｐｃ＊ｓｉｎ（Ｖ＊ｔ／ｒ）−ｄｂｅｌｔ＊ｓｉｎ（Ｖ＊ｔ／ｒ））
＝ω＊（ｄｐｃ−ｄｂｅｌｔ）＊ｓｉｎ（Ｖ＊ｔ／ｒ）
となり、位相が一致していれば線速差は偏心量の差に応じたサイン周期となり、偏心が生じても速度差を非常に小さくすることができることになる。
【００４０】
（請求項７記載の発明の実施形態）
カラープリンタなどの画像形成装置においては複数の感光体を用いる、いわゆるタンデム構成を採用することがあるが、この場合は感光体の本数分、複数の一次転写位置が必要となる。
上述の位相合わせによって一次転写部での線速差を小さく抑える手法を、各一次転写部で破綻無く実施するためには、各々の一次転写部での位相が全て揃っている必要がある。ただし、中間転写体は１つであり、この速度の変動周期は固定であるので、感光体側の方を中間転写体の速度変動に合わせる必要がある。よって、各一次転写位置での速度変動に対して感光体の位相が合うようにすれば、タンデム構成においても画像の伸びの少ない機会とすることができることになる。
【００４１】
この場合、中間転写体がベルトの場合はベルトの駆動ローラの径と感光体の径が等しければ、感光体間距離（ピッチ）には特に制限が無く、例えば４本の感光体が中間転写ベルトに接するタンデム機の場合、一次転写部間距離を感光体外周の半分とすれば、一番、３番目の感光体に対して、２，４番目の感光体は逆位相で駆動すればよく、一次転写位置が各々中途半端な位置にある場合も駆動ローラからの距離を計算して位相を合わせればよい。
【００４２】
しかし、機械の構成上、各一次転写部間の距離が感光体の外周と一致していれば、全ての感光体が同位相で駆動すればよくなるので、機械設計上、位相の検出、調整が楽になる。一般的には各一次転写部間距離と最外一次転写位置から駆動ローラまでの距離が感光体外周長と一致しているのが望ましい。
【００４３】
（請求項８，９，１０記載の発明の実施形態）
一方、発明者は、感光体３とトナーおよび、トナーと中間転写体２との表面の摩擦係数にも着目して、図１と同様の実験を行った。
すなわち、図１の結果が得られた実験では感光体のトナーに対する摩擦係数が中間転写ベルトとほぼ同等の条件（感光体＝０．５４、中間転写ベルト＝０．４７）での実験結果だったのに対して、両者の摩擦係数を共に下げた場合と、感光体の摩擦係数に対して、中間転写ベルトの摩擦係数が高い場合との条件下での実験を行ったところ、図７に示すように、中間転写体の摩擦係数が感光体よりも高い場合において、線速差による画像伸びが理論値よりも抑えられる現象が確認された。特に中間転写体の速度が感光体に対して遅い場合が顕著に現れているが、共に摩擦係数が高い場合に比べて、伸び量が約半分以下程度に緩和されている。
【００４４】
図８は、摩擦係数でのトナーの挙動を説明するための図であり、図８（Ａ）は感光体と中間転写体にはさまれた直後でのトナー層の状態を示し、図８（Ｂ）は感光体３と中間転写体２とが同程度の表面摩擦係数を有している場合を示し、図８（Ｃ）は感光体の方が中間転写体よりも表面摩擦係数が小さい場合を示している。
【００４５】
図８において、感光体３と中間転写体２との間に速度差が生じた際に、滑る界面がトナー層のどの位置に現れるかで決定されると考えられている。つまり、感光体３と中間転写体２の双方がトナーに対して同じ程度の摩擦係数である場合（図８（Ｂ）参照）には、速度差を吸収する剪断位置が一意に決まらず、トナー全体が崩れるように引き延ばされてしまう。これに対して感光体３と中間転写体２との摩擦係数に差がある場合（図８（Ｃ）参照）には、滑りやすい位置で、同図の場合には感光体側で滑りやすくなることにより速度差を吸収することができる。しかも、トナー自身も凝集力があるので、感光体３の摩擦係数が低い場合には感光体３の表面とトナー層の表面との間に剪断が生じることで余りトナー層自体が崩れることなく転写されるためと考えられる。
【００４６】
図７に示した結果は、摩擦係数に大きく差がある場合の例であるが、本実験で使用した画像形成装置は、リコー製デジタルカラー複写機ＩｍａｇｉｏＣｏｌｏｒ５１００であり、この装置は感光体３および中間転写体２の表面摩擦係数を下げるために、固体潤滑剤であるステアリン酸亜鉛の塗布を定期的に行っている。塗布の状態を変えて実験を繰り返したが、概ね全く固体潤滑剤の塗布がない場合に対して、中間転写体側へのみ固体潤滑剤を塗布した場合には画像の伸びの余裕度がおよそ１．５倍になっていることが判明した。
【００４７】
すなわち、感光体３のトナーに対する表面摩擦係数に対して、中間転写体２の摩擦係数が低い場合においては、所望の画素の伸び許容度を達成するために、請求項１，２にある式の条件が１．５倍に緩和されることになる。ただし、この効果を求めるあまりに中間転写体の表面摩擦係数を上げることは、一次転写の後のプロセスである、中間転写体上から紙へのトナー転写（二次転写）時において、転写率を落としてしまいかねない。よって、このような条件を達成して且つ良好な紙転写を保つためには中間転写体のトナーに対する表面摩擦係数が、紙の摩擦係数よりも低い状態に有る必要があり、
感光体での摩擦係数＜中間転写体での摩擦係数＜紙での摩擦係数
の順列を守ることが必要である。
【００４８】
【実施例】
（実施例１）
以下、本発明を画像形成装置の一つであるフルカラープリンタ（以下、「カラープリンタ」という）の実施例について説明する。なお、以下に説明するカラープリンタでは、中間転写体としてベルトが用いられる場合を挙げるが、中間転写体としては、ベルトに限らずドラムを用いることも可能である。
【００４９】
図９は、この発明による一例である、カラープリンタにおける内部機構の全体概略構成図である。なお、図１０は、図９に示したカラーレーザプリンタでの感光体周りの構成を示す図であり、図１１は、図９に示した内部機構を装備した画像形成システム全体の構成図である。
【００５０】
図９乃至図１１において、カラープリンタ１は、画像形成部１Ａが縦方向の中央部に位置し、その下方には給紙部１Ｂが、さらに画像形成部１Ａの上方には原稿載置台１Ｃ１を備えた原稿走査部１Ｃがそれぞれ配置されている。
画像形成部１Ａには、水平方向に展張面を有する中間転写ベルト２が配置されており、中間転写ベルト２の上位には、色分解色と補色関係にある色の画像を形成するための構成が設けられている。
【００５１】
画像形成部１Ａには、補色関係にある色のトナー（イエロー、マゼンタ、シアン、ブラック）による画像を担持可能なドラム状とされたＯＰＣ感光体３Ｂ、３Ｙ、３Ｃ、３Ｍが中間転写体２の展張面に沿って並置されている。なお、以下の説明において、全ての感光体に共通する内容の場合には感光体を符号３により示す。
各感光体３Ｂ、３Ｙ、３Ｃ、３Ｍは、それぞれ同じ方向（図１では、反時計方向）に回転可能なドラムで構成されており、その周辺には、回転過程において画像形成処理を実行する帯電装置４，書き込み装置５，２成分磁気ブラシ現像方式を採用した現像装置６，１次転写装置７，およびクリーニング装置８が配置されている（便宜上、感光体３Ｂを対象として、各装置の符号にＢを付して示してある）。
【００５２】
図９において中間転写ベルト２は、各感光体を備えた作像ユニットからの可視像を順次転写される１次転写部に相当しており、複数のローラ２Ａ〜２Ｃに掛け回されて感光体との対峙位置において同方向に移動可能な構成を備え、展張面を構成するローラ２Ａ、２Ｂとは別のローラ２Ｃは、中間転写ベルト２を挟んで２次転写装置９に対峙している。なお、図９，１１中、符号１０は、中間転写ベルト２のクリーニング装置を示している。
【００５３】
２次転写装置９は、帯電駆動ローラ９Ａおよび従動ローラ９Ｂに掛け回されて２次転写装置９が位置する２次転写位置において中間転写ベルト２と同方向に移動可能な転写ベルト９Ｃを備えており、転写ベルト９Ｃを帯電駆動ローラ９Ａにより帯電させることで記録シートを静電吸着しながら搬送する過程で中間転写ベルト２に重畳された多色画像を一括転写によってあるいは担持されている単一色の画像をそれぞれ記録シートに転写することができる。
【００５４】
２次転写位置には給紙部１Ｂから記録シートが給送されるようになっている。給紙部１Ｂは、複数の給紙カセット１Ｂ１と、給紙カセット１Ｂ１から繰り出される記録シートの搬送路に配置された複数の搬送ローラ１Ｂ２と、２次転写位置前方に位置するレジストローラ１Ｂ３とを備えている。本実施例では、給紙部１Ｂには、給紙トレイ１Ｂ１から繰り出される記録シートの搬送路に加えて給紙カセット１Ｂ１内に収容されていない種類の記録シートを２次転写位置に向け給送できる構成が備えられており、この構成は、画像形成部１Ａの壁面の一部を起倒可能に設けた手差しトレイ１Ａ１と繰り出しコロ１Ａ２とを備えている。
給紙カセット１Ｂ１からレジストローラ１Ｂ３に向けた記録シートの搬送路途中には、手差しトレイ１Ａ１から繰り出された記録シートの搬送路が合流し、いずれの搬送路から給送される記録シートもレジストローラ１Ｂ３によってレジストタイミングが設定されるようになっている。
【００５５】
書き込み装置５（図１１では、便宜上、符号５Ｂで示してある）は、原稿走査部１Ｃに有する原稿載置台１Ｃ１上の原稿を走査することにより得られる画像情報あるいは図示しないコンピュータから出力される画像情報により書き込み光が制御されて感光体３Ｂ、３Ｙ、３Ｃ、３Ｍに対して画像情報に応じた静電潜像を形成するようになっている。書き込み装置５の構成としては、レーザ光源と、回転多面鏡等の偏向器と、走査結像光学系、及びミラー群、からなるレーザ走査光学系や、一次元もしくは二次元に多数のＬＥＤが配列したＬＥＤアレイと、結像光学系からなる、ＬＥＤ書込み系等が用いられ、各書き込み光路末端に位置する感光体に対して主走査方向および副走査方向の書き込み操作により感光体３に対して潜像を形成することができる。
【００５６】
原稿走査部１Ｃには、原稿載置台１Ｃ１上の原稿を露光走査するスキャナ１Ｃ２が備えられており、さらに原稿載置台１Ｃ１の上面には、自動原稿給送装置１Ｃ３が配置されている。自動原稿給送装置１Ｃ３は、原稿載置台１Ｃ１上に繰り出される原稿を反転可能な構成を備え、原稿の表裏各面での走査が行えるようになっている。
【００５７】
書き込み装置５により形成された感光体３（図１１において符号３Ｂ、３Ｙ、３Ｃ、３Ｍで示す部材）上の静電潜像は、現像装置６（図１１では、便宜上、符号６Ｂで示してある）によって可視像処理され、中間転写ベルト２に１次転写される。中間転写ベルト２に対して各色毎のトナー像が重畳転写されると、２次転写装置９により記録シートに対して一括して２次転写される。
【００５８】
２次転写された記録シートは、表面に担持している未定着画像を定着装置１１によって定着される。定着装置１１は、詳細を図示しないが加熱ローラにより加熱される定着ベルトと定着ベルトに対向当接する加圧ローラとを備えたベルト定着構造を備えており、定着ベルトと加圧ローラとの当接領域、つまりニップ領域を設けることにより別ローラ方式の定着構造に比べて記録シートへの加熱領域を広げることができるようになっている。
定着装置１１を通過した記録シートは、定着装置１１の後方に配置されている搬送路切り換え爪１２によって搬送方向が切り換えられるようになっており、排紙トレイ１３に向けた搬送路と、反転搬送路ＲＰとに搬送方向が選択される。
【００５９】
以上のような構成を備えたカラープリンタ１では、原稿載置台１Ｃ１上に載置された原稿を露光走査することによりあるいはコンピュータからの画像情報により、一様帯電された感光体３に対して静電潜像が形成され、静電潜像が現像装置６によって可視像処理された後、トナー像が中間転写ベルト２に１次転写される。
【００６０】
中間転写ベルト２に転写されたトナー像は、単一色画像の場合にはそのまま給紙部１Ｂから繰り出された記録シートに対して転写され、多色画像の場合には１次転写が繰り返されることで重畳された上で記録シートに対して一括して２次転写される。２次転写後の記録シートは定着装置１１により未定着画像を定着された後、排紙トレイ１３あるいは、反転されて再度レジストローラ１Ｂ３に向けて給送される。
【００６１】
図１０において現像装置６はユニット構成とされており、シアンを対象とする現像ユニット（符号の数字にＣを添付したユニット）で説明すると、現像ユニットは現像ローラ６Ｃ１とドクタブレード６Ｃ２、２本の搬送スクリュー６Ｃ３Ａおよび６Ｃ３Ｂ、トナー濃度センサ６Ｃ４と外ケース６Ｃ’とからなる。現像ローラ６Ｃ１と搬送スクリュー６Ｃ３Ａ、６Ｃ３Ｂとの位置関係は、現像ローラ６Ｃ１に対して搬送スクリュー６Ｃ３Ａ、６Ｃ３Ｂの方が斜め下方向の位置にあり、２本の搬送スクリュー６Ｃ３Ａ、６Ｃ３Ｂは水平方向で並列に配設されている。外ケース６Ｃ’は、２本の搬送スクリュー６Ｃ３Ａ、６Ｃ３の２室に分ける仕切り板６Ｃ５が設けられている。この仕切り板６Ｃ５の紙面に対する奥側と手前側とは、現像剤が２本の搬送スクリュー６Ｃ３Ａ、６Ｃ３Ｂ間を循環できるように切り欠かれている。また、外ケース６Ｃ’は感光体３Ｃと対面する部分が開口しており、この開口部から現像ローラ６Ｃ１の一部が露出するようになっている。このように外ケース６Ｃ’は、図１０のように現像ローラ６Ｃ１の横で第１の搬送スクリュー６Ｃ３Ａの上の空間を少し多めにして現像ローラ６Ｃ１、搬送スクリュー６Ｃ３Ａおよびドクタブレード６Ｃ６を囲っている。現像ローラ６Ｃ１は、回転可能な非磁性の現像スリーブと内側に磁界発生手段であるマグネットが固定されて構成されている。現像剤は非磁性トナーと磁性キャリアのからなる２成分現像剤である。
【００６２】
現像剤は送り方向が反対の搬送スクリュー６Ｃ３Ａ、６Ｃ３Ｂによって攪拌されながら搬送され、２室を常に循環している。攪拌搬送されて循環している現像剤は、現像ローラ６Ｃ１側に位置する搬送スクリュー６Ｃ３Ａによって現像スリーブ表面に供給され、マグネットの磁力によって表面に磁気ブラシ状で保持されて現像スリーブの回転方向に汲み上げられる。汲み上げられた磁気ブラシ上の現像剤はドクターブレード６Ｃ５によって適正な量に穂切りされて感光体３Ｃと対向している現像部へと送られる。
【００６３】
ドクターブレード６Ｃ５で穂切りされて残った現像剤は重力で現像ローラ６Ｃ１の現像スリーブ表面で磁気ブラシ状の外側を落ちて搬送スクリュー６Ｃ３Ａに戻され、再度攪拌搬送されながら現像スリーブに供給することが繰り返される。
一方、現像部に送られた現像剤は感光体３Ｃ上の静電潜像にトナーが移行して顕像化される。顕像化に使われなかった現像剤は外ケース６Ｃ’内に戻り、マグネットの磁力が働かない部分で現像スリーブから離れて搬送スクリュー６Ｃ３Ａに回収される。このように現像剤は搬送スクリュー６Ｃ３Ａ、および６Ｃ３Ｂを攪拌搬送されて循環しながら現像スリーブに供給、回収される。また画像が繰り返し出力されるとトナー濃度が薄くなるのでトナー濃度センサ６Ｃ４で検知しながら一定濃度になるようにトナー補給（不図示）する。
【００６４】
次にクリーニング装置８について説明する。なお、図１０においては、便宜上、現像装置と同様にシアンを対象とする感光体３Ｃに付設されているクリーニング装置を対象として符号Ｃを付けて説明する）について説明するが他の色のユニットにおいても同様な構成である。
図１０において、クリーニング装置８はユニットで構成され、一次転写後、感光体３Ｃ上に残留したトナーを除去するもので、弾性体のクリーニングブレードやファーブラシあるいはそれらを併用したクリーニング部材が用いられている。本実施例では弾性体、例えばポリウレタンゴムのクリーニングブレード８Ｃ１とファーブラシ８Ｃ２及びファーブラシ８Ｃ２に接触して配設された電界ローラ８Ｃ３と電界ローラ８Ｃ３のスクレーパ８Ｃ４、さらに回収スクリュー８Ｃ５が備えられて構成されている。ファーブラシ８Ｃ２は導電性で電界ローラ８Ｃ３は金属である。
【００６５】
以上のような構成のクリーニング装置では、まず感光体３Ｃの回転方向とは逆方向のカウンタで回転しているファーブラシ８Ｃ２で、感光体３Ｃ上の残留トナーを掻き落とし、ファーブラシ８Ｃ２に付着したトナーはファーブラシ８Ｃ２に対してカウンタで回転している電界ローラ８Ｃ３で取り除き、電界ローラ８Ｃ３はスクレーパ８Ｃ４でクリーニングされる。このとき電界ローラ８Ｃ３にはバイアスが印可されており、静電気力で残留トナーが感光体３Ｃからファーブラシ８Ｃ２、ファーブラシ８Ｃ２から電界ローラ８Ｃ３へと移動させて最終的にスクレーパ８Ｃ４で掻き落とすことにより回収スクリュー８Ｃ５で廃トナーボトル（不図示）に回収される。あるいは現像ユニットに戻して再利用する。
【００６６】
クリーニング装置８Ｃと現像装置６Ｃとの位置関係はクリーニング装置８Ｃでの回収スクリュー８Ｃ５の部分が現像装置６Ｃの搬送スクリューのうちで現像ローラ６Ｃ１から遠い位置にある搬送スクリュー６Ｃ３Ｂの上側で外ケース６Ｃ’に対して上側に重なって配設されている。
【００６７】
中間転写ベルト２は各感光体３Ｍ、３Ｃ、３Ｙ、３Ｂと各転写装置の間に介在し、この間に各感光体から各色のトナー像が順次重ね合わせて転写され、感光体上のトナー顕像を坦持する。転写の方式としてはベルトを挟み込む様に感光体と対向して設けられた、転写電荷付与手段２Ｄが転写電界を発生させて静電的に転写する。図９乃至１１に示す構成においては、導電性発泡ＥＰＤＭゴム（ゴム硬度ＪＩＳ−Ａ　３０度、体積抵抗１０^８Ωｃｍ）で構成されたローラからなる静電電化付与手段２Ｄに対して約１．５ｋＶの電圧を印加して転写電界を発生させている。そのためニップは多少広くならざるを得ない。ニップは硬度と加圧力によるくい込み量で決定されている。
【００６８】
中間転写ベルト２は様々な材質のものを使用可能であるが、ここでは耐久性にすぐれ、ヤング率の高いポリイミド製のベルトや、表面平滑性に優れたＰＶＤＦ製ベルト、もしくはポリウレタン樹脂層の上にポリウレタンゴム層をもち、さらに上にフッ素成分を含有したコート層をもつ、表面が弾性の多層構造ベルトなどを使用している。特にポリウレタン多層構造ベルトは表面が弾性を持つため、感光体表面や紙表面との密着性が良く、一次転写、二次転写に優れている。いずれのベルトも１０^１０〜１０^１２Ωｃｍ程度の体積抵抗を持ち、トナーの載る部分の表面抵抗が１０^１２Ω／□以上の特性値となっており、転写性に優れている。また、本実施例では中間転写ベルトを使用する例を載せたが、機械レイアウトや、求められる精度、大きさなどから中間転写ドラム方式を採用しても構わない。
【００６９】
本実施例では感光体の半径は３０ｍｍである。本装置は主副走査方向共に６００ｄｐｉ書込解像度を持つ機械であり、感光体上の副走査方向の最小画素長さは４２．３μｍである。
上述の説明のように、伸びの許容度は元の線幅の最大６０％であるので、２５．４μｍの伸びまで許容可能であるとする。この機械の感光体および中間転写ベルトの駆動速度はともに平均速度が２４５ｍｍ／ｓｅｃに設定してある。
【００７０】
当初本装置の一次転写ニップ幅は８ｍｍであった。この条件下において、前述した「式５‘」より、線速比変動は±０．３２％以内に納めないと所望の６００ｄｐｉの１ｄｏｔ画像を解像できないことが分かる。
本実施例に用いられる中間転写体である中間転写ベルトの駆動ローラ径は３０φであり、ローラは０．５ｍｍの厚みのゴムローラとなっている。このローラはゴムローラということもあり、加工精度を余り上げることが出来ない。振れ精度は５０μｍ程度までが限界であり、このときローラの振れによるベルト速度の変動は±０．１６％にもなる。さらにベルトはＰＶＤＦ製であったが、ベルト厚みの誤差、ヤング率のムラなどによる、速度の変動が上乗せされている。実際にこの系でベルトの表面速度をレーザードップラー変位計にて測定したところ、±０．２５％程度の速度偏差があった。また、感光体の加工精度も同様に振れ５０μｍ程度であり、このときの外周速度変化はやはり±０．０８％程度である。こちらも実測したところ±０．１％程度の速度変動が生じていた。
【００７１】
この二つの速度変動を合わせると、最大で±０．３５％程度となり、先に求めた許容度±０．３２％を越えている。実際にこのとき感光体上に副走査方向に１ｄｏｔライン作像し、１ｄｏｔライン作像しない繰り返しのパターンを作って中間転写ベルト上でどのようになるか観察したが、満足な解像度が得られなかった。
【００７２】
そこで、ベルト側の速度変動をこれ以上抑えるのは困難であったので、感光体の加工精度を上げ、振れ量を１０μｍまで抑えたところ、感光体の速度変動が実測で０．０４％程度まで減少し、ベルトの変動と合わせた速度変動が±０．２９％程度となり、解像することが可能となった。
【００７３】
しかし、感光体の加工精度を上げるとコストアップに直結したので従来の感光体の加工精度で解像度を上げるために、機械の構成を変更し、ニップ幅を減少させることとした。請求項２に有るように、ニップ幅を従来の８ｍｍでは「式８−２」より、
Ｎｗ　＜ΔＩｗ＊Ｖｐｃ／（ΔＶｐｃ＋ΔＶｂｅｌｔ）−Ｉｗ
これを変形して線速変動の形に直すと
Ｎｗ＜ΔＩｗ／（ΔＶｐｃ／Ｖｐｃ＋ΔＶｂｅｌｔ／Ｖｐｃ）−Ｉｗ…式９
ここで、すでにΔＶｐｃ／Ｖｐｃ、ΔＶｂｅｌｔ／Ｖｐｃが各々０．１％、０．２５％と分かっているのでこのときに解像する為の必要なニップ幅条件は
Ｎｗ＜２５．４μｍ／（０．１％＋０．２５％）−４２．３μｍ＝７．２（ｍｍ）
と求められるので、この装置でニップ幅を８ｍｍから６ｍｍへ変更したところ、感光体の加工精度を上げなくとも良好な解像度の画像を得られるようになった。
【００７４】
この後、この実験に用いた装置には６００ＤＰＩのかわりに１２００ＤＰＩの解像度を持つレーザー書き込み系が搭載された。今後更なる高画質化のために、機械のレーザー書き込み系は一層高密度化されていく。それに対応した実験系にするためである。この解像により、感光体上の最小画素が長さは６００ｄｐｉに比べ半分の２１．２μｍとなった。よって、許容される画像の伸びも半分となり、ΔＩＷ＝１２．７μｍとなり、従来の感光体やベルトの部品精度、駆動精度では十分な解像が得られなくなった。
【００７５】
従来の部品精度はそのままに、１２００ｄｐｉを解像すべく、以下のごとく対応した。
１つはニップ幅の更なる低減である。部品精度そのままであれば、同じく式９を用いて、
Ｎｗ＜１２．７μｍ／（０．１％＋０．２５％）−２１．２μｍ〜３．６ｍｍ
となる。
しかし、機械構成上、確実に感光体と中間転写体が接触するような条件で、ニップを３ｍｍ以下に安定に取るのは難しく、この機械では４ｍｍまでが限界であった。これでは「式９」を満足せず、１２００ｄｐｉ画像を解像できないことなる。そこで、さらに感光体へ固体潤滑剤の塗布を行い、感光体の表面摩擦係数を低下させた。固体潤滑剤にはステアリン酸亜鉛の粉体を用いて、塗布の方法としては感光体クリーニングブラシにステアリン酸亜鉛の集合体を擦りつけ、微粉体としてブラシに削り取られたステアリン酸亜鉛を感光体に微量塗布することで行った。この方法以外にも、供給するトナーの中にあらかじめ微粉体としたステアリン酸亜鉛を重量比０．７％程度混入させ、トナー表面に付着して現像器の磁気ブラシ穂を介して感光体に供給する方法なども考えられる。
【００７６】
この状態で１００枚ほどＡ４用紙にテスト印刷した後の感光体の表面摩擦係数は０．１５程度であり、ベルトの方は０．４程度であった。元々の値は感光体が０．４程度、ベルトは０．４５程度であるので、感光体を介してベルト表面にも多少の潤滑剤の混入が見られる。
このように感光体と中間転写ベルトとの表面摩擦係数に差を付けることで、画像の伸びに余裕度が生じる。これは前述の実験結果にも明かである。よって、ニップ幅の更なる低減（６ｍｍ→４ｍｍへ）と感光体への固体潤滑剤塗布によって、機械設計の許容度があがり、１２００ｄｐｉの解像度を中間転写ベルト上に忠実に再現できるようになり、十分な解像度を達成できた。
【００７７】
（実施例２）
前述のように、感光体３と中間転写体２の一次転写位置における変動の位相を抑えることで、画像の伸びも抑えることが出来る。そこで、図１２に示すように、感光体３と転写ベルトの駆動ローラ（図１２中、符号２Ａ’で示すローラ）の径を共にφ６０ｍｍした例を示す。こうすることで、感光体および駆動ローラの偏心によるそれぞれの一次転写位置の外周速度の変動周期が、符号ＰＬで示すピッチにおいて一致する（なお、駆動ローラ２Ａ’とブラック画像用感光体３Ｂとの間は紙面の都合で一部が省略されているが、感光体間でのピッチＰＬに設定されている。）。このようにしておいて、感光体および駆動ローラの製造時に、部品の精度検査を行い、回転中心が幾何中心に対してどの位置にきているかを全数検査し、図１３において符号ＭＰで示すように、感光体３および駆動ローラ２Ａの外周位置に印を施す。
製造ラインにおいて各部品を組み付ける際に、それらの印ＭＰを目安にして一次転写位置でのベルトと感光体の速度変動の位相が合うように組み付けることで、速度変動の影響を極小にすることが出来る。
【００７８】
しかしながら長期的に使用を継続していくと、ベルトのスリップなどにより位相がずれてくる可能性もあるので、定期的メンテナンスの際には、メンテナンス員が随時調整するようにすればよい。また、各感光体および中間転写ベルト駆動ローラにマーク検出機能を付け、作像を開始する前に自動で位相を調整できるように機械を構成すれば尚よい。具体的には感光体や駆動ローラの偏心方向のマークとして、ローラの端部に反射率の異なる状態を作り出したり、切り欠きを設けたりして、反射型光センサによって検出したり、物理的な突起やへこみなどをつけ、接触センサなどにより検知する方法などが挙げられる。このようにして位相を検知し、各々の感光体を独立に駆動制御し、所定の位相で動くように停止位置や起動タイミングを調整する。この場合は中間転写ベルトと感光体の接離機構があることが望ましく、ベルトが感光体から離れた状態で位相あわせした方が感光体や中間転写ベルト表面への摩擦による傷などの発生が防止されてよい。
【００７９】
【発明の効果】
請求項１乃至５記載の発明によれば、関係式に設定された条件を満たすように機械を設計することで潜像担持体およびこれからの画像の転写体との間に線速差が生じた場合にその線速差を規定することにより画像の引きずりによる伸びを抑えることができ、所望の解像度を維持したまま潜像担持体上のトナー像を転写体上に忠実に転写し、良好な画像を得ることが可能となる。
【００８０】
請求項６および７記載の発明によれば、一次転写位置での潜像担持体および転写体との間での速度変動移送を揃えることにより偏心などの機械的な速度変動要因が存在していても画像の伸びが発生する機会を少なくできるように感光体と中間転写体の駆動速度の部品精度に依存する変動の影響を最小限にして、所望の解像度を維持したまま潜像担持体上のトナー像を転写体上に忠実に転写し、良好な画像を得ることが可能となる。
【００８１】
請求項８および９記載の発明によれば、感光体の表面摩擦係数を中間転写体のものをよりも小さくすることで、請求項１乃至５記載の発明において設定した条件を緩和し、より部品の精度を落とし低コストに機械を設計しても、良好な画像を得ることが可能となる。
【００８２】
請求項１０記載の発明によれば、請求項８，９記載の発明において設定した条件下においても、転写率を低下させないようにできるので、所望の解像度を維持したまま潜像担持体上から転写された転写体上のトナー像を効率よく紙に転写し、良好な画像を得ることが可能となる。
【図面の簡単な説明】
【図１】請求項１記載の発明の実施形態を説明するための線速里画像の伸びとの関係を示す線図である。
【図２】潜像担持体と転写体との間の線速差による画像の伸びに対する影響を説明するための模式図である。
【図３】潜像担持体上での理想的な細線が線速差により転写位置が変化することを説明するための模式図である。
【図４】潜像担持体上での定ピッチの細線が線速差の傾向によりどのような転写状態の変化となるかを実際の画像を対象として説明するための模式図である。
【図５】潜像担持体の偏心状態を説明するための模式図である。
【図６】理想的な細線の集合が細線として認識できる状態の限界を説明するための模式図である。
【図７】潜像担持体と転写体との表面での摩擦係数と画像の伸びとの関係を説明するための線図である。
【図８】潜像担持体および転写体の表面での摩擦係数と画像の伸びとの関係を説明するための模式図である。
【図９】本発明の実施例による画像形成装置の内部機構を説明するための模式図である。
【図１０】図９に示した内部機構のうちの一部を拡大して示した模式図である。
【図１１】図９に示した内部機構を装備した画像形成システムの全体構成を示す模式図である。
【図１２】図９に示した構成の一部変形例を説明するための模式図である。
【図１３】図５に示した偏心状態に対する対策を説明するための模式図である。
【符号の説明】
１　カラープリンタ
２　中間転写体
２Ａ　駆動ローラ
３　感光体[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus, and more particularly to deterioration of a transferred image from a latent image carrier, particularly due to a difference in linear velocity or a difference in friction coefficient generated between the latent image carrier and a transfer member. The present invention relates to a configuration for preventing a decrease in resolution due to dragging an image.
[0002]
[Prior art]
An image forming apparatus is one of the apparatuses in which an image writing process is adopted by using an optical scanning mechanism. The image forming apparatus includes a copying machine, a printer or a facsimile apparatus using an electrostatic recording method or an electrophotographic method. Further, there is an image forming apparatus such as a printing machine.
[0003]
Some of these image forming apparatuses have a configuration capable of forming a single color image by providing a single latent image carrier and a configuration capable of forming a multicolor image. An image forming apparatus having the latter configuration is known as a full-color printer or a full-color copying machine. In these apparatuses, the following configuration is adopted as a configuration employing the above-described electrostatic recording system or electrophotographic system. is there.
That is, a plurality of developing devices are arranged so as to be in contact with a photoconductor as one latent image carrier, and a toner image of each color is created for each rotation of the photoconductor, and the images are transferred from the photoconductor to an intermediate transfer member. Alternatively, there is a so-called one-drum system in which a color toner image is created by sequentially transferring the image onto paper held on a transfer drum or the like (for example, Non-Patent Document 1).
[0004]
The transfer method in an image forming apparatus capable of forming a full color includes a primary transfer process in which toner images of a plurality of colors are superimposed on the above-described intermediate transfer body, and a toner image superimposed in the primary transfer process is collectively transferred to paper. There are an intermediate transfer method in which the secondary transfer process is combined, and a direct transfer method in which a color toner image is created by sequentially transferring the image onto paper held on a transfer drum or the like. The direct transfer method has a simple structure and is low in cost, but it is difficult to perform stable image formation when transferring to paper a plurality of times because the conditions vary depending on the resistance and moisture content of the paper. The intermediate transfer method has a feature that the image is transferred to paper only once, so that the image quality is stable and the paper type is good.
[0005]
However, in any of the configurations, in order to obtain a color image using four colors, the photosensitive member had to be rotated four times, and productivity was not improved. Therefore, for the purpose of responding to high speed, the number of photoconductors is increased by the number of colors (usually three or four), and corresponding developing devices are arranged so that the paper continuously contacts the photoconductor. An image forming apparatus employing a so-called tandem type or in-line type configuration capable of obtaining a color image has also been proposed (Non-Patent Document 2).
[0006]
In a configuration using a plurality of photoconductors, if the peripheral speed of the photoconductor is the same as that of the one-drum system, it is possible to form an image at a speed four times or more. However, in the case of a direct transfer method in which a plurality of photoconductors are sequentially transferred directly from a photoconductor to a paper in a configuration using a plurality of photoconductors, instability at the time of paper transfer and alignment at the time of paper conveyance are not considered. Failure may occur. Therefore, a so-called tandem intermediate transfer system using a tandem system and using an intermediate transfer member has been proposed (for example, Patent Document 1).
On the other hand, in the above-described image forming apparatus, an image is written on a photoconductor, that is, a laser diode (hereinafter, referred to as an LD) as an exposure device, an electric circuit for driving the laser diode, and a laser beam emitted from the LD are guided to the photoconductor. An optical system such as a lens is used, the design accuracy of each of these members is increased, and further improvement in the writing density is seen in conjunction with the demand for higher image quality by the user.
[0007]
[Non-patent document 1]
The Electrographic Society of Japan, "Continuing Basics and Applications of Electrophotographic Technology", November 15, 1996, First Edition, First Edition, p34, Figure 1.23.
[Non-patent document 2]
The Electrographic Society of Japan, "Continuing Basics and Applications of Electrophotographic Technology", November 15, 1996, First Edition, First Edition, p36, Fig. 1.25
[Patent Document 1]
JP 2002-182451 A (“0037” column to “0038” column, FIG. 1)
[0008]
[Problems to be solved by the invention]
With respect to the above-mentioned writing density, a device having an output resolution of about 300 to 400 DPI had the highest image quality ten years ago, but in recent years, 600 DPI has become a standard, and some devices have achieved 1200 DPI. At such a high resolution, the size of the minimum dot formed on the photoreceptor is as very small as about 21 μm square. For this reason, there is a possibility that the smallest pixel cannot be resolved due to a slight impact or unevenness in the linear velocity at the time of transfer. For example, as a model declaring a resolution of 1200 DPI in the sub-scanning direction, there are two lines (approximately 21 μm in thickness, ideally about 21 μm) which are transferred on paper and drawn on the side of the main scanning direction at an interval of about 42 μm between centers. Is desirable), but it is an essential condition that the boundary can be identified well. Conventionally, in the case of a resolution of 600 DPI, it has been said that it is only necessary to be able to identify two lines with a center distance of about 85 μm, but in the case of the above-described high resolution, such an identification state is insufficient.
[0009]
As described above, as image quality and resolution have been improved, it has been desired to reproduce an image formed on a photoreceptor without deterioration. However, as represented by an intermediate transfer belt, In a color image forming apparatus using an intermediate transfer member, the resolution of an image is most likely to be lost during transfer (primary transfer) from a photosensitive member to an intermediate transfer member.
This is because an image is rubbed due to a linear velocity difference generated at a contact portion (referred to as a primary transfer nip) between the photosensitive member and the intermediate transfer member, and the minimum pixel cannot be reproduced. There are various reasons for the linear velocity difference, but the most problem is the eccentricity of the photoconductor. The photoreceptor usually obtains rotational force by transmitting power from a drive motor provided in the machine body to a photoreceptor shaft via a gear or a belt. At this time, if the rotation center of the photoconductor does not coincide with the rotation axis, even if the rotation axis of the drive rotates at a constant angular velocity, if the rotation center of the photoconductor does not coincide with the rotation center, the outer peripheral speed of the photoconductor becomes It changes every rotation of the photoconductor.
[0010]
Similarly, for example, in the case of a drum-shaped intermediate transfer member, it is considered that the eccentricity is affected similarly to the photoreceptor, and also in a belt member such as an intermediate transfer belt, a belt driving roller for driving the belt is used. It is conceivable that the circling speed of the belt fluctuates every one rotation of the drive roller due to the eccentricity. As described above, when the two rotating bodies independently obtain the driving force, a linear velocity difference is inevitably generated at the contact portion. Further, as described above, in recent years, in order to increase the speed, a so-called tandem system using a plurality of photoconductors is becoming mainstream. In this case, since a plurality of photoconductors are independently driven, Further, it is more susceptible to the speed difference at the transfer nip, and for these reasons, it is presently difficult to obtain a desired high-resolution image reproducibility.
[0011]
An object of the present invention is to provide a structure capable of securing high-resolution image reproducibility at low cost in the above-described conventional image forming apparatus, particularly in an apparatus having a structure for transferring an image from a plurality of latent image carriers. To provide an image forming apparatus.
[0012]
[Means for Solving the Problems]
The invention according to claim 1 is an image forming apparatus having a configuration for obtaining a final recording output through a process of transferring an image from a latent image carrier,
Let ΔIw be the allowable variation width of the desired minimum pixel length Iw in the sub-scanning direction, Nw be the contact width between the latent image carrier and the transfer body used in the transfer process, and Vpc be the average value of the peripheral image carrier outer peripheral speed. Then, with respect to the absolute value of the difference between the outer peripheral speed Vpc (t) and the speed Velt (t) of the transfer member at the contact position between the latent image carrier and the transfer member at a certain time t,
| Vpc (t) −Vbelt (t) | <ΔIw * Vpc / (Nw−Iw) (1)
It is characterized by satisfying.
[0013]
According to a second aspect of the present invention, there is provided an image forming apparatus having a configuration in which a final print output is obtained through a process of transferring an image from a latent image carrier, a permissible variation of a desired minimum pixel length Iw in the sub-scanning direction. When the width is ΔIw, the natural peripheral speed variation width of the latent image carrier is ΔVpc, and the natural speed variation width of the surface of the transfer body used in the transfer process is ΔVb, the contact width Nw between the latent image carrier and the transfer body is ,
Nw <ΔIw * Vpc / (ΔVpc + ΔVbelt) −Iw (2)
Is satisfied.
[0014]
According to a third aspect of the present invention, in addition to the first or second aspect, the allowable variation width is set within 60% of the minimum pixel length in the sub-scanning direction.
[0015]
According to a fourth aspect of the present invention, in addition to the first or second aspect, the minimum pixel length in the sub-scanning direction is a minimum resolution length of a latent image on a photoconductor.
[0016]
According to a fifth aspect of the present invention, in addition to the first aspect of the present invention, when there is a difference between the average speed of the photosensitive member and the intermediate transfer member, the intermediate transfer member side speed decreases. It is characterized by such setting.
[0017]
According to a sixth aspect of the present invention, in addition to the first aspect of the invention, the transfer member used in the transfer process is an intermediate transfer member constituted by a belt, and a driving source of the belt is provided. The latent image carrier and the transfer member are set to have substantially the same outer diameter as the outer peripheral speed due to the eccentricity of the latent image carrier. It is characterized in that the phase of the speed fluctuation at the transfer position and the phase at the primary transfer position in the fluctuation of the belt speed due to the eccentricity of the transfer body by the drive roller are matched.
[0018]
According to a seventh aspect of the present invention, in addition to the sixth aspect, the plurality of latent image carriers are in contact with the transfer member, and the toner developed on each of the latent image carriers is sequentially transferred to the transfer member. When a color image is obtained by transfer, the speed variation phase of each latent image carrier is aligned with the speed variation phase of the transfer body at the primary transfer position of each latent image carrier.
[0019]
According to an eighth aspect of the present invention, in addition to the first aspect, when the friction coefficient of the transfer member with respect to the toner is higher than the friction coefficient of the latent image carrier with the toner. Is given by
| Vpc (t) −Vbelt (t) | <1.5 * ΔIw * Vpc / (Nw−Iw) (3)
It is characterized by satisfying.
[0020]
According to a ninth aspect of the present invention, in addition to the first aspect of the present invention, when the friction coefficient of the transfer body with respect to the toner is higher than the friction coefficient of the latent image carrier with the toner. , (2),
Nw <1.5 * ΔIw * Vpc / (ΔVpc + ΔVbelt) −Iw (4)
It is characterized by satisfying.
[0021]
According to a tenth aspect of the present invention, in addition to the eighth or ninth aspect, the friction coefficient of the toner on paper is higher than the friction coefficient of the transfer body on toner. I have.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
(Embodiment of the invention described in claim 1)
The inventor of the present invention experimented by arbitrarily changing the speed of a photoreceptor used as a latent image carrier and a speed of a transfer member (hereinafter, referred to as an intermediate transfer member) used in a primary transfer step which is one of the transfer steps. Was repeated to examine how much the fine line formed on the photoconductor was stretched by the linear velocity difference between the photoconductor and the intermediate transfer member and transferred to the intermediate transfer member. According to this experiment, as shown in FIG. 1, the amount by which the thin line is stretched has a correlation not only with the speed difference between the photosensitive member and the intermediate transfer member but also with the contact width (primary transfer nip width) between the photosensitive member and the intermediate transfer member. I found it.
[0023]
FIG. 1 shows the transfer state of an image in the primary transfer nip width region. In FIG. 1, the thin line is elongated even if the intermediate transfer member is faster or slower than the photosensitive member with respect to the speed difference. I understand that there is.
[0024]
The reason why such a phenomenon occurs is as follows.
First, as shown in FIGS. 2 and 3, an ideal fine line having a thickness of “0” is formed on a photoconductor (for convenience, reference numeral 3 shown in FIG. 9 used for describing the embodiment) in the sub-scanning direction. Suppose it exists along. It is assumed that this is in contact with the photosensitive member 3 and the intermediate transfer member (for convenience, the reference numeral 2 shown in FIG. 9 used in the description of the embodiment is used) with a contact width (nip width) Nw. It is assumed that the transfer body 2 is driven at constant outer peripheral speeds Vpc and Vbelt.
At this time, if there is no speed difference, at the entrance of the nip, the point a ′ on the intermediate transfer body 2 where the thin line a on the photoreceptor 3 first contacts remains the same after passing through the nip. However, when there is a linear velocity difference (in this case, the belt is assumed to be fast), the thin line is elongated in the nip, and at the exit of the nip, a point a ′ that first contacts as shown in FIG. The thin line a on the photoreceptor is still in the nip even after coming out of the nip, and if the point on the intermediate transfer member which was in contact when the fine line a was separated by further rotation is b ', the intermediate transfer The thin line stretched and transferred on the body has a length of a ′ to b ′, and when this is Tw,
Tw = Nw * (Vbelt−Vpc) / Vpc
It becomes.
[0025]
Even when the intermediate transfer member is slower than the photosensitive member, the image similarly expands as indicated by the arrow in FIG.
Tw = Nw * (Vpc−Vbelt) / Vpc
In both cases, take the absolute value of the linear velocity difference,
Tw = Nw * | Vpc−Vbelt | / Vpc
Can be expressed as
[0026]
Next, consider an image having the actual length. An image having an actual length is considered to be a state where the above-described ideal thin lines having a width of “0”, that is, the thin lines are overlapped so as to have an image length Iw on the photoconductor 3 as shown in FIG. In FIG. 4, the top row shows a thin line with a constant pitch on the photoconductor 3, the middle row shows the case where the intermediate transfer member (indicated as “medium rotation” in the figure) is slower than the photoconductor, The transfer length of the image when the speed is faster than the photoconductor is shown.
Fine lines arranged at a constant pitch on the photoreceptor, when transferred to the intermediate transfer member, change in pitch interval depending on the speed ratio. For example, if Vbelt is 1/10 faster than Vpc, the image on the PC is reduced to 1/10 on the belt, and conversely, if the speed is 10 times, it is 10 times longer.
If there is a speed difference, each ideal thin line causes a drag elongation according to the speed difference and the nip width as described above, so the length of the image finally transferred to the intermediate transfer member is:
Iw '= Tw + Iw * Vblet / Vpc
Becomes
The elongation of the image (ΔIw)
ΔIw = Iw′−Iw = Tw + Iw * Vbelt / Vpc−Iw
= Iw * Vbelt / Vpc + Nw * | Vbelt-Vpc | / Vpc
It becomes. Here, it is divided into two cases, and if ΔV = | Vbelt−Vpc |

It can be expressed as.
[0027]
Here, the amount of elongation of the thin line is directly linked to the proposition that the transfer is performed while resolving the required resolution. In other words, in order to transfer a photoconductor image of, for example, 600 dpi onto the intermediate transfer body as it is, only a slight extension is allowed for the thin line.
[0028]
From the formulas 1-1 and 1-2 of the nip and the speed difference obtained in this study, the speed difference ΔV is
ΔV = Vpc * ΔIw / (Nw ± Iw) Equation 2
It can be expressed as.
From the equations obtained from these experimental results, it is possible to estimate the amount of elongation of the thin line when a speed difference occurs from the relationship between the average speed of the photoconductor, the average speed of the intermediate transfer body, and the primary transfer nip width. Conversely, in order to keep the amount of elongation within a certain value, the difference in the outer peripheral speed between the photoconductor 3 at the primary transfer position and the intermediate transfer body 2 must be kept within a certain value.
[0029]
The above result is based on the premise that the outer peripheral speed between the photosensitive member 3 and the intermediate transfer member 2 is constant. However, the speed of the primary transfer portion is not constant due to the eccentricity of the shaft. There are many. Therefore, it can be seen that even when the average velocities are matched, the image expands due to the constant fluctuation in the velocities.
[0030]
Here, as shown in FIG. 5, when only the eccentricity of the rotation shaft is considered, the outer peripheral speed at the primary transfer nip of the photosensitive member and the intermediate transfer member at time t can be expressed as follows. In FIG. 5, a solid line indicates a circle based on the geometric center of the photoconductor, and a two-dot chain line indicates a circle based on the driving rotation center of the photoconductor.
Vpc (t) = ωpc * (rpc + dpc * sin (Vpc * t / rpc + A)) Equation 3
However,
Vpc: average peripheral speed of the photoconductor
ωpc: angular velocity of photoconductor drive shaft
rpc: photoconductor drum radius
dpc: distance between the rotation center of the drum and the geometric center
A: Initial phase
Vbelt (t) = ωbelt * (rbelt + dbelt * sin (Vbelt * t / rbelt + B)) Equation 4
However,
Vbelt: average peripheral speed of the intermediate transfer member
ωbelt: angular velocity of the drive roller shaft of the intermediate transfer member (here, belt)
rbelt: radius of the intermediate transfer belt driving roller
dpc: distance between the rotation center of the intermediate transfer belt drive roller and the geometric center
B: Initial phase
Therefore, from the equation 2, if the linear velocity difference is always as shown in the equation 5 as the strictest condition, it is possible to prevent image expansion.
ΔV = | Vpc (t) −Vbelt (t) | <Vpc * ΔIw / (Nw−Iw) Equation 5
In terms of the linear velocity ratio, Equation 5 is transformed
ΔV / Vpc <ΔIw / (Nw−Iw) Equation 5 ′
It is expressed as
The above is the reason why the mathematical formula specified in the first aspect is set.
[0031]
(Embodiment of Claim 2)
The amount of elongation when targeting the smallest pixel is also correlated with the nip width, and as the nip width increases, the amount of elongation also increases, and conversely, the linear velocity difference between the photoconductor and the intermediate transfer body becomes a constant value. In this case, it has been found by experiments by the present inventor that the amount of elongation of the fine wire is determined by the nip width.
[0032]
When only the shake of the photoconductor is considered, the width ΔVpc of the speed fluctuation of the photoconductor is obtained from Expressions 3 and 4.
ΔVpc = ωpc * dpc * sin (Vpc * t / rpc) Equation 6
Similarly, the width of the speed fluctuation of the intermediate transfer member is
ΔVbelt = ωbelt * dbelt * sin (Vbelt * t / rbelt) Equation 7
It can be expressed as. Such speed fluctuations due to eccentricity cannot be eliminated at all because they are determined by the accuracy of the individual components, and the accuracy is determined by a balance between cost and function. Therefore, it is difficult to keep this value within a certain value, and in that case, it is understood from Expression 1 that it is desirable to reduce the nip width Nw in order to reduce ΔIw. From this, the desired amount of elongation of the image, the photoconductor, the average speed of the intermediate transfer body coincides, if a certain amount of speed fluctuation is allowed,
Nw <ΔIw * Vpc / (ΔVpc ± ΔVbelt) ± Iw Equation 7-1
And as the most severe conditions,
Nw <ΔIw * Vpc / (ΔVpc + ΔVbelt) −Iw Equation 8-2
It is preferable that the setting is made as follows.
[0033]
In the above-described example, the diameter of the photosensitive member 3 is a drum shape, and the diameter of the intermediate transfer member 2 is a belt shape. This can be similarly considered even when the photosensitive member 3 is a belt, the intermediate transfer member 2 is a drum, both are belts, and both are drums. In the above-described example, only the eccentricity of the rotation axis of the belt used for the photoconductor 3 and the intermediate transfer body 2 is considered as a factor of the speed fluctuation. Thickness unevenness, roundness of the belt drive roller, fluctuation of the angular velocity of the rotation center axis, etc. must also be considered in the actual diameter.However, the one that is periodic and has the greatest effect is the eccentricity of the shaft. Speed fluctuations can be similarly considered.
[0034]
(Embodiments of the invention described in claims 3 and 4)
When discussing the resolution, the resolution of 600 dpi in the sub-scanning direction is determined by whether two lines with a center interval of 84.6 μm drawn in the main scanning direction appear separately or invisible as shown in FIG. Is often determined.
That is, ideally, lines having a thickness of 42.3 μm are arranged at an ideal line interval with an interval of 42.3 μm (see FIG. 6A), and 42 lines on the photoreceptor are formed. When a 0.3 μm line extends 42.3 μm on the intermediate transfer member to 84.6 μm, that is, when the image portion: non-image portion = 10: 0, the adjacent line comes into contact with the adjacent line and two lines (See FIG. 6C). In this case, the permissible elongation of the fine line allowed in the primary transfer portion of this system is 42.3 μm, and it is necessary to design the machine so that the fine line does not extend further in the primary transfer portion. Based on the actual permissible amount and the resolution on the paper finally obtained, the elongation of the image at the primary transfer portion is further reduced, and as shown in FIG. In many cases, the ratio of the parts is up to about 8: 2. In other words, only an extension of about 30 to 60% with respect to the width of the original line is allowed.
[0035]
Note that a line in the sub-scanning direction (a direction orthogonal to the paper transport direction) is affected by the line speed difference, and the smallest pixel length that is the thinnest is most affected. It is desirable that
[0036]
Normally, it often corresponds to the minimum resolution of a latent image formed on the photoconductor 3 as in the above example.
[0037]
(Embodiment of Claim 5)
Here, comparing the expressions 1-1 and 1-2, the image elongation ΔIw is obtained by the expression 1-1 when the speed of the intermediate transfer member is lower than the speed of the photosensitive member. It can be seen that the amount of elongation is smaller than that. That is, in the above description, the elongation effect by the nip width of the ideal thin line is determined by the absolute value of the linear velocity difference both when the photoconductor is fast and when the photoconductor is slow, but the overall pitch is When the intermediate transfer member is slow, the line shrinks because of the ratio. At a certain value, the shrinkage and the extension are canceled out, and the elongation of the image is suppressed. Therefore, when a speed difference is inevitably attached or must be applied (for example, when a relative speed difference is provided to increase the transfer rate or suppress reverse transfer), the intermediate transfer member is not attached to the photosensitive member. It would be good to make the speed of the program slower.
[0038]
(Embodiment of Claim 6)
Here, attention is paid to Expressions 3 and 4 described above. When attention is paid to Expression 5 shown in the embodiment of the present invention, the speed difference between the photosensitive member 3 and the intermediate transfer member 2 at the primary transfer portion at a certain time t mainly depends on the photosensitive member 3 and the intermediate transfer member. In the case of the eccentricity of the rotation center of No. 2, it can be seen that each speed fluctuates at a constant period. As long as there is no difference in the linear velocities, the image does not expand, so that it can be seen that the respective velocities may vary.
However, it can be seen that the linear velocity difference cannot always be minimized unless the fluctuation periods match each other. For example, when the photoconductor 3 is in the form of a drum and the intermediate transfer body 2 is in the form of a belt, the photoconductor diameter and the drive roller diameter of the intermediate transfer belt are the same, and the phases match. Need to be.
[0039]
In that case, from Equations 3 and 4, when Rpc = rbelt = r, ωpc = ωbelt = ω, Vpc = Vbelt = V, and A = B = 0,
| Vpc (t) -Vbelt (t) |
= Ωpc * (rpc + dpc * sin (Vpc * t / rpc + A)) − ωbel
t * (rbelt + dbelt * sin (Vbelt * t / rbelt + B))
= Ω * (r + dpc * sin (V * t / r)) − ω * (r + dbelt * sin
(V * t / r))
= Ω * (dpc * sin (V * t / r) -dbelt * sin (V * t / r))
= Ω * (dpc-dbelt) * sin (V * t / r)
If the phases match, the linear velocity difference becomes a sine cycle corresponding to the difference in the amount of eccentricity, and even if eccentricity occurs, the velocity difference can be made very small.
[0040]
(Embodiment of Claim 7)
An image forming apparatus such as a color printer may employ a so-called tandem configuration in which a plurality of photoconductors are used. In this case, a plurality of primary transfer positions are required for the number of photoconductors.
In order for the above-described phase matching to reduce the linear velocity difference in the primary transfer unit to be small without failure in each primary transfer unit, it is necessary that all the phases in each primary transfer unit are aligned. However, there is one intermediate transfer member, and the speed fluctuation cycle is fixed. Therefore, it is necessary to adjust the speed of the photoconductor side to the speed fluctuation of the intermediate transfer member. Therefore, if the phase of the photoconductor is adjusted to the speed fluctuation at each primary transfer position, it is possible to provide an opportunity for less image expansion even in the tandem configuration.
[0041]
In this case, when the intermediate transfer member is a belt, the distance (pitch) between the photosensitive members is not particularly limited as long as the diameter of the drive roller of the belt is equal to the diameter of the photosensitive member. In the case of a tandem machine in contact with the photoconductor, if the distance between the primary transfer units is set to half of the outer circumference of the photoconductor, the second and fourth photoconductors may be driven in opposite phases with respect to the first and third photoconductors. Even when the primary transfer positions are halfway, the distance from the driving roller may be calculated to match the phases.
[0042]
However, if the distance between the primary transfer portions is equal to the outer circumference of the photoconductor due to the configuration of the machine, all the photoconductors need to be driven in the same phase. It will be easier. Generally, it is desirable that the distance between each primary transfer portion and the distance from the outermost primary transfer position to the drive roller match the outer peripheral length of the photoconductor.
[0043]
(Embodiments of the invention described in claims 8, 9, and 10)
On the other hand, the inventor focused on the friction coefficient of the surface between the photoconductor 3 and the toner and between the toner and the intermediate transfer body 2 and conducted an experiment similar to that of FIG.
That is, in the experiment in which the result of FIG. 1 was obtained, the experiment result was obtained under the condition that the friction coefficient of the photoconductor with respect to the toner was almost equal to that of the intermediate transfer belt (photoconductor = 0.54, intermediate transfer belt = 0.47). On the other hand, FIG. 7 shows the results of experiments conducted under the conditions where both friction coefficients were reduced and when the friction coefficient of the intermediate transfer belt was higher than the friction coefficient of the photosensitive member. As described above, when the friction coefficient of the intermediate transfer member is higher than that of the photoreceptor, the phenomenon that the image elongation due to the difference in linear velocity is suppressed below the theoretical value was confirmed. In particular, the case where the speed of the intermediate transfer member is slower than that of the photosensitive member appears remarkably, but the elongation is reduced to about half or less as compared with the case where the friction coefficient is high.
[0044]
FIG. 8 is a diagram for explaining the behavior of the toner at the friction coefficient. FIG. 8A shows the state of the toner layer immediately after being sandwiched between the photoconductor and the intermediate transfer member. FIG. 8B shows a case where the photosensitive member 3 and the intermediate transfer member 2 have the same surface friction coefficient. FIG. 8C shows a case where the photosensitive member has a smaller surface friction coefficient than the intermediate transfer member. Is shown.
[0045]
In FIG. 8, when a speed difference occurs between the photoconductor 3 and the intermediate transfer body 2, it is considered that the speed is determined by a position in the toner layer where a slipping interface appears. That is, when both the photoconductor 3 and the intermediate transfer body 2 have the same degree of friction coefficient with respect to the toner (see FIG. 8B), the shear position for absorbing the speed difference is not uniquely determined, and It is stretched so that the whole collapses. On the other hand, when there is a difference in the coefficient of friction between the photoconductor 3 and the intermediate transfer body 2 (see FIG. 8C), the photoconductor 3 becomes slippery at the slippery position. Can absorb the speed difference. In addition, since the toner itself has a cohesive force, when the friction coefficient of the photoreceptor 3 is low, a shear occurs between the surface of the photoreceptor 3 and the surface of the toner layer, so that the toner layer itself is transferred without collapsing. It is thought to be.
[0046]
The result shown in FIG. 7 is an example in the case where there is a large difference in the coefficient of friction. The image forming apparatus used in this experiment is a digital color copier ImagioColor 5100 manufactured by Ricoh, and this apparatus is a photoconductor 3 and an intermediate unit. In order to reduce the surface friction coefficient of the transfer body 2, zinc stearate, which is a solid lubricant, is regularly applied. The experiment was repeated while changing the application state. However, when the solid lubricant was applied only to the intermediate transfer member side, the margin of image elongation was about 1. It turned out to be 5 times.
[0047]
That is, when the friction coefficient of the intermediate transfer member 2 is lower than the surface friction coefficient of the photosensitive member 3 with respect to the toner, in order to achieve a desired elongation allowance of the pixel, the formulas in claims 1 and 2 are used. The condition is reduced to 1.5 times. However, increasing the surface friction coefficient of the intermediate transfer body too much to obtain this effect is a process after the primary transfer, which lowers the transfer rate during toner transfer from the intermediate transfer body to paper (secondary transfer). I could end up. Therefore, in order to achieve such a condition and maintain good paper transfer, the surface friction coefficient of the intermediate transfer body with respect to the toner must be lower than the friction coefficient of the paper.
Coefficient of friction on photoconductor <Coefficient of friction on intermediate transfer member <Coefficient of friction on paper
It is necessary to keep the permutation of
[0048]
【Example】
(Example 1)
Hereinafter, an embodiment of a full-color printer (hereinafter, referred to as a “color printer”) as one of image forming apparatuses according to the present invention will be described. In the color printer described below, a case is described in which a belt is used as an intermediate transfer member. However, the intermediate transfer member is not limited to a belt, and a drum can be used.
[0049]
FIG. 9 is an overall schematic configuration diagram of an internal mechanism in a color printer, which is an example according to the present invention. FIG. 10 is a diagram showing the configuration around the photoconductor in the color laser printer shown in FIG. 9, and FIG. 11 is a configuration diagram of the entire image forming system equipped with the internal mechanism shown in FIG. .
[0050]
9 to 11, in the color printer 1, an image forming section 1A is located at a central portion in a vertical direction, a sheet feeding section 1B is provided below the image forming section 1A, and a document table 1C1 is provided above the image forming section 1A. Provided document scanning units 1C are arranged.
An intermediate transfer belt 2 having a horizontally extending surface is arranged in the image forming section 1A. A structure for forming an image of a color complementary to the color separation color is provided above the intermediate transfer belt 2. Is provided.
[0051]
In the image forming unit 1A, drum-shaped OPC photoconductors 3B, 3Y, 3C, and 3M capable of holding an image with toners of complementary colors (yellow, magenta, cyan, and black) are provided on the intermediate transfer body 2. They are juxtaposed along the extension surface. In the following description, the photoconductor is denoted by reference numeral 3 if the content is common to all the photoconductors.
Each of the photoconductors 3B, 3Y, 3C, and 3M is formed of a drum that can rotate in the same direction (counterclockwise in FIG. 1). A device 4, a writing device 5, a developing device 6, which employs a two-component magnetic brush developing system, a primary transfer device 7, and a cleaning device 8 are arranged (for convenience, the reference numerals of the respective devices are set for the photosensitive member 3B). B).
[0052]
In FIG. 9, an intermediate transfer belt 2 corresponds to a primary transfer portion to which a visible image from an image forming unit provided with each photoconductor is sequentially transferred, and is wrapped around a plurality of rollers 2A to 2C to be exposed. A roller 2C, which is configured to be movable in the same direction at a position facing the body and is different from the rollers 2A and 2B constituting the expansion surface, faces the secondary transfer device 9 with the intermediate transfer belt 2 interposed therebetween. . 9 and 11, reference numeral 10 denotes a cleaning device for the intermediate transfer belt 2.
[0053]
The secondary transfer device 9 includes a transfer belt 9C that is wrapped around a charging drive roller 9A and a driven roller 9B and is movable in the same direction as the intermediate transfer belt 2 at a secondary transfer position where the secondary transfer device 9 is located. The transfer belt 9C is charged by the charging drive roller 9A to transfer the recording sheet while electrostatically adsorbing the recording sheet. The images can each be transferred to a recording sheet.
[0054]
The recording sheet is fed from the sheet feeding unit 1B to the secondary transfer position. The paper feed unit 1B includes a plurality of paper feed cassettes 1B1, a plurality of transport rollers 1B2 arranged on a transport path of a recording sheet fed from the paper feed cassette 1B1, and a registration roller 1B3 located in front of the secondary transfer position. Have. In the present embodiment, in addition to the conveyance path of the recording sheet fed from the sheet feeding tray 1B1, a type of recording sheet not contained in the sheet feeding cassette 1B1 is fed to the sheet feeding section 1B toward the secondary transfer position. The configuration includes a manual tray 1A1 provided with a part of the wall surface of the image forming unit 1A so as to be able to be turned upside down and a feeding roller 1A2.
The conveyance path of the recording sheet fed from the manual feed tray 1A1 joins the conveyance path of the recording sheet from the paper feed cassette 1B1 to the registration roller 1B3, and the recording sheet fed from any of the conveyance paths is also registered by the registration roller. The registration timing is set by 1B3.
[0055]
The writing device 5 (in FIG. 11, indicated by a reference numeral 5B for convenience) is configured to scan image data obtained by scanning a document on a document table 1C1 included in the document scanning unit 1C or to output an image output from a computer (not shown). The writing light is controlled by the information so that an electrostatic latent image corresponding to the image information is formed on the photoconductors 3B, 3Y, 3C, and 3M. The configuration of the writing device 5 includes a laser light source, a deflector such as a rotary polygon mirror, a scanning image forming optical system, a laser scanning optical system including a mirror group, and a large number of LEDs arranged one-dimensionally or two-dimensionally. An LED writing system or the like including an LED array and an imaging optical system is used, and the photosensitive member located at the end of each writing optical path is written to the photosensitive member 3 by a writing operation in the main scanning direction and the sub-scanning direction. An image can be formed.
[0056]
The original scanning section 1C is provided with a scanner 1C2 for exposing and scanning the original on the original placing table 1C1, and an automatic original feeder 1C3 is arranged on the upper surface of the original placing table 1C1. The automatic document feeder 1C3 has a configuration in which a document fed out onto the document table 1C1 can be inverted, and can scan the front and back surfaces of the document.
[0057]
The electrostatic latent image formed on the photoconductor 3 (members indicated by reference numerals 3B, 3Y, 3C, and 3M in FIG. 11) formed by the writing device 5 is indicated by reference numeral 6B in FIG. 11 for convenience. ), And is primary-transferred to the intermediate transfer belt 2. When the toner images of the respective colors are superimposedly transferred onto the intermediate transfer belt 2, the toner images are secondarily transferred collectively to the recording sheet by the secondary transfer device 9.
[0058]
The unfixed image carried on the surface of the recording sheet on which the secondary transfer has been performed is fixed by the fixing device 11. Although not shown in detail, the fixing device 11 has a belt fixing structure including a fixing belt heated by a heating roller and a pressure roller that is in contact with and contacts the fixing belt. By providing an area, that is, a nip area, a heating area for the recording sheet can be expanded as compared with a fixing structure using a separate roller.
The conveyance direction of the recording sheet that has passed through the fixing device 11 is switched by a conveyance path switching claw 12 disposed behind the fixing device 11. The transport direction is selected for the path RP.
[0059]
In the color printer 1 having the above-described configuration, the uniformly charged photoconductor 3 is statically exposed by scanning and exposing the original placed on the original placing table 1C1 or by image information from a computer. After an electrostatic latent image is formed and the electrostatic latent image is subjected to visible image processing by the developing device 6, the toner image is primarily transferred to the intermediate transfer belt 2.
[0060]
In the case of a single color image, the toner image transferred to the intermediate transfer belt 2 is directly transferred to a recording sheet fed from the sheet feeding unit 1B, and in the case of a multicolor image, primary transfer is repeated. Are superimposed on the recording sheet, and are collectively transferred to the recording sheet. After the untransferred image is fixed on the recording sheet after the secondary transfer by the fixing device 11, the recording sheet is re-fed to the sheet discharge tray 13 or the reversal and again to the registration roller 1B3.
[0061]
In FIG. 10, the developing device 6 has a unit configuration. In the case of a developing unit for cyan (a unit to which C is attached to the numeral), the developing unit is composed of a developing roller 6C1 and a doctor blade 6C2. The transport screws 6C3A and 6C3B include a toner density sensor 6C4 and an outer case 6C '. The positional relationship between the developing roller 6C1 and the conveying screws 6C3A and 6C3B is such that the conveying screws 6C3A and 6C3B are positioned obliquely downward with respect to the developing roller 6C1, and the two conveying screws 6C3A and 6C3B are parallel in the horizontal direction. It is arranged in. The outer case 6C ′ is provided with a partition plate 6C5 that divides into two chambers of two conveying screws 6C3A and 6C3. The rear side and the front side of the partition plate 6C5 with respect to the paper surface are notched so that the developer can circulate between the two conveying screws 6C3A and 6C3B. The outer case 6C 'has an opening at a portion facing the photoconductor 3C, and a part of the developing roller 6C1 is exposed from this opening. As described above, the outer case 6C 'surrounds the developing roller 6C1, the conveying screw 6C3A, and the doctor blade 6C6 with a slightly larger space above the first conveying screw 6C3A beside the developing roller 6C1 as shown in FIG. . The developing roller 6C1 includes a rotatable non-magnetic developing sleeve and a magnet serving as a magnetic field generating means fixed inside. The developer is a two-component developer composed of a non-magnetic toner and a magnetic carrier.
[0062]
The developer is transported while being agitated by the transporting screws 6C3A and 6C3B whose feeding directions are opposite, and is constantly circulating in the two chambers. The circulating developer agitated and conveyed is supplied to the surface of the developing sleeve by the conveying screw 6C3A located on the side of the developing roller 6C1, and is held in a magnetic brush shape on the surface by the magnetic force of the magnet, and is pumped up in the rotation direction of the developing sleeve. Can be The pumped-up developer on the magnetic brush is cut into an appropriate amount by a doctor blade 6C5 and sent to a developing section facing the photoconductor 3C.
[0063]
The remaining developer, which has been cut off by the doctor blade 6C5, falls off the outside of the magnetic brush on the surface of the developing sleeve of the developing roller 6C1 due to gravity, is returned to the transport screw 6C3A, and can be supplied to the developing sleeve while being stirred and transported again. Repeated.
On the other hand, in the developer sent to the developing unit, the toner is transferred to the electrostatic latent image on the photoconductor 3C and is visualized. The developer not used for visualization returns to the outer case 6C ′, and is separated from the developing sleeve at a portion where the magnetic force of the magnet does not work and is collected by the transport screw 6C3A. As described above, the developer is supplied to and recovered from the developing sleeve while being circulated and circulated through the transport screws 6C3A and 6C3B. Further, when the image is repeatedly output, the toner density becomes low. Therefore, the toner is supplied (not shown) so that the toner density becomes constant while being detected by the toner density sensor 6C4.
[0064]
Next, the cleaning device 8 will be described. In FIG. 10, for convenience, a cleaning device attached to the photoconductor 3 </ b> C for cyan will be described with reference numeral C as in the case of the developing device. Has a similar configuration.
In FIG. 10, the cleaning device 8 is a unit, which removes the toner remaining on the photoconductor 3C after the primary transfer, and uses an elastic cleaning blade, a fur brush or a cleaning member using them in combination. I have. In this embodiment, a cleaning blade 8C1 made of an elastic material such as polyurethane rubber, a fur brush 8C2, an electric field roller 8C3 disposed in contact with the fur brush 8C2, a scraper 8C4 of the electric field roller 8C3, and a recovery screw 8C5 are provided. Have been. The fur brush 8C2 is conductive and the electric field roller 8C3 is metal.
[0065]
In the cleaning device having the above-described configuration, first, the residual toner on the photoconductor 3C is scraped off by the fur brush 8C2 rotating by a counter in a direction opposite to the rotation direction of the photoconductor 3C, and adheres to the fur brush 8C2. The toner is removed by the electric field roller 8C3 rotating by the counter with respect to the fur brush 8C2, and the electric field roller 8C3 is cleaned by the scraper 8C4. At this time, a bias is applied to the electric field roller 8C3, and the residual toner is moved from the photoreceptor 3C to the fur brush 8C2 and from the fur brush 8C2 to the electric field roller 8C3 by electrostatic force, and finally scraped off by the scraper 8C4. The toner is collected in a waste toner bottle (not shown) by the collection screw 8C5. Alternatively, it is returned to the developing unit and reused.
[0066]
The positional relationship between the cleaning device 8C and the developing device 6C is such that the portion of the recovery screw 8C5 in the cleaning device 8C is above the transport screw 6C3B that is far from the developing roller 6C1 among the transport screws of the developing device 6C, and the outer case 6C ′. Are arranged on top of each other.
[0067]
The intermediate transfer belt 2 is interposed between each of the photoconductors 3M, 3C, 3Y, and 3B and each of the transfer devices. During this period, the toner images of each color are sequentially transferred from each of the photoconductors in a superimposed manner. To carry. As a transfer method, a transfer charge applying means 2D provided opposite to the photoreceptor so as to sandwich the belt generates a transfer electric field and performs electrostatic transfer. 9 to 11, the conductive foamed EPDM rubber (rubber hardness JIS-A 30 degrees, volume resistance 10 ⁸ .OMEGA.cm), a transfer electric field is generated by applying a voltage of about 1.5 kV to the electrostatic electrification applying means 2D composed of a roller composed of rollers. Therefore, the nip has to be slightly wider. The nip is determined by the amount of penetration by the hardness and the pressing force.
[0068]
The intermediate transfer belt 2 can be made of various materials. Here, a polyimide belt having excellent durability and a high Young's modulus, a PVDF belt having excellent surface smoothness, or a polyurethane resin layer is used. A multi-layer belt having an elastic surface with a polyurethane rubber layer and a coat layer containing a fluorine component thereon is used. In particular, since the surface of the polyurethane multilayer structure belt has elasticity, it has good adhesion to the photoreceptor surface and the paper surface, and is excellent in primary transfer and secondary transfer. Each belt is 10 ¹⁰ -10 ¹² It has a volume resistance of about Ωcm and a surface resistance of 10 ¹² It has a characteristic value of Ω / □ or more and has excellent transferability. In this embodiment, an example in which an intermediate transfer belt is used has been described. However, an intermediate transfer drum method may be adopted in consideration of a machine layout, required accuracy, size, and the like.
[0069]
In this embodiment, the radius of the photoconductor is 30 mm. This apparatus is a machine having a writing resolution of 600 dpi in both the main and sub scanning directions, and the minimum pixel length in the sub scanning direction on the photoconductor is 42.3 μm.
As described above, since the elongation tolerance is a maximum of 60% of the original line width, it is assumed that elongation of 25.4 μm is permissible. The driving speeds of the photosensitive member and the intermediate transfer belt of this machine are both set to an average speed of 245 mm / sec.
[0070]
Initially, the primary transfer nip width of this apparatus was 8 mm. Under this condition, it is understood from the above-mentioned "Equation 5 '" that the desired 600 dpi 1-dot image cannot be resolved unless the linear velocity ratio variation is within ± 0.32%.
The drive roller diameter of the intermediate transfer belt, which is the intermediate transfer member used in this embodiment, is 30φ, and the roller is a rubber roller having a thickness of 0.5 mm. Since this roller is a rubber roller, the processing accuracy cannot be improved much. The run-out accuracy is limited to about 50 μm, and at this time, the fluctuation of the belt speed due to the run-out of the roller becomes ± 0.16%. Further, although the belt was made of PVDF, fluctuations in speed due to errors in belt thickness, unevenness in Young's modulus, and the like are added. When the surface speed of the belt was actually measured by a laser Doppler displacement meter in this system, there was a speed deviation of about ± 0.25%. The processing accuracy of the photoconductor is also about 50 μm, and the change in the outer peripheral speed at this time is also about ± 0.08%. When this was also actually measured, a speed fluctuation of about ± 0.1% occurred.
[0071]
When these two speed fluctuations are combined, the maximum is about ± 0.35%, which exceeds the previously obtained tolerance of ± 0.32%. Actually, at this time, one dot line image was formed on the photoreceptor in the sub-scanning direction, and a repetitive pattern in which one dot line was not formed was formed and observed on the intermediate transfer belt, but satisfactory resolution was not obtained. Was.
[0072]
Therefore, it was difficult to further suppress the speed fluctuation on the belt side. Therefore, when the processing accuracy of the photoconductor was increased and the amount of deflection was suppressed to 10 μm, the speed fluctuation of the photoconductor was reduced to about 0.04% by actual measurement. As a result, the speed fluctuation along with the fluctuation of the belt became about ± 0.29%, and the resolution became possible.
[0073]
However, increasing the processing accuracy of the photoconductor directly led to an increase in cost. Therefore, in order to increase the resolution with the processing accuracy of the conventional photoconductor, the configuration of the machine was changed to reduce the nip width. As described in claim 2, when the nip width is 8 mm in the related art, according to “Equation 8-2”,
Nw <ΔIw * Vpc / (ΔVpc + ΔVbelt) −Iw
If this is transformed into a linear velocity fluctuation,
Nw <ΔIw / (ΔVpc / Vpc + ΔVbelt / Vpc) −Iw Equation 9
Here, since ΔVpc / Vpc and ΔVbelt / Vpc are already known to be 0.1% and 0.25%, respectively, the nip width conditions necessary for resolution at this time are as follows.
Nw <25.4 μm / (0.1% + 0.25%) − 42.3 μm = 7.2 (mm)
Therefore, when the nip width was changed from 8 mm to 6 mm with this apparatus, an image with a good resolution could be obtained without increasing the processing accuracy of the photoreceptor.
[0074]
Thereafter, the device used in this experiment was equipped with a laser writing system having a resolution of 1200 DPI instead of 600 DPI. In the future, the laser writing system of machines will be further densified in order to achieve higher image quality. This is to make the experimental system corresponding to it. Due to this resolution, the minimum pixel length on the photoreceptor is 21.2 μm, which is half of 600 dpi. Therefore, the allowable elongation of the image is also halved, ΔIW = 12.7 μm, and sufficient resolution cannot be obtained with the conventional accuracy of the components and drive of the photosensitive member and the belt.
[0075]
In order to resolve 1200 dpi while keeping the conventional component accuracy, the following measures were taken.
One is a further reduction in the nip width. If the accuracy of the parts is the same, similarly using Equation 9,
Nw <12.7 μm / (0.1% + 0.25%) − 21.2 μm to 3.6 mm
It becomes.
However, due to the mechanical configuration, it is difficult to stably reduce the nip to 3 mm or less under the condition that the photoconductor and the intermediate transfer member surely come into contact with each other. In this case, “Equation 9” is not satisfied, and a 1200 dpi image cannot be resolved. Therefore, a solid lubricant was further applied to the photoconductor to lower the surface friction coefficient of the photoconductor. As a solid lubricant, zinc stearate powder is used, and as a method of application, an assembly of zinc stearate is rubbed against a photoreceptor cleaning brush, and the zinc stearate shaved off by the brush as fine powder is applied to the photoreceptor. This was done by applying a small amount. In addition to this method, a fine powder of zinc stearate is preliminarily mixed into the toner to be supplied in an amount of about 0.7% by weight, adheres to the toner surface, and is supplied to the photoreceptor via a magnetic brush spike of a developing device. There are also ways to do this.
[0076]
The surface friction coefficient of the photoreceptor after test printing on about 100 sheets of A4 paper in this state was about 0.15, and that of the belt was about 0.4. The original values are about 0.4 for the photoconductor and about 0.45 for the belt, so that some lubricant is mixed into the belt surface via the photoconductor.
By giving a difference in the surface friction coefficient between the photoconductor and the intermediate transfer belt in this way, a margin occurs in the elongation of the image. This is evident from the experimental results described above. Therefore, by further reducing the nip width (from 6 mm to 4 mm) and applying a solid lubricant to the photoreceptor, the tolerance of the machine design is increased, and the resolution of 1200 dpi can be faithfully reproduced on the intermediate transfer belt. Sufficient resolution could be achieved.
[0077]
(Example 2)
As described above, by suppressing the phase of the fluctuation at the primary transfer position of the photosensitive member 3 and the intermediate transfer member 2, the elongation of the image can be suppressed. Therefore, as shown in FIG. 12, an example is shown in which the diameters of both the photoconductor 3 and the driving roller of the transfer belt (the roller indicated by reference numeral 2A ′ in FIG. 12) are both φ60 mm. By doing so, the fluctuation period of the outer peripheral speed of each primary transfer position due to the eccentricity of the photoconductor and the drive roller matches at the pitch indicated by the symbol PL (note that the drive roller 2A 'and the black image photoconductor 3B do not match). Although a part of the space is omitted for the sake of space, it is set to a pitch PL between the photoconductors.) In this way, during the manufacture of the photoconductor and the drive roller, the accuracy of the components is inspected, and the position of the center of rotation with respect to the geometric center is completely inspected. As shown in FIG. Then, marks are made on the outer peripheral positions of the photoconductor 3 and the driving roller 2A.
When assembling each part on the production line, by using the mark MP as a guide and assembling the belt and the photoconductor at the primary transfer position so that the phase of the speed fluctuation matches, the influence of the speed fluctuation can be minimized. I can do it.
[0078]
However, if the use is continued for a long period of time, the phase may be shifted due to slippage of the belt or the like. Therefore, at the time of periodic maintenance, the maintenance staff may adjust the timing as needed. Further, it is more preferable to provide a mark detection function for each photoconductor and the intermediate transfer belt driving roller, and to configure the machine so that the phase can be automatically adjusted before image formation is started. Specifically, as a mark of the eccentric direction of the photoconductor and the drive roller, a state in which the reflectivity is different at the end of the roller, or a notch is provided, which is detected by a reflection type optical sensor, There is a method in which a projection or a dent is provided, and a detection is performed by a contact sensor or the like. In this way, the phase is detected, and each photoconductor is independently driven and controlled, and the stop position and the start timing are adjusted so as to move at a predetermined phase. In this case, it is desirable to have a mechanism for contacting and separating the intermediate transfer belt and the photoreceptor. It is better to adjust the phase while the belt is separated from the photoreceptor to prevent damage to the photoreceptor and the surface of the intermediate transfer belt due to friction. May be.
[0079]
【The invention's effect】
According to the first to fifth aspects of the present invention, a linear velocity difference is generated between the latent image carrier and the image transfer body from now on by designing the machine so as to satisfy the conditions set in the relational expressions. In such a case, by defining the difference in the linear velocity, the elongation due to the drag of the image can be suppressed, and the toner image on the latent image carrier is faithfully transferred onto the transfer body while maintaining the desired resolution, thereby obtaining a good image. Can be obtained.
[0080]
According to the present invention, mechanical speed fluctuation factors such as eccentricity exist by aligning the speed fluctuation transfer between the latent image carrier and the transfer body at the primary transfer position. Also, the influence of fluctuations depending on the component accuracy of the drive speed of the photoreceptor and the intermediate transfer member is minimized so that the opportunity for image elongation can be reduced, and the desired resolution can be maintained on the latent image carrier. The toner image can be faithfully transferred onto the transfer member, and a good image can be obtained.
[0081]
According to the eighth and ninth aspects of the present invention, the conditions set in the first to fifth aspects of the present invention are relaxed by making the surface friction coefficient of the photoreceptor smaller than that of the intermediate transfer body. Even if the machine is designed at a low cost by lowering the precision of the machine, a good image can be obtained.
[0082]
According to the tenth aspect, even under the conditions set in the eighth and ninth aspects, the transfer rate can be prevented from lowering, so that the transfer from the latent image carrier is performed while maintaining the desired resolution. The transferred toner image on the transfer body can be efficiently transferred to paper, and a good image can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining an embodiment of the invention described in claim 1, showing the relationship with the elongation of a linear speed image.
FIG. 2 is a schematic diagram for explaining an effect on image elongation due to a difference in linear velocity between a latent image carrier and a transfer member.
FIG. 3 is a schematic diagram for explaining that an ideal fine line on a latent image carrier changes a transfer position due to a difference in linear velocity.
FIG. 4 is a schematic diagram for explaining how the transfer state of a thin line having a constant pitch on a latent image carrier changes due to a tendency of a difference in linear velocity, with respect to an actual image.
FIG. 5 is a schematic diagram for explaining an eccentric state of the latent image carrier.
FIG. 6 is a schematic diagram for explaining a limit of a state where an ideal set of thin lines can be recognized as a thin line.
FIG. 7 is a diagram for explaining the relationship between the coefficient of friction on the surface of the latent image carrier and the transfer member and the elongation of the image.
FIG. 8 is a schematic diagram for explaining the relationship between the coefficient of friction on the surface of a latent image carrier and a transfer body and the elongation of an image.
FIG. 9 is a schematic diagram for explaining an internal mechanism of the image forming apparatus according to the embodiment of the present invention.
FIG. 10 is an enlarged schematic view showing a part of the internal mechanism shown in FIG. 9;
FIG. 11 is a schematic diagram showing an overall configuration of an image forming system equipped with the internal mechanism shown in FIG.
FIG. 12 is a schematic diagram for explaining a partially modified example of the configuration shown in FIG. 9;
FIG. 13 is a schematic diagram for explaining a countermeasure against the eccentric state shown in FIG. 5;
[Explanation of symbols]
1 color printer
2 Intermediate transfer member
2A drive roller
3 Photoconductor

Claims (10)

An image forming apparatus having a configuration for obtaining a final recording output through a process of transferring an image from a latent image carrier,
Let ΔIw be the allowable variation width of the desired minimum pixel length Iw in the sub-scanning direction, Nw be the contact width between the latent image carrier and the transfer body used in the transfer process, and Vpc be the average value of the peripheral image carrier outer peripheral speed. Then, with respect to the absolute value of the difference between the outer peripheral speed Vpc (t) and the speed Velt (t) of the transfer member at the contact position between the latent image carrier and the transfer member at a certain time t,
| Vpc (t) −Vbelt (t) | <ΔIw * Vpc / (Nw−Iw) (1)
An image forming apparatus characterized by satisfying the following. An image forming apparatus having a configuration for obtaining a final recording output through a process of transferring an image from a latent image carrier,
When the allowable variation width of the desired minimum pixel length Iw in the sub-scanning direction is ΔIw, the natural peripheral speed variation width of the latent image carrier is ΔVpc, and the natural speed variation width of the transfer member surface used in the transfer process is ΔVb. The contact width Nw between the latent image carrier and the transfer member is
Nw <ΔIw * Vpc / (ΔVpc + ΔVbelt) −Iw (2)
An image forming apparatus characterized by satisfying the following. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein the allowable variation width is set within 60% of a minimum pixel length in the sub-scanning direction. The image forming apparatus according to claim 1, wherein
The image forming apparatus according to claim 1, wherein the minimum pixel length in the sub-scanning direction is a minimum resolution length of the latent image on the photoconductor. The image forming apparatus according to claim 1, wherein
An image forming apparatus characterized in that, when a difference occurs between the average speed of the photosensitive member and the average speed of the intermediate transfer member, the intermediate transfer member side speed is set to be low. The image forming apparatus according to claim 1, wherein
The transfer member used in the transfer process is an intermediate transfer member formed of a belt, and has a configuration in which a driving force is generated by a roller in contact with the belt as a driving source of the belt,
The outer diameter of the drive roller of the latent image carrier and the transfer body is set to be substantially the same, and the phase of the speed fluctuation at the primary transfer position at the outer peripheral speed due to the eccentricity of the latent image carrier is determined by the drive roller of the transfer body. An image forming apparatus wherein the phase at the primary transfer position in the fluctuation of the belt speed due to the eccentricity is made uniform. The image forming apparatus according to claim 6,
When a plurality of latent image carriers are in contact with a transfer body, and the toner developed on each latent image carrier is sequentially transferred to the transfer body to obtain a color image, primary transfer on each latent image carrier is performed. An image forming apparatus, wherein the speed fluctuation phase of each latent image carrier is aligned with the speed fluctuation phase of a transfer body at a position. The image forming apparatus according to claim 1, wherein
When the friction coefficient of the transfer member with respect to the toner is higher than the friction coefficient of the latent image carrier with respect to the toner,
| Vpc (t) −Vbelt (t) | <1.5 * ΔIw * Vpc / (Nw−Iw) (3)
An image forming apparatus characterized by satisfying the following. The image forming apparatus according to claim 1, wherein
If the friction coefficient of the transfer member with respect to the toner is higher than the friction coefficient of the latent image bearing member with respect to the toner,
Nw <1.5 * ΔIw * Vpc / (ΔVpc + ΔVbelt) −Iw (4)
An image forming apparatus characterized by satisfying the following. The image forming apparatus according to claim 8, wherein
An image forming apparatus characterized in that the friction coefficient of the toner on paper is higher than the friction coefficient of the transfer body on toner.

Images