JP2004109702A - Device and method for forming image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress transfer dust, to stably suppress irregularities in dots regardless of variation in lapse of time or irregularities in a developer and to improve image quality by improving graininess in an image forming device such as a copying machine, a printer and a facsimile. <P>SOLUTION: A voltage applying member 15 for applying voltage which has the same polarity as a charge polarity of a photosensitive body 3 and also has an absolute value larger than latent image voltage V<SB>L</SB>is provided in an intermediate transfer belt 2 of a part corresponding to a gap on the upstream side of a transferring area in the rotating direction of the photosensitive body 3, and transferring is performed by a transferring bias roller. An optical writer 5 has a static beam spot wherein the relation between a dot pitch P of resolution X (dpi) in a vertical scanning direction and a static beam spot diameter ω is ω ≤ 2P. In this case, the static beam spot diameter ω is 1/e<SP>2</SP>of maximum beam intensity. <P>COPYRIGHT: (C)2004,JPO

Description

【００１５】
を満たす、という構成を採っている。
【００１６】
請求項６記載の発明では、請求項１乃至３のうちの何れか１つに記載の画像形成装置において、上記像担持体上のドット面積の変動量ΔＳが、ΔＳ≦２００μｍ^２である、という構成を採っている。
【００１７】
請求項７記載の発明では、像担持体を帯電させ、露光することで上記像担持体上に画像情報に基づいて電位Ｖ_Ｌの静電潜像を形成し、上記像担持体上の静電潜像を該静電潜像と同極性のトナーで顕像化し、上記転写体と上記像担持体が接触した転写領域で上記トナー像を上記転写体上に転写する画像形成方法において、上記転写領域よりも上記像担持体の回転方向上流側の空隙に対応する部分の上記転写体に上記像担持体の帯電極性と同極性で且つ絶対値がＶ_Ｌより大きい電圧を印加することとした。
【００１８】
請求項８記載の発明では、請求項７記載の画像形成方法において、上記静電潜像の上記像担持体表面での電界強度が３×１０^７Ｖ／ｍ以上であることとした。
【００１９】
請求項９記載の発明では、請求項８記載の画像形成方法において、解像度Ｘ（ｄｐｉ）の副走査方向ドットピッチをＰ、上記像担持体上のドット径をＤとするとき、下記式
【００２０】
【数５】

【００２１】
を満たすこととした。
【００２２】
請求項１０記載の発明では、請求項８記載の画像形成方法において、上記像担持体上のドット面積の変動量ΔＳが、ΔＳ≦２００μｍ^２であることとした。
【００２３】
請求項１１記載の発明では、請求項８の画像形成方法において、上記像担持体として、ＣＴＬ層でのキャリア移動度をμｐ、μｎ、ＣＧＭでの量子効率をηとするとき、下記式
【００２４】
【数６】

【００２５】
を満たす感光体を用い、スキャン速度Ｖｘ＝１８９０〜１９２８ｍ／ｓ（主走査）、０．２５３８〜０．３１０２ｍ／ｓ（副走査）、主走査・副走査ともに１２００ｄｐｉ間隔の書き込みにおいて、静止ビームの主走査方向径が３０μｍ、副走査方向のビーム径が４０μｍとすることとした。
【００２６】
【発明の実施の形態】
以下、本発明の実施形態を図１乃至図９に基づいて説明する。
まず、図１に基づいて本実施形態における画像形成装置としてのタンデム型のカラー複写機の構成及び動作の概要を説明する。カラー複写機１は、装置本体中央部に位置する画像形成部１Ａと、該画像形成部１Ａの下方に位置する給紙部１Ｂと、画像形成部１Ａの上方に位置する画像読取部１Ｃを有している。
画像形成部１Ａには、水平方向に延びる転写面を有する転写体としての中間転写ベルト２が配置されており、該中間転写ベルト２の上面には、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。すなわち、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）による像を担持可能な像担持体としての感光体３Ｙ、３Ｍ、３Ｃ、３Ｂが中間転写ベルト２の転写面に沿って並置されている。
【００２７】
各感光体３Ｙ、３Ｍ、３Ｃ、３Ｂはそれぞれ同じ反時計回り方向に回転可能なドラムで構成されており、その周りには、回転過程において画像形成処理を実行する帯電手段としての帯電装置４、各感光体３Ｙ、３Ｍ、３Ｃ、３Ｂ上に画像情報に基づいて電位Ｖ_Ｌの静電潜像を形成するための露光手段としての光書込装置５、各感光体３上の静電潜像を該静電潜像と同極性のトナーで現像する現像手段としての現像装置６、一次転写手段としての転写バイアスローラ７、印加電圧部材１５、クリーニング装置８が配置されている。各符号に付記しているアルファベットは、感光体３と同様、トナーの色別に対応している。各現像装置６にはそれぞれのカラートナーが収容されている。
中間転写ベルト２は、複数のローラ２Ａ〜２Ｃに掛け回されて感光体３Ｙ、３Ｍ、３Ｃ、３Ｂとの対峙位置において同方向に移動可能な構成を備えている。転写面を支持するローラ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に対向している。図１中、符号１０は中間転写ベルト２を対象としたクリーニング装置を示している。
【００２８】
感光体３Ｙの表面が帯電装置４Ｙにより一様に帯電され、画像読取部１Ｃからの画像情報に基づいて感光体３Ｙ状に静電潜像が形成される。該静電潜像はイエローのトナーを収容した現像装置６Ｙによりトナー像として可視像化され、該トナー像は第１の転写工程として、中間転写ベルト２上に、転写バイアスローラ７Ｙに印加された電圧による電界で引き付けられて転写される。
印加電圧部材１５Ｙは感光体３Ｙの回転方向における転写バイアスローラ７Ｙの上流側に設けられている（図２参照）。印加電圧部材１５Ｙにより、中間転写ベルト２に感光体３Ｙの帯電極性と同極性で且つ絶対値がベタ時Ｖ_Ｌより大きい電圧を印加し、転写領域にトナー像が入る以前に感光体３Ｙから中間転写ベルト２へトナーが転写することを防止して、感光体３Ｙから中間転写ベルト２へのトナーの転写時のチリによる乱れを防止する。以下、これをカウンター方式転写と称する。
【００２９】
他の感光体３Ｍ、３Ｃ、３Ｂでもトナーの色が異なるだけで同様の画像形成がなされ、それぞれの色のトナー像が中間転写ベルト２上に順に転写されて重ね合わせられる。
転写後感光体３上に残留したトナーはクリーニング装置８により除去され、また、転写後図示しない除電ランプにより感光体３の電位が初期化され、次の作像工程に備えられる。
２次転写装置９は、帯電駆動ローラ９Ａ及び従動ローラ９Ｂに掛け回されて中間転写ベルト２と同方向に移動する転写ベルト９Ｃを有している。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、中間転写ベルト２に重畳された多色画像あるいは担持されている単一色の画像を転写材としての用紙１６に転写することができる。
【００３０】
２次転写位置には給紙部１Ｂから用紙１６が給送されるようになっている。給紙部１Ｂには用紙１６が積載収容される複数の給紙カセット１Ｂ１と、給紙カセット１Ｂ１に収容された用紙１６を最上のものから順に１枚ずつ分離して給紙する給紙コロ１Ｂ２と、搬送ローラ対１Ｂ３と、２次転写位置の上流に位置するレジストローラ対１Ｂ４等が設けられている。
給紙カセット１Ｂ１から給紙された用紙１６は、レジストローラ対１Ｂ４で一旦停止され、斜めずれ等を修正された後、中間転写ベルト２上のトナー像の先端と搬送方向先端部の所定位置とが一致するタイイングでレジストローラ対１Ｂ４により２次転写位置に送られる。装置本体の右側には起倒可能に手差しトレイ５０が設けられており、該手差しトレイ５０に収容された用紙１６は給紙コロ５２により給送された給紙カセット１Ｂ１からの用紙搬送路と合流する搬送路によりレジストローラ対１Ｂ４に向けて送られる。
【００３１】
光書込装置５では、画像読取部１Ｃからの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３Ｙ、３Ｍ、３Ｃ、３Ｂに対して画像情報に応じた書き込み光を出射して静電潜像を形成するようになっている。
画像読取部１Ｃは、自動原稿給送装置１Ｃ１と、原稿載置台としてのコンタクトガラス５４を有するスキャナ１Ｃ２等を有している。自動原稿給送装置１Ｃ１は、コンタクトガラス５４上に繰り出される原稿を反転可能な構成を有し、原稿の表裏各面での走査が行えるようになっている。
光書込装置５により形成された感光体３上の静電潜像は現像装置６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により用紙１６上に一括して２次転写される。２次転写された用紙１６は定着装置１１へ送られ、ここで熱と圧力により未定着画像を定着される。２次転写後の中間転写ベルト２上の残留トナーは、クリーニング装置１０により除去される。
【００３２】
定着装置１１を通過した用紙１６は、定着装置１１の下流側に設けられた搬送路切り換え爪１２により、排紙トレイ１３に向けた搬送路と反転搬送路ＲＰとに選択的に案内される。排紙トレイ１３に向けて搬送された場合には、排紙ローラ対５６により排紙トレイ１３上に排出され、スタックされる。反転搬送路ＲＰへ案内された場合には反転装置５８により反転され、再度レジストローラ対１Ｂ４に向けて送られる。
【００３３】
以上の構成により、カラー複写機１では、コンタクトガラス５４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、一様に帯電された感光体３（以下、単に感光体又は像担持体ともいう）に対して静電潜像が形成され、該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。
中間転写ベルト２に転写されたトナー像は、単一画像の場合にはそのまま給紙部１Ｂから繰り出された用紙１６に転写される。多色画像の場合には１次転写が繰り返されることにより重畳された後、用紙１６に一括して２次転写される。
２次転写後の用紙１６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に排出され、あるいは反転されて両面画像形成のために再度レジストローラ対１Ｂ４に向けて送られる。
【００３４】
上記カウンター方式転写により画像の乱れを低減できるが、エッジ周辺のみが転写されにくくドットがばらつく現象が発生した。これは、図１１で説明したように、画像エッジ部の潜像電位が画像中央部と比較して電位の絶対値が大きく、エッジ部と中間転写ベルト２との間で放電が発生して感光体３上のトナー極性が変わるためであり、従来の転写ではチリのためにベルト上エッジ部の乱れは目立たなかったと考えられる。
これを防止するために、露光ビームスポット径を変化させて潜像のプロファイルを変え、確認を行った。
本実施形態では感光体３上の電位分布の計測が困難なことから、感光体３上の電荷密度分布、電界強度分布、電位分布を計算で算出している。
潜像形成では、帯電された感光体３上にある光エネルギーを照射（露光）することにより、感光体３内部のキャリア発生層（ＣＧＬ）に発生したキャリア（電荷）が、感光体３中のキャリア移動層（ＣＴＬ）を移動して、感光体３上の帯電電荷と中和して潜像を形成しているが、キャリアの発生量、キャリア間のクーロン反発力、キャリア同士の再結合、キャリアの移動度を考慮して計算を行っている。
【００３５】
［実施例１］
書き込み密度：１２００ｄｐｉ
発光波長：７８０ｎｍ
静止ビームスポット径：３０μｍ×３０μｍ、４５μｍ×４５μｍ、５５μｍ×７０μｍ
（静止ビームスポット径とは最大ビーム強度の１／ｅ^２径とする。）
像担持体：感光体
感光体量子効率：η＝２．８×１０^−５×Ｅ^{０．５９３３}
感光体吸収係数：０．７
感光体膜厚：２８μｍ
必要露光エネルギー：７ｅｒｇ／ｃｍ^２
露光手段：走査型レーザービーム
線速０．２８５ｍ／ｓ
走査スキャンスピード１９０９ｍ／ｓ
初期帯電電位：−６５０Ｖ
【００３６】
各ビームスポット径における転写部での潜像電位分布を図３に示す。図３より、ビームスポット径を小径化することにより、潜像電位のプロファイルがシャープになってくることが確認できた。感光体３の帯電特性とトナーの極性が同極性の場合、感光体３の帯電電位の絶対値が小さくなると、トナーの現像装置６の現像スリーブへの付着力より感光体３への付着力が大きくなり、現像量が増加する。
小径ビームスポットの方が、ドット中央部の潜像電位の絶対値が小さくなっていることよりトナーの顕像化がされやすく、低電位現像の可能性がある。また、トナーの付着量を一定にして比較した場合、小径ビームの方が潜像部の電位の絶対値が低いことから、中間転写ベルト２との放電が発生しにくくなる。
【００３７】
実際にカウンター転写方式で形成した画像で確認したところ、エッジ周辺のみが転写されにくくドットがばらつく現象が発生した。これは、画像エッジ部の潜像電位が画像中央部と比較して電位の絶対値が大きく、エッジ部で中間転写体と放電が発生し、像担持体上のトナー極性が変わって転写されていないためと考えられる。現像バイアスを下げて確認したところ、エッジ周辺の転写不良を解消することが可能となった。
従来は像担持体上のドット変動量より画像の乱れや画像チリ等の転写部での変動の方が大きかったが、転写部での変動低減により像担持体上のトナー像を忠実に再現するため像担持体上のドット変動量が画像にセンシティブに影響することになり、良好な画像を形成するためには像担持体上でのドット変動量を抑える必要がある。
【００３８】
ドット径は感光体特性、現像剤特性を含めたプロセスの経時劣化や剤バラツキ等により変動して画像が劣化する。ここではビームスポット径を変化させ、感光体上ドット径の変動量を確認した。その結果を図４示す。ここでは感光体上ドット径およびその変動量を計算で算出している。算出方法について以下に示す。
現像電界強度分布において、ある閾値電界強度以上の領域にトナーが付着することで、感光体上にトナー画像が形成されるが、この閾値電界強度を臨界現像電界強度（Ｅｔｈ）と定義し、現像プロセスや現像剤の条件によって決まる値とする。
この臨界現像電界強度が分かれば、感光体上のドット径（以下、像担持体上ドット径Ｄと呼ぶ）が算出可能である。
要するに、ここでいうドット径とは、現像時電界強度分布において臨界現像電界強度以上の領域の、感光体上ドットの主走査、副走査方向の長さである。
【００３９】
また、ドット径の変動量はドット算出時の臨界現像電界強度の変動と置き換えられ、臨界現像電界強度を変動させることにより算出した。
図４から判るように、ビームスポット径の小径化に伴いドット変動量が低下している。ドット変動量が１ドットの半分以下に抑えられると画像のざらつき感が低減されるといわれており、ビームスポット径ωが解像度Ｘ（ｄｐｉ）の副走査方向ドットピッチＰの２倍程度になると、ドット変動量が１ドットの半分程度になる。
よって、ビームスポット径をω≦２Ｐ（Ｐは解像度Ｘ（ｄｐｉ）の副走査方向ドットピッチ）として潜像を形成することにより、トナーの付着した部分での感光体３と中間転写ベルト２との放電を防止し、エッジ部の転写を阻害することなくドット面積のバラツキが小さく粒状性が向上するような画像を形成することが判明した。
【００４０】
［実施例２］
像担持体に潜像を形成する場合、必要な露光エネルギー量は像担持体の量子効率と光減衰特性から決定される。像担持体として感光体を使用した場合、高解像度（例えば１２００ｄｐｉ）な画像を形成するためには高密度な書込が必要となってくるが、それに伴い感光体のキャリア生成量が増加し、キャリアの再結合が発生し、生成キャリア量が減少する現象が発生する。
よって、高解像度の画像を形成する場合、画像を形成するための必要露光エネルギー量だけではなく、必要露光エネルギー量がどれくらいの領域に露光されているかが問題となる。レーザービームの強度プロファイルはガウシアン分布に近似できるが、ここでは露光面積当たりの平均露光エネルギー量として露光エネルギーを考え、それにおける感光体上のドット変動量について考察を行った。その結果を図５に示す。
【００４１】
書き込み密度：１２００ｄｐｉ
発光波長：７８０ｎｍ
静止ビームスポット径：３０μｍ〜７０μｍまで可変
（静止ビームスポット径とは最大ビーム強度の１／ｅ^２径とする。）
像担持体：感光体
感光体量子効率：η＝５．６×１０^−６×Ｅ^{０．７０２２}
感光体吸収係数：０．７
感光体膜厚：３０μｍ
必要露光エネルギー：７ｅｒｇ／ｃｍ^２
露光手段：走査型レーザービーム
線速０．３６２ｍ／ｓ
走査スキャンスピード１９８３．８７ｍ／ｓ
初期帯電電位：−８００Ｖ
【００４２】
平均露光エネルギー量の増加に伴いドットの変動量が減少することが確認された。従来は像担持体上のドット変動量より画像の乱れや画像チリ等の転写部での変動の方が大きかったが、転写部での変動低減により像担持体上のトナー像を忠実に再現するため像担持体上のドット変動量が画像にセンシティブに影響することになり、良好な画像を形成するためには像担持体上でのドット変動量を抑える必要がある。
ドット変動量が１ドットの半分程度に抑えられると画像のざらつき感が低減されるといわれている。例えば、１２００ｄｐｉにおいては感光体上のドット変動量は１５μｍ以下に抑える必要があるため、図５より平均露光エネルギー量が１．９×１０^−３Ｊ／ｍ^２以上であることが望ましいことが判った。
また、必要露光エネルギーは像担持体の感度と現像プロセスによって決まり、例えば感光体を使用した場合、その光減衰特性から算出される。光減衰特性は露光エネルギーの関数で減衰していき、像担持体それぞれの残留電位Ｖｓに飽和する。光減衰特性の式は以下の通りである。
【００４３】
【数７】

【００４４】
ここで、Ｖ_０は初期帯電電位、ｅは電荷量、Ｘは露光エネルギー、Ｃは感光体の静電容量、Ｌは膜厚、ｎおよびαは感光体量子効率の定数である。
量子効率　η＝α・Ｅ^ｎ
また、従来は、転写部での画像の乱れの影響で像担持体上の画像より転写後の画像が太るような現象があったが、転写部での変動低減により像担持体上のトナー像を忠実に再現するため、黒ベタ、斜めライン、縦ライン画像も像担持体上で再現されていないといけないことが判った。
そこで、像担持体上のドット径と副走査方向のドットピッチとの関係Ｄ＝αＰについて、１ドットライン画像で検討を行った。現像バイアスを変えて像担持体上のドット径ＤとＰの関係を算出し、その現像条件でカウンターバイアス方式を用いて画像出しを行ない、ラインの途切れとライン幅を観察した。その結果を表１に示す。
【００４５】
【表１】

【００４６】
表１から、ライン幅が小さく、かつ途切れないラインを形成するにはが良いことが判る。よって、解像度Ｘ（ｄｐｉ）の副走査方向のドットピッチをＰとすると、像担持体上のドット径Ｄが、下記式
【００４７】
【数８】

【００４８】
になるような露光を行うことにより、黒ベタ、斜めライン、縦ライン画像が再現できることが判明した。
【００４９】
［実施例３］
図６は、さまざまの装置での、画像評価項目の一つである粒状度と、紙上の１ドット面積変動量の関係を表したものである。たとえば粒状度０．２を達成するためには△Ｓ＝２００μｍ^２、０．１だと△Ｓ＝１５０μｍ^２以下のドット面積変動量（標準偏差）が必要であることを示している。
一方、図７は、粒状度の異なるさまざまな画像サンプルでの画質満足度調査を行った結果を示したものである。これより大部分の被験者の満足を得るには、粒状度０．２以下、できれば０．１以下が望ましいということが判る。図６、図７から△Ｓ＝２００μｍ^２以下が必要と判断される。
カウンターバイアス方式では紙上画像のばらつきはＯＰＣ（感光体）上画像ばらつきとほぼ等しい。よって、ＯＰＣ上の画像ばらつきを△Ｓ＝２００μｍ^２以下にする必要がある。
主走査方向ビームスポット径を３０μｍに固定したときのビームスポット径とドット面積変動量の関係を図９に示す。例えば粒状度０．２を達成するには、ビームスポット径３０ｘ４０μｍより小径が望ましいことが判る。
【００５０】
［実施例４］
従来、トナーを転写する際、感光体３と中間転写ベルト２とが接触する以前に、感光体３上のトナーが中間転写ベルト２方向に移動することにより、画像の乱れが発生していたが、上述のように、転写部上流側に設けられた印加電圧部材１５により中間転写ベルト２に感光体３の帯電極性と同極性で且つ絶対値がＶ_Ｌより大きい電圧を印加し、転写領域にトナー像が入る以前に感光体３から中間転写ベルト２へ転写することを防止して、感光体３から中間転写ベルト２へのトナーの転写時のチリによる乱れを防止している。しかしながら、カウンター転写方式では充分でない。
【００５１】
本発明者らは、カウンター転写方式において、画像のチリと潜像の感光体上電界強度の最大値（計算値）とに相関があることを確認した。これを図８に示す。チリ量はライン画像のエッジ部から地肌部へはみ出したトナーの面積を指標として値を設定しており、値の増加によりチリが多くなり、画像が悪くなる。
潜像の電界強度増加に伴い画像チリは増加し、３×１０^７Ｖ／ｍ以上になると飽和する。よって、潜像の電界強度を３×１０^７Ｖ／ｍ以上にすることより、画像チリを防止し、経時変動や現像剤のバラツキによらず安定した画像を形成することができることを確認した。
【００５２】
また、上記構成で下記式
【００５３】
【数９】

【００５４】
を満たす画像を形成したところ、感光体３上で黒ベタ、斜めライン、縦ライン画像の一部が欠損することなく形成することができ、高画質画像の形成が可能となった。
また、上記構成で像担持体上ドット面積変動量ΔＳをΔＳ≦２００μｍ^２に抑えることにより、感光体３上で粒状性の高い顕像を形成することにより、粒状性の高い高画質な画像を形成することができた。
【００５５】
［実施例５］
書き込み密度：１２００ｄｐｉ
発光波長：７８０ｎｍ
静止ビームスポット径：３０〜７０μｍ
（静止ビームスポット径とは最大ビーム強度の１／ｅ^２径とする。）
像担持体：感光体
感光体量子効率：η＝２．８×１０^−５×Ｅ^{０．５９３３}
感光体移動度：下記式
【００５６】
【数１０】

【００５７】
感光体吸収係数：０．７
感光体膜厚：２８μｍ
必要露光エネルギー：７ｅｒｇ／ｃｍ^２
露光手段：走査型レーザービーム
線速０．２８５ｍ／ｓ
走査スキャンスピード１９０９ｍ／ｓ
初期帯電電位：−６５０Ｖ
【００５８】
ビーム径に対する感光体上ドット変動量の関係を図９に示す。静止ビームの主走査方向径が３０μｍ、副走査方向ビーム径が４０μｍのとき、△Ｓ＝２００μｍ^２を達成し、黒ベタ、斜めライン、縦ライン画像の一部が欠損することなく、高画質画像を形成可能であることを確認した。
【００５９】
上記実施形態では、転写体として中間転写ベルト２について説明したが、像担持体から転写材（用紙）への転写においても同様の思想で実施できる。
【００６０】
【発明の効果】
請求項１記載の発明によれば、像担持体と、該を像担持体を帯電する帯電手段と、上記像担持体上に画像情報に基づいて電位Ｖ_Ｌの静電潜像を形成するための露光手段と、上記像担持体上の静電潜像を該静電潜像と同極性のトナーで現像する現像手段を有し、上記転写体と上記像担持体が接触した転写領域で上記トナー像を上記転写体上に転写する画像形成装置において、上記転写領域よりも上記像担持体の回転方向上流側の空隙に対応する部分の上記転写体に上記像担持体の帯電極性と同極性で且つ絶対値がＶ_Ｌより大きい電圧を印加する部材を有する構成としたので、転写領域前でのトナーの転写体への移動による画像チリを低減することができる。
【００６１】
請求項２記載の発明によれば、請求項１記載の画像形成装置において、上記転写体が中間転写体である構成としたので、転写領域前でのトナーの中間転写ベルトへの移動による画像チリを低減することができる。
【００６２】
請求項３記載の発明によれば、請求項１又は２記載の画像形成装置において、上記露光手段として、解像度Ｘ（ｄｐｉ）の副走査方向ドットピッチＰと、静止ビームスポット径ωの関係が、ω≦２Ｐである静止ビームスポットを有する構成としたので、転写前の像担持体上のトナーのプレ転写や、転写前の像担持体上のトナーへの放電を防止し、画像チリを防止できて経時変動や現像剤のバラツキによらず安定した画像を形成することができる。
【００６３】
請求項４記載の発明によれば、請求項１乃至３のうちの何れか１つに記載の画像形成装置において、解像度Ｘ＝１２００（ｄｐｉ）であり、且つ、上記露光手段の出力露光エネルギー量が画像を形成するための必要露光エネルギー量以上であり、且つ、露光領域の単位面積あたりの平均露光エネルギー量Ｊが、Ｊ≧１．９×１０−３（Ｊ／ｍ^２）になるような露光エネルギーを出力する構成としたので、画像チリを防止でき、経時変動や現像剤のバラツキによらず安定した画像を形成することができる。
【００６４】
請求項５記載の発明によれば、請求項１乃至３のうちの何れか１つに記載の画像形成装置において、解像度Ｘ（ｄｐｉ）の副走査方向のドットピッチをＰ、上記像担持体上のドット径をＤとするとき、下記式
【００６５】
【数１１】

【００６６】
を満たす構成としたので、像担持体上で黒ベタ、斜めライン、縦ラインをきちんと形成することができ、高品質の画像を形成することができる。
【００６７】
請求項６記載の発明によれば、請求項１乃至３のうちの何れか１つに記載の画像形成装置において、上記像担持体上のドット面積の変動量ΔＳが、ΔＳ≦２００μｍ^２である構成としたので、像担持体上で粒状性の高い顕像を形成でき、高品質の画像を形成することができる。
【００６８】
請求項７記載の発明によれば、像担持体を帯電させ、露光することで上記像担持体上に画像情報に基づいて電位Ｖ_Ｌの静電潜像を形成し、上記像担持体上の静電潜像を該静電潜像と同極性のトナーで顕像化し、上記転写体と上記像担持体が接触した転写領域で上記トナー像を上記転写体上に転写する画像形成方法において、上記転写領域よりも上記像担持体の回転方向上流側の空隙に対応する部分の上記転写体に上記像担持体の帯電極性と同極性で且つ絶対値がＶ_Ｌより大きい電圧を印加することとしたので、転写領域前でのトナーの転写体への移動による画像チリを低減することができる。
【００６９】
請求項８記載の発明によれば、請求項７記載の画像形成方法において、上記静電潜像の上記像担持体表面での電界強度が３×１０^７Ｖ／ｍ以上であることとしたので、画像チリを防止でき、経時変動や現像剤のバラツキによらず安定した画像を形成することができる。
【００７０】
請求項９記載の発明によれば、請求項８記載の画像形成方法において、解像度Ｘ（ｄｐｉ）の副走査方向ドットピッチをＰ、上記像担持体上のドット径をＤとするとき、下記式
【００７１】
【数１２】

【００７２】
を満たすこととしたので、像担持体上で黒ベタ、斜めライン、縦ラインをきちんと形成することができ、高品質の画像を形成することができる。
【００７３】
請求項１０記載の発明によれば、請求項８記載の画像形成方法において、上記像担持体上のドット面積の変動量ΔＳが、ΔＳ≦２００μｍ^２であることとしたので、像担持体上で粒状性の高い顕像を形成でき、高品質の画像を形成することができる。
【００７４】
請求項１１記載の発明によれば、請求項８の画像形成方法において、上記像担持体として、ＣＴＬ層でのキャリア移動度をμｐ、μｎ、ＣＧＭでの量子効率をηとするとき、下記式
【００７５】
【数１３】

【００７６】
を満たす感光体を用い、スキャン速度Ｖｘ＝１８９０〜１９２８ｍ／ｓ（主走査）、０．２５３８〜０．３１０２ｍ／ｓ（副走査）、主走査・副走査ともに１２００ｄｐｉ間隔の書き込みにおいて、静止ビームの主走査方向径が３０μｍ、副走査方向のビーム径が４０μｍとしたので、転写前の像担持体上のトナーのプレ転写や、転写前の像担持体上のトナーへの放電を防止し、画像チリを防止できて経時変動や現像剤のバラツキによらず安定した画像を形成することができ、像担持体の画像の一部が欠損することなく粒状性の高い高品質の画像を形成することができる。
【図面の簡単な説明】
【図１】本発明の実施形態における画像形成装置としてのカラー複写機の概要正面図である。
【図２】像担持体に対する転写バイアスローラと印加電圧部材との位置関係を示す図である。
【図３】各ビームスポット径における転写部での潜像電位分布を示す図である。
【図４】ビーム径とドット変動量の関係を示す図である。
【図５】平均露光エネルギーとドット変動量の関係を示す図である。
【図６】ドット面積標準偏差と粒状度の関係を示す図である。
【図７】粒状度と画質満足度の関係を示す図である。
【図８】画像のチリと感光体上電界強度の最大値間の相関を示す図である。
【図９】副走査ビームスポット径とドット面積標準偏差の関係を示す図である。
【図１０】感光体上のトナー像に対する転写体が接触する前の部分での電位状態を示す図である。
【図１１】ドット画像の像担持体上電位と、像担持体との接触部より上流側の中間転写体表面電位の関係を示す図である。
【符号の説明】
２　転写体としての中間転写ベルト
３　像担持体としての感光体
４　帯電手段としての帯電装置
５　露光手段としての光書込装置
６　現像手段としての現像装置[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile, and more particularly, to an image forming apparatus in which a toner image formed on an image carrier is temporarily transferred to an intermediate transfer body and then transferred to a transfer material again to obtain an image. Equipment related.
[0002]
[Prior art]
In an image forming apparatus using an electrophotographic process, when toner is transferred from an image carrier to an intermediate transfer member or a transfer material, disturbance due to dust has conventionally been a problem.
It is considered that this is mainly because transfer from the image bearing member to the transfer material or the intermediate transfer member, that is, pre-transfer occurs before the toner image enters the transfer area.
As a countermeasure, for example, Japanese Patent Application Laid-Open No. 8-166728 discloses that a member for pressing the image carrier and the transfer member is provided on the upstream side of the transfer portion to reduce a gap before the transfer, to prevent the flying, and to provide a pressing member. Describes a technique for applying a polarity opposite to the charging polarity of the toner to cancel the electric field in the transfer area.
Japanese Patent Application Laid-Open No. 10-188688 describes a configuration in which a conductive member having a potential of the same polarity as the charged polarity of the image carrier or a zero potential is provided upstream of a transfer nip portion in a so-called negative-positive image forming system. ing. According to this, since a bias having the same polarity as that of the toner used for image formation is applied, a force for actively attracting the toner is hardly generated.
[0003]
[Patent Document 1]
JP-A-6-202497
[Patent Document 2]
JP-A-8-30119
[Patent Document 3]
JP-A-8-166728
[Patent Document 4]
JP-A-10-186878
[Patent Document 5]
JP-A-2000-89583
[Patent Document 6]
JP-A-2000-221800
[Patent Document 7]
JP 2001-242724 A
[0004]
[Problems to be solved by the invention]
In the configuration described in Japanese Patent Application Laid-Open No. 8-166728, since the electric field formed by the pressing member is in the direction of causing the toner to fly toward the transfer member, image dust due to pre-transfer cannot be completely prevented.
In the configuration described in Japanese Patent Application Laid-Open No. H10-186878, a force for actively attracting the toner is unlikely to be generated, but the potential of the image portion where the toner is attached to a general image carrier is not zero, and some There is a residual potential of the same polarity as the charged polarity of the body. Therefore, when the potential of the conductive member is zero or the like, toner scattering has occurred.
[0005]
As described above, in the conventional method, the phenomenon in which the toner moves before the photosensitive member as the image carrier and the intermediate transfer member, for example, cannot be completely prevented.
The present inventors conducted experiments by changing the process conditions such as the charging potential of the photoconductor, the exposure intensity, and the developing bias. As a result, it was found that the amount of toner turbulence was not determined only by the voltage conditions applied to the conductive member. Was.
However, when the experimental results were analyzed in detail, it was found that the results were related to the latent image potential of the image portion (toner-attached portion) of the photoconductor formed under the process conditions.
[0006]
FIG. 10 shows the state of the potential at a portion before the intermediate transfer member contacts the transfer portion (transfer region) with respect to the toner image on the photosensitive member. V is the potential of the non-image area on the photoconductor, V_LIs the image portion potential. Since the polarity of the toner is negative and the image is formed by the negative-positive method, the toner is attached to a portion of the latent image having the same polarity having a smaller absolute value. In this figure, the upward direction is shown to be negative. The negative toner is more likely to move downward in the figure in terms of potential.
Here, considering the case of GND (ground) as the potential of the intermediate transfer member, for the toner, V_LIt is more stable, and the toner moves in the GND direction and adheres to the intermediate transfer member. When the exposure is sufficiently performed in consideration of the ideal photosensitive member, the potential of the image portion becomes GND._LIs −100 to −150 V.
[0007]
Therefore, with respect to the contact portion between the image carrier and the intermediate transfer member, the intermediate transfer member in a portion corresponding to the gap on the upstream side in the image carrier rotation direction has the same polarity as the image carrier charge polarity and an absolute value of V._LIf a higher voltage is applied, transfer dust can be suppressed.
Specifically, V_LIs determined by an initial value determined by the type of the photoconductor and a change due to deterioration over time.
Also, in this method, since the potential of the latent image at the edge of the image is higher than that at the center of the image, discharge occurs between the intermediate transfer member and the toner polarity on the image carrier changes, and the edge of the image changes. The phenomenon that the transfer was hard to occur in the periphery occurred.
FIG. 11 shows experimental relationship data between the potential of the dot image on the image carrier and the surface potential of the intermediate transfer member upstream of the contact portion with the image carrier. Compared to the potential difference between the center of the dot and the intermediate transfer member, the potential difference between the edge portion of the dot and the intermediate transfer member is large. When the potential difference increases, discharge occurs, and the image carrier adheres around the edge. It is considered that the transfer of the toner becomes difficult due to the change in the toner polarity, and the dot area varies, which causes the graininess to deteriorate.
[0008]
Accordingly, the present invention provides an image forming method capable of suppressing transfer dust, stably suppressing variation in dots regardless of variation over time and variation in developer, and improving image quality by improving graininess. The main purpose is to provide an apparatus and an image forming method.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the present invention suppresses transfer dust (image dust) in relation to the state of an electrostatic latent image.
Specifically, according to the first aspect of the present invention, the image carrier, a charging unit for charging the image carrier, and a potential V on the image carrier based on image information._LExposing means for forming an electrostatic latent image on the image carrier, and developing means for developing the electrostatic latent image on the image carrier with toner having the same polarity as the electrostatic latent image. In the image forming apparatus for transferring the toner image onto the transfer body in the transfer area where the carrier is in contact, the above-mentioned transfer body is provided with a portion corresponding to a gap on the upstream side in the rotation direction of the image carrier from the transfer area. The same polarity as the charging polarity of the image carrier and the absolute value is V_LThe configuration has a member for applying a higher voltage.
[0010]
According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the transfer body is an intermediate transfer body.
[0011]
According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the relationship between the dot pitch P of the resolution X (dpi) in the sub-scanning direction and the stationary beam spot diameter ω is ω ≦ It has a 2P static beam spot configuration.
Here, the stationary beam spot diameter ω is 1 / e of the maximum beam intensity.²Diameter.
[0012]
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the resolution X is 1200 (dpi) and the output exposure energy amount of the exposure unit is an image. And the average exposure energy J per unit area of the exposure area is J ≧ 1.9 × 10 −3 (J / m²) Is output.
[0013]
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the dot pitch in the sub-scanning direction of the resolution X (dpi) is P, and the dot on the image carrier is P. When the diameter is D, the following equation
[0014]
(Equation 4)

[0015]
Is satisfied.
[0016]
According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the variation ΔS of the dot area on the image carrier is ΔS ≦ 200 μm²Is adopted.
[0017]
In the invention according to claim 7, the image carrier is charged and exposed, so that the potential V is set on the image carrier based on image information._LForming an electrostatic latent image on the image carrier, visualizing the electrostatic latent image on the image carrier with toner having the same polarity as the electrostatic latent image, and forming the electrostatic latent image on the transfer area where the transfer body and the image carrier are in contact with each other. In the image forming method for transferring a toner image onto the transfer body, the same polarity as the charge polarity of the image carrier is applied to a portion of the transfer body corresponding to a gap upstream of the transfer area in the rotation direction of the image carrier. And the absolute value is V_LA higher voltage was applied.
[0018]
According to an eighth aspect of the present invention, in the image forming method of the seventh aspect, the electric field intensity of the electrostatic latent image on the surface of the image carrier is 3 × 10⁷V / m or more.
[0019]
According to a ninth aspect of the present invention, in the image forming method of the eighth aspect, when the dot pitch in the sub-scanning direction of the resolution X (dpi) is P and the dot diameter on the image carrier is D, the following expression is used.
[0020]
(Equation 5)

[0021]
It was decided to satisfy.
[0022]
According to a tenth aspect of the present invention, in the image forming method of the eighth aspect, the variation ΔS of the dot area on the image carrier is ΔS ≦ 200 μm²It was decided.
[0023]
According to an eleventh aspect of the present invention, in the image forming method of the eighth aspect, when the carrier mobility in the CTL layer is μp and μn, and the quantum efficiency in CGM is η,
[0024]
(Equation 6)

[0025]
Scan speed Vx = 1890 to 1928 m / s (main scanning), 0.2538 to 0.3102 m / s (sub scanning), and writing at 1200 dpi intervals in both main scanning and sub scanning. The diameter in the main scanning direction was 30 μm, and the beam diameter in the sub-scanning direction was 40 μm.
[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 9.
First, the configuration and operation of a tandem-type color copying machine as an image forming apparatus according to the present embodiment will be described with reference to FIG. The color copying machine 1 has an image forming unit 1A located at the center of the apparatus main body, a paper feeding unit 1B located below the image forming unit 1A, and an image reading unit 1C located above the image forming unit 1A. are doing.
In the image forming section 1A, an intermediate transfer belt 2 as a transfer body having a transfer surface extending in the horizontal direction is arranged. On the upper surface of the intermediate transfer belt 2, an image of a color having a complementary color relationship with the color separation color is provided. Are provided. That is, the photoconductors 3Y, 3M, 3C, and 3B as image carriers capable of carrying an image with toners of complementary colors (yellow, magenta, cyan, and black) are juxtaposed along the transfer surface of the intermediate transfer belt 2. Have been.
[0027]
Each of the photoconductors 3Y, 3M, 3C, and 3B is formed of a drum that is rotatable in the same counterclockwise direction. Around the charging device 4, a charging device 4 serving as a charging unit that performs an image forming process in a rotation process is provided. The potential V is set on each of the photoconductors 3Y, 3M, 3C and 3B based on the image information._LAn optical writing device 5 as an exposure device for forming an electrostatic latent image, and a developing device as a developing device for developing the electrostatic latent image on each photoconductor 3 with toner having the same polarity as the electrostatic latent image. 6, a transfer bias roller 7 as a primary transfer unit, an applied voltage member 15, and a cleaning device 8 are arranged. The alphabet attached to each code corresponds to the color of the toner similarly to the photoconductor 3. Each developing device 6 contains a respective color toner.
The intermediate transfer belt 2 is configured to be wound around a plurality of rollers 2A to 2C and move in the same direction at a position facing the photoconductors 3Y, 3M, 3C, and 3B. A roller 2C different from the rollers 2A and 2B supporting the transfer surface is opposed to the secondary transfer device 9 with the intermediate transfer belt 2 interposed therebetween. In FIG. 1, reference numeral 10 indicates a cleaning device for the intermediate transfer belt 2.
[0028]
The surface of the photoconductor 3Y is uniformly charged by the charging device 4Y, and an electrostatic latent image is formed on the photoconductor 3Y based on image information from the image reading unit 1C. The electrostatic latent image is visualized as a toner image by a developing device 6Y containing yellow toner, and the toner image is applied to a transfer bias roller 7Y on the intermediate transfer belt 2 as a first transfer process. Is attracted and transferred by the electric field due to the applied voltage.
The applied voltage member 15Y is provided on the upstream side of the transfer bias roller 7Y in the rotation direction of the photoconductor 3Y (see FIG. 2). The applied voltage member 15Y causes the intermediate transfer belt 2 to have the same polarity as the charging polarity of the photoreceptor 3Y and an absolute value of V_LA larger voltage is applied to prevent the toner from transferring from the photoconductor 3Y to the intermediate transfer belt 2 before the toner image enters the transfer area. Prevent turbulence caused by dust. Hereinafter, this is referred to as counter-type transfer.
[0029]
Similar image formation is performed on the other photoconductors 3M, 3C, and 3B except for the color of the toner, and the toner images of the respective colors are sequentially transferred onto the intermediate transfer belt 2 and superimposed.
After the transfer, the toner remaining on the photoconductor 3 is removed by the cleaning device 8, and after the transfer, the potential of the photoconductor 3 is initialized by a discharging lamp (not shown) to prepare for the next image forming process.
The secondary transfer device 9 has a transfer belt 9C that is wrapped around a charging drive roller 9A and a driven roller 9B and moves in the same direction as the intermediate transfer belt 2. By charging the transfer belt 9C by the charging drive roller 9A, a multicolor image or a single-color image carried on the intermediate transfer belt 2 can be transferred onto a sheet 16 as a transfer material.
[0030]
The paper 16 is fed from the paper feed unit 1B to the secondary transfer position. A plurality of paper feed cassettes 1B1 in which paper 16 is stacked and stored in the paper feed unit 1B, and a paper feed roller 1B2 which separates and feeds the paper 16 stored in the paper feed cassette 1B1 one by one from the top. , A pair of conveying rollers 1B3, a pair of registration rollers 1B4 located upstream of the secondary transfer position, and the like.
The paper 16 fed from the paper feed cassette 1B1 is temporarily stopped by the pair of registration rollers 1B4, and after correcting the oblique displacement or the like, the position of the front end of the toner image on the intermediate transfer belt 2 and the predetermined position of the front end in the conveyance direction are determined. Are sent to the secondary transfer position by the registration roller pair 1B4 with the matching tying. A manual tray 50 is provided on the right side of the apparatus main body so as to be able to be turned upside down. The paper 16 stored in the manual tray 50 merges with a paper transport path from the paper feed cassette 1B1 fed by the paper feed roller 52. The sheet is sent toward the registration roller pair 1B4 by the convey path.
[0031]
In the optical writing device 5, writing light is controlled by image information from the image reading unit 1C or image information output from a computer (not shown) to write the photosensitive members 3Y, 3M, 3C, and 3B according to the image information. Light is emitted to form an electrostatic latent image.
The image reading unit 1C includes an automatic document feeder 1C1, a scanner 1C2 having a contact glass 54 as a document table, and the like. The automatic document feeder 1C1 has a configuration in which a document fed out onto the contact glass 54 can be inverted, and can scan the front and back surfaces of the document.
The electrostatic latent image on the photoconductor 3 formed by the optical writing device 5 is subjected to visible image processing by the developing device 6, and is primarily transferred to the intermediate transfer belt 2. When the toner images of each color are superimposedly transferred onto the intermediate transfer belt 2, they are collectively and secondarily transferred onto the paper 16 by the secondary transfer device 9. The secondary-transferred sheet 16 is sent to the fixing device 11, where the unfixed image is fixed by heat and pressure. The residual toner on the intermediate transfer belt 2 after the secondary transfer is removed by the cleaning device 10.
[0032]
The sheet 16 that has passed through the fixing device 11 is selectively guided to a conveyance path toward the paper discharge tray 13 and a reverse conveyance path RP by a conveyance path switching claw 12 provided on the downstream side of the fixing apparatus 11. When the sheet is conveyed toward the sheet discharge tray 13, the sheet is discharged onto the sheet discharge tray 13 by the sheet discharge roller pair 56 and stacked. When the sheet is guided to the reversing conveyance path RP, the sheet is reversed by the reversing device 58 and is again sent to the registration roller pair 1B4.
[0033]
With the configuration described above, in the color copying machine 1, the photosensitive member 3 (hereinafter simply referred to as the photosensitive member 3) uniformly charged by exposing and scanning the original placed on the contact glass 54 or by image information from a computer. An electrostatic latent image is formed on the intermediate transfer belt 2 after the electrostatic latent image is formed into a visible image by the developing device 6. .
In the case of a single image, the toner image transferred to the intermediate transfer belt 2 is transferred as it is to the sheet 16 fed from the sheet feeding unit 1B. In the case of a multicolor image, after the primary transfer is repeated and superimposed, the secondary transfer is collectively performed on the paper 16.
The sheet 16 after the secondary transfer has the unfixed image fixed thereon by the fixing device 11, and is then discharged to the sheet discharge tray 13 or reversed and sent again to the registration roller pair 1B4 for double-sided image formation.
[0034]
Although the image disturbance can be reduced by the above-described counter transfer, it is difficult to transfer only the periphery of the edge, and a phenomenon in which dots vary occurs. This is because, as described with reference to FIG. 11, the latent image potential at the edge of the image has a greater absolute value than the central portion of the image, and a discharge occurs between the edge and the intermediate transfer belt 2 so that the photosensitive This is because the polarity of the toner on the body 3 is changed, and it is considered that the disturbance at the upper edge portion of the belt was not conspicuous due to dust in the conventional transfer.
In order to prevent this, the profile of the latent image was changed by changing the exposure beam spot diameter, and confirmation was performed.
In this embodiment, since it is difficult to measure the potential distribution on the photoconductor 3, the charge density distribution, the electric field intensity distribution, and the potential distribution on the photoconductor 3 are calculated.
In the latent image formation, carriers (charges) generated in a carrier generation layer (CGL) inside the photoconductor 3 by irradiating (exposure) light energy existing on the charged photoconductor 3 are transferred to the photoconductor 3. The carrier transfer layer (CTL) is moved to neutralize the charge on the photoreceptor 3 to form a latent image, but the amount of generated carriers, Coulomb repulsion between carriers, recombination of carriers, The calculation is performed in consideration of carrier mobility.
[0035]
[Example 1]
Writing density: 1200 dpi
Emission wavelength: 780 nm
Still beam spot diameter: 30 μm × 30 μm, 45 μm × 45 μm, 55 μm × 70 μm
(The stationary beam spot diameter is 1 / e of the maximum beam intensity.²Diameter. )
Image carrier: Photoconductor
Photoconductor quantum efficiency: η = 2.8 × 10^-5× E^0.5933
Photoconductor absorption coefficient: 0.7
Photoconductor thickness: 28 μm
Required exposure energy: 7 erg / cm²
Exposure means: scanning laser beam
Linear velocity 0.285m / s
Scanning scan speed 1909m / s
Initial charging potential: -650V
[0036]
FIG. 3 shows the latent image potential distribution at the transfer portion for each beam spot diameter. From FIG. 3, it was confirmed that the profile of the latent image potential became sharper by reducing the beam spot diameter. When the charging characteristic of the photoconductor 3 and the polarity of the toner are the same, if the absolute value of the charging potential of the photoconductor 3 decreases, the adhesion of the toner to the developing sleeve of the developing device 6 becomes smaller than the adhesion of the toner to the developing sleeve of the developing device 6. And the amount of development increases.
The small-diameter beam spot has a smaller absolute value of the latent image potential at the center of the dot, so that the toner can be easily visualized, and there is a possibility of low-potential development. Further, when the comparison is made with the toner adhesion amount constant, the discharge with the intermediate transfer belt 2 is less likely to occur because the small diameter beam has a lower absolute value of the potential of the latent image portion.
[0037]
When actually confirmed by an image formed by the counter transfer method, it was found that only the periphery of the edge was hardly transferred, and a phenomenon that dots varied. This is because the latent image potential at the edge of the image has a larger absolute value of the potential than that at the center of the image, the intermediate transfer body is discharged at the edge, and the toner polarity on the image carrier changes, and the image is transferred. Probably because there is no. When confirmed by lowering the developing bias, it was possible to eliminate the transfer failure near the edge.
Conventionally, image fluctuations and image dust fluctuations in the transfer unit were larger than dot fluctuation amounts on the image carrier.However, by reducing fluctuations in the transfer unit, the toner image on the image carrier is faithfully reproduced. Therefore, the amount of dot fluctuation on the image carrier sensitively affects the image, and it is necessary to suppress the amount of dot fluctuation on the image carrier in order to form a good image.
[0038]
The dot diameter fluctuates due to the aging of the process including the characteristics of the photosensitive member and the characteristics of the developer and the variation in the agent, and the image is deteriorated. Here, the beam spot diameter was changed, and the fluctuation amount of the dot diameter on the photosensitive member was confirmed. The result is shown in FIG. Here, the diameter of the dot on the photoreceptor and the variation thereof are calculated. The calculation method is described below.
In the developing electric field intensity distribution, a toner image is formed on a photoconductor by toner adhering to a region above a certain threshold electric field intensity. This threshold electric field intensity is defined as a critical developing electric field intensity (Eth). The value is determined by the process and the condition of the developer.
If the critical developing electric field strength is known, the dot diameter on the photoconductor (hereinafter, referred to as dot diameter D on the image carrier) can be calculated.
In short, the term “dot diameter” as used herein refers to the length of the dot on the photoconductor in the main scanning direction and the sub-scanning direction in a region where the development electric field intensity distribution is equal to or higher than the critical developing electric field intensity.
[0039]
The variation in the dot diameter was replaced with the variation in the critical developing electric field strength at the time of calculating the dots, and was calculated by varying the critical developing electric field strength.
As can be seen from FIG. 4, the dot fluctuation amount decreases as the beam spot diameter decreases. It is said that if the dot variation is suppressed to half or less of one dot, the roughness of the image is reduced. When the beam spot diameter ω becomes about twice the dot pitch P in the sub-scanning direction of the resolution X (dpi), The dot fluctuation amount becomes about half of one dot.
Therefore, by forming a latent image with a beam spot diameter of ω ≦ 2P (P is a dot pitch in the sub-scanning direction of the resolution X (dpi)), the photoconductor 3 and the intermediate transfer belt 2 at the portion where the toner adheres are formed. It has been found that an image is formed in which discharge is prevented and variation in dot area is small and graininess is improved without hindering transfer of an edge portion.
[0040]
[Example 2]
When a latent image is formed on an image carrier, the necessary amount of exposure energy is determined from the quantum efficiency and light attenuation characteristics of the image carrier. When a photoconductor is used as an image carrier, high-density writing is required to form a high-resolution (for example, 1200 dpi) image. Carrier recombination occurs, and a phenomenon occurs in which the amount of generated carriers decreases.
Therefore, when forming a high-resolution image, there is a problem not only in the amount of exposure energy required to form an image but also in what area the required amount of exposure energy is exposed. Although the intensity profile of the laser beam can be approximated to a Gaussian distribution, here, the exposure energy was considered as the average exposure energy amount per exposure area, and the dot fluctuation amount on the photoconductor at that time was considered. The result is shown in FIG.
[0041]
Writing density: 1200 dpi
Emission wavelength: 780 nm
Static beam spot diameter: variable from 30 μm to 70 μm
(The stationary beam spot diameter is 1 / e of the maximum beam intensity.²Diameter. )
Image carrier: Photoconductor
Photoconductor quantum efficiency: η = 5.6 × 10^-6× E^0.7022
Photoconductor absorption coefficient: 0.7
Photoconductor thickness: 30 μm
Required exposure energy: 7 erg / cm²
Exposure means: scanning laser beam
Linear velocity 0.362m / s
Scanning scan speed 1983.87m / s
Initial charging potential: -800V
[0042]
It was confirmed that the variation amount of the dots decreased with the increase in the average exposure energy amount. Conventionally, fluctuations in the transfer section such as image disturbance and image dust were larger than the amount of dot fluctuation on the image carrier.However, by reducing the fluctuations in the transfer section, the toner image on the image carrier is faithfully reproduced. Therefore, the amount of dot fluctuation on the image carrier sensitively affects the image, and it is necessary to suppress the amount of dot fluctuation on the image carrier in order to form a good image.
It is said that if the dot fluctuation amount is suppressed to about half of one dot, the roughness of the image is reduced. For example, at 1200 dpi, the amount of dot fluctuation on the photoreceptor needs to be suppressed to 15 μm or less.^-3J / m²It turned out that it is desirable to be above.
The required exposure energy is determined by the sensitivity of the image carrier and the development process. For example, when a photoconductor is used, it is calculated from its light attenuation characteristics. The light attenuation characteristic attenuates as a function of the exposure energy, and saturates to the residual potential Vs of each image carrier. The equation of the light attenuation characteristic is as follows.
[0043]
(Equation 7)

[0044]
Where V₀Is the initial charge potential, e is the amount of charge, X is the exposure energy, C is the capacitance of the photoreceptor, L is the film thickness, and n and α are constants of the photoreceptor quantum efficiency.
Quantum efficiency η = α · Eⁿ
Conventionally, there has been a phenomenon in which the image after transfer is thicker than the image on the image carrier due to the influence of disturbance of the image in the transfer unit. However, the fluctuation in the transfer unit has reduced the toner image on the image carrier. It has been found that in order to faithfully reproduce the image, the solid black image, the oblique line, and the vertical line image must also be reproduced on the image carrier.
Therefore, the relationship D = αP between the dot diameter on the image carrier and the dot pitch in the sub-scanning direction was examined for one dot line image. The relationship between the dot diameters D and P on the image bearing member was calculated by changing the developing bias, and an image was produced using the counter bias method under the developing conditions, and line breaks and line widths were observed. Table 1 shows the results.
[0045]
[Table 1]

[0046]
Table 1 shows that it is preferable to form a line having a small line width and no break. Therefore, assuming that the dot pitch in the sub-scanning direction at the resolution X (dpi) is P, the dot diameter D on the image carrier is
[0047]
(Equation 8)

[0048]
It has been found that by performing the exposure such that the image becomes black solid, diagonal line and vertical line images can be reproduced.
[0049]
[Example 3]
FIG. 6 shows the relationship between the granularity, which is one of the image evaluation items, and the amount of change in the area of one dot on paper in various apparatuses. For example, to achieve a granularity of 0.2, ΔS = 200 μm², 0.1 is ΔS = 150 μm²This indicates that the following dot area variation (standard deviation) is required.
On the other hand, FIG. 7 shows the results of an image quality satisfaction survey conducted on various image samples having different granularities. It can be seen that in order to obtain the satisfaction of most subjects, the granularity is preferably 0.2 or less, and preferably 0.1 or less. 6 and FIG. 7, ΔS = 200 μm²It is determined that the following is necessary.
In the counter bias method, the variation of the image on the paper is substantially equal to the variation of the image on the OPC (photoconductor). Therefore, the variation in the image on the OPC is ΔS = 200 μm²It must be:
FIG. 9 shows the relationship between the beam spot diameter and the dot area variation when the beam spot diameter in the main scanning direction is fixed at 30 μm. For example, in order to achieve a granularity of 0.2, it is found that a beam spot diameter smaller than 30 × 40 μm is desirable.
[0050]
[Example 4]
Conventionally, when transferring the toner, the toner on the photoconductor 3 moves in the direction of the intermediate transfer belt 2 before the photoconductor 3 and the intermediate transfer belt 2 come into contact with each other, so that the image is disturbed. As described above, the voltage applied to the intermediate transfer belt 2 by the applied voltage member 15 provided on the upstream side of the transfer unit has the same polarity as the charge polarity of the photoconductor 3 and the absolute value is V._LA higher voltage is applied to prevent the toner image from being transferred from the photoconductor 3 to the intermediate transfer belt 2 before the toner image enters the transfer area, and the toner is transferred from the photoconductor 3 to the intermediate transfer belt 2 due to dust. Prevents disturbance. However, the counter transfer method is not sufficient.
[0051]
The present inventors have confirmed that, in the counter transfer method, there is a correlation between the dust of an image and the maximum value (calculated value) of the electric field strength of the latent image on the photoconductor. This is shown in FIG. The amount of dust is set as a value using the area of the toner that has protruded from the edge portion of the line image to the background portion as an index, and as the value increases, the amount of dust increases and the image deteriorates.
The image dust increases with an increase in the electric field strength of the latent image, and becomes 3 × 10⁷When it exceeds V / m, it saturates. Therefore, the electric field strength of the latent image is set to 3 × 10⁷It was confirmed that by setting V / m or more, image dust was prevented, and a stable image could be formed irrespective of variation with time and variation in developer.
[0052]
In the above configuration,
[0053]
(Equation 9)

[0054]
When the image satisfying the above conditions was formed, it was possible to form a black solid, a diagonal line, and a part of the vertical line image on the photoreceptor 3 without loss, and a high quality image could be formed.
In the above configuration, the dot area variation ΔS on the image carrier is set to ΔS ≦ 200 μm²As a result, a high-grained visible image with high granularity was formed on the photoreceptor 3, whereby a high-quality image with high granularity could be formed.
[0055]
[Example 5]
Writing density: 1200 dpi
Emission wavelength: 780 nm
Static beam spot diameter: 30 to 70 μm
(The stationary beam spot diameter is 1 / e of the maximum beam intensity.²Diameter. )
Image carrier: Photoconductor
Photoconductor quantum efficiency: η = 2.8 × 10^-5× E^0.5933
Photoreceptor mobility:
[0056]
(Equation 10)

[0057]
Photoconductor absorption coefficient: 0.7
Photoconductor thickness: 28 μm
Required exposure energy: 7 erg / cm²
Exposure means: scanning laser beam
Linear velocity 0.285m / s
Scanning scan speed 1909m / s
Initial charging potential: -650V
[0058]
FIG. 9 shows the relationship between the dot diameter on the photosensitive member and the beam diameter. When the diameter of the stationary beam in the main scanning direction is 30 μm and the beam diameter in the sub-scanning direction is 40 μm, ΔS = 200 μm²Was achieved, and it was confirmed that a high-quality image could be formed without any loss of black solid, oblique line, and part of the vertical line image.
[0059]
In the above embodiment, the intermediate transfer belt 2 has been described as a transfer member. However, the transfer from an image carrier to a transfer material (paper) can be performed with the same concept.
[0060]
【The invention's effect】
According to the first aspect of the present invention, the image carrier, a charging unit for charging the image carrier, and a potential V on the image carrier based on image information._LExposing means for forming an electrostatic latent image on the image carrier, and developing means for developing the electrostatic latent image on the image carrier with toner having the same polarity as the electrostatic latent image. In the image forming apparatus for transferring the toner image onto the transfer body in the transfer area where the carrier is in contact, the above-mentioned transfer body is provided with a portion corresponding to a gap on the upstream side in the rotation direction of the image carrier from the transfer area. The same polarity as the charging polarity of the image carrier and the absolute value is V_LSince the structure having the member for applying a higher voltage is used, it is possible to reduce the image dust due to the movement of the toner to the transfer body before the transfer area.
[0061]
According to the second aspect of the present invention, in the image forming apparatus according to the first aspect, since the transfer body is an intermediate transfer body, an image dust due to movement of the toner to the intermediate transfer belt in front of the transfer area is provided. Can be reduced.
[0062]
According to the third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the relationship between the dot pitch P of the resolution X (dpi) in the sub-scanning direction and the diameter of the stationary beam spot ω is as follows. With a configuration having a stationary beam spot where ω ≦ 2P, it is possible to prevent pre-transfer of toner on the image carrier before transfer and discharge to the toner on the image carrier before transfer, thereby preventing image dust. As a result, a stable image can be formed irrespective of variation over time or variation in the developer.
[0063]
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the resolution X = 1200 (dpi) and the output exposure energy amount of the exposure unit. Is greater than or equal to the exposure energy required to form an image, and the average exposure energy J per unit area of the exposure area is J ≧ 1.9 × 10 −3 (J / m²Since the configuration is such that the exposure energy is output as described in (1), image dust can be prevented, and a stable image can be formed regardless of variation over time and variations in the developer.
[0064]
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the dot pitch in the sub-scanning direction of the resolution X (dpi) is P, Where D is the dot diameter of
[0065]
[Equation 11]

[0066]
Is satisfied, black solid, oblique lines, and vertical lines can be properly formed on the image carrier, and a high-quality image can be formed.
[0067]
According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the variation ΔS of the dot area on the image carrier is ΔS ≦ 200 μm²With this configuration, a visible image with high granularity can be formed on the image carrier, and a high-quality image can be formed.
[0068]
According to the seventh aspect of the present invention, the image carrier is charged and exposed, so that the potential V is set on the image carrier based on image information._LForming an electrostatic latent image on the image carrier, visualizing the electrostatic latent image on the image carrier with toner having the same polarity as the electrostatic latent image, and forming the electrostatic latent image on the transfer area where the transfer body and the image carrier are in contact with each other. In the image forming method for transferring a toner image onto the transfer body, the same polarity as the charge polarity of the image carrier is applied to a portion of the transfer body corresponding to a gap upstream of the transfer area in the rotation direction of the image carrier. And the absolute value is V_LSince a higher voltage is applied, image dust due to movement of the toner to the transfer member in front of the transfer area can be reduced.
[0069]
According to an eighth aspect of the present invention, in the image forming method according to the seventh aspect, the electric field intensity of the electrostatic latent image on the surface of the image carrier is 3 × 10⁷Since V / m or more, image dust can be prevented, and a stable image can be formed irrespective of variation over time and variation in the developer.
[0070]
According to the ninth aspect of the present invention, in the image forming method of the eighth aspect, when the dot pitch in the sub-scanning direction of the resolution X (dpi) is P and the dot diameter on the image carrier is D,
[0071]
(Equation 12)

[0072]
Is satisfied, black solid, oblique lines, and vertical lines can be properly formed on the image carrier, and a high-quality image can be formed.
[0073]
According to a tenth aspect of the present invention, in the image forming method of the eighth aspect, the variation ΔS of the dot area on the image carrier is ΔS ≦ 200 μm²Therefore, a visible image with high granularity can be formed on the image carrier, and a high-quality image can be formed.
[0074]
According to an eleventh aspect of the present invention, in the image forming method of the eighth aspect, when the carrier mobility in the CTL layer is μp and μn, and the quantum efficiency in the CGM is η, the following expression is used.
[0075]
(Equation 13)

[0076]
Scan speed Vx = 1890 to 1928 m / s (main scanning), 0.2538 to 0.3102 m / s (sub scanning), and writing at 1200 dpi intervals in both main scanning and sub scanning. Since the diameter in the main scanning direction is 30 μm and the beam diameter in the sub-scanning direction is 40 μm, pre-transfer of toner on the image carrier before transfer and discharge to the toner on the image carrier before transfer are prevented. To prevent dust, to form a stable image regardless of variation over time and variations in developer, and to form a high-quality image with high granularity without partial loss of the image on the image carrier Can be.
[Brief description of the drawings]
FIG. 1 is a schematic front view of a color copying machine as an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a diagram illustrating a positional relationship between a transfer bias roller and an applied voltage member with respect to an image carrier.
FIG. 3 is a diagram showing a latent image potential distribution at a transfer portion at each beam spot diameter.
FIG. 4 is a diagram illustrating a relationship between a beam diameter and a dot variation amount.
FIG. 5 is a diagram illustrating a relationship between an average exposure energy and a dot variation amount.
FIG. 6 is a diagram showing a relationship between dot area standard deviation and granularity.
FIG. 7 is a diagram showing a relationship between granularity and image quality satisfaction.
FIG. 8 is a diagram illustrating a correlation between dust of an image and a maximum value of an electric field intensity on a photoconductor.
FIG. 9 is a diagram showing a relationship between a sub-scanning beam spot diameter and a dot area standard deviation.
FIG. 10 is a diagram illustrating a potential state in a portion before a transfer member contacts a toner image on a photosensitive member.
FIG. 11 is a diagram illustrating a relationship between a potential of a dot image on an image carrier and a surface potential of an intermediate transfer body upstream of a contact portion with the image carrier.
[Explanation of symbols]
2) Intermediate transfer belt as transfer body
3. Photoconductor as image carrier
4 Charging device as charging means
5. Optical writing device as exposure means
6. Developing device as developing means

Claims (11)

An image carrier; a charging unit configured to charge the image carrier; an exposure unit configured to form an electrostatic latent image having a potential _VL on the image carrier based on image information; Developing means for developing the electrostatic latent image with toner having the same polarity as the electrostatic latent image, and transferring the toner image onto the transfer body in a transfer area where the transfer body and the image carrier are in contact with each other. In the image forming apparatus,
A member that applies a voltage having the same polarity as the charging polarity of the image carrier and an absolute value greater than _{VL to the} transfer body in a portion corresponding to a gap on the rotation direction upstream side of the image carrier from the transfer area. An image forming apparatus comprising: The image forming apparatus according to claim 1,
An image forming apparatus, wherein the transfer member is an intermediate transfer member. The image forming apparatus according to claim 1, wherein
An image forming apparatus as described above, wherein the exposure means has a stationary beam spot in which a relationship between a dot pitch P in the sub-scanning direction at a resolution X (dpi) and a stationary beam spot diameter ω is ω ≦ 2P. The image forming apparatus according to any one of claims 1 to 3,
The resolution X = 1200 (dpi), the output exposure energy of the exposure means is equal to or greater than the exposure energy required for forming an image, and the average exposure energy J per unit area of the exposure region is , J ≧ 1.9 × 10 −3 (J / m ² ). The image forming apparatus according to any one of claims 1 to 3,
When the dot pitch of the resolution X (dpi) in the sub-scanning direction is P and the dot diameter on the image carrier is D, the following equation is used.

An image forming apparatus characterized by satisfying the following. The image forming apparatus according to any one of claims 1 to 3,
The image forming apparatus according to claim ¹ , wherein a variation amount ΔS of the dot area on the image carrier is ΔS ≦ 200 μm ² . By charging and exposing the image carrier, an electrostatic latent image having a potential _VL is formed on the image carrier based on image information, and the electrostatic latent image on the image carrier is replaced with the electrostatic latent image. And an image forming method of transferring the toner image onto the transfer body in a transfer area where the transfer body and the image carrier are in contact with each other,
Applying a voltage having the same polarity as the charging polarity of the image carrier and an absolute value larger than _{VL to the} transfer body in a portion corresponding to a gap on the upstream side in the rotation direction of the image carrier from the transfer area. A characteristic image forming method. The image forming method according to claim 7,
An image forming method, wherein an electric field intensity of the electrostatic latent image on the surface of the image carrier is 3 × 10 ⁷ V / m or more. The image forming method according to claim 8, wherein
When the dot pitch in the sub-scanning direction of the resolution X (dpi) is P and the dot diameter on the image carrier is D, the following formula is used.

An image forming apparatus characterized by satisfying the following. The image forming method according to claim 8, wherein
An image forming method, wherein the variation amount ΔS of the dot area on the image carrier is ΔS ≦ 200 μm ² . The image forming method according to claim 8,
When the carrier mobility in the CTL layer is μp and μn and the quantum efficiency in CGM is η, the following expression is used.

Scan speed Vx = 1890 to 1928 m / s (main scanning), 0.2538 to 0.3102 m / s (sub scanning), and writing at 1200 dpi intervals in both main scanning and sub scanning. An image forming method, wherein the diameter in the main scanning direction is 30 μm and the beam diameter in the sub-scanning direction is 40 μm.

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