JP2004004783A - Charging system, process cartridge and image forming apparatus - Google Patents

Abstract

【選択図】図１Provided are a charging system, a process cartridge, and an image forming apparatus that optimize various conditions relating to charging in a method of performing charging using conductive particles.
An image carrier includes an image carrier, a charging member that forms a nip portion with the image carrier, and charges the image carrier. The nip portion is provided with conductive particles m. , The capacitance of the image carrier is Cd (F / mm ² ), the particle size of the conductive particles is D (mm), and the density of the conductive particles m on the charging member is Nc. (Pcs / mm ² ), the surface moving speed of the charging member is Vc (mm / sec), the surface moving speed of the image carrier is Vd (mm / sec), k = Vc / Vd, the nip in the image carrier moving direction. Is W (mm), the time coefficient α represented by the following equation is α> 15.

[Selection diagram] Fig. 1

Description

【００２０】
また、上記の目的を達成するための本発明に係るプロセスカートリッジの代表的な構成は、画像形成装置本体に着脱可能なプロセスカートリッジであり、像担持体と、前記像担持体とニップ部を形成し、前記像担持体を帯電する帯電部材と、を有し、前記ニップ部には導電粒子が設けられ、前記像担持体の表面の抵抗率をρ（Ω）、像担持体の静電容量をＣｄ（Ｆ／ｍｍ^２）、前記導電粒子の粒径をＤ（ｍｍ）、前記帯電部材上にある前記導電粒子の密度をＮｃ（個／ｍｍ^２）、前記帯電部材の表面移動速度をＶｃ（ｍｍ／ｓｅｃ）、前記像担持体の表面移動速度をＶｄ（ｍｍ／ｓｅｃ）、ｋ＝Ｖｃ／Ｖｄ、前記像担持体移動方向における前記ニップ部の幅をＷ（ｍｍ）とすると、下記の式で表わされる時間係数αが、α＞１５であることを特徴とするプロセスカートリッジ、である。
【００２１】
記
【００２２】
【式５】

【００２３】
また、上記の目的を達成するための本発明に係る帯電システムの代表的な構成は、被帯電体と、前記被帯電体とニップ部を形成し、前記被帯電体を帯電する帯電部材と、とを有し、前記ニップ部には導電粒子が設けられ、前記被帯電体の表面の抵抗率をρ（Ω）、前記被帯電体の静電容量をＣｄ（Ｆ／ｍｍ^２）、前記導電粒子の粒径をＤ（ｍｍ）、前記帯電部材上にある前記導電粒子の密度をＮｃ（個／ｍｍ^２）、前記帯電部材の表面移動速度をＶｃ（ｍｍ／ｓｅｃ）、前記被帯電体の表面移動速度をＶｄ（ｍｍ／ｓｅｃ）、ｋ＝Ｖｃ／Ｖｄ、前記被帯電体移動方向における前記ニップ部の幅をＷ（ｍｍ）とすると、下記の式で表わされる時間係数αが、α＞１５であることを特徴とする帯電システム、である。
【００２４】
記
【００２５】
【式６】

【００２６】
【発明の実施の形態】
以下、図１〜３を用いて本発明に従う、帯電システム、プロセスカートリッジ及び画像形成装置を説明する。
【００２７】
図１は本実施例を示す画像形成装置の概略構成模型図である。本実施例の画像形成装置は、転写方式電子写真プロセス利用、導電性粒子（帯電促進粒子）を用いた直接注入帯電方式、反転現像方式、プロセスカートリッジ方式のレーザービームプリンタである。
【００２８】
（１）プリンタの全体的概略構成
図１中、１は被帯電体（像担持体）たる直径３０ｍｍの感光ドラム、２は接触帯電部材たる直径１８ｍｍの帯電ローラ、３は現像装置、４は転写帯電装置たる転写ローラ、５は定着装置、６は露光装置たるレーザービームスキャナである。
Ａ．帯電工程；感光ドラム１は図１中矢印Ａの方向に所定の周面速度（プロセススピード）Ｖｄ（ｍｍ／ｓｅｃ）をもって回転駆動される。帯電ローラ２は、装置が未使用の状態であらかじめ導電性粒子ｍが塗布され、感光ドラム１に対して所定の押圧力をもって圧接されており、感光ドラム１の回転方向Ａに対して、圧接部（帯電ニップ）Ｎにおいて逆方向（図１中矢印Ｂ）の方向に所定の周面速度Ｖｃ（ｍｍ／ｓｅｃ）をもって回転駆動される。帯電ローラ２は感光ドラム１との圧接部Ｎにおいて、その外周面を所定の極性・電位に一様に注入帯電する。本実施例では、ほぼ−７００Ｖになるように帯電した。帯電システムは、被帯電体である感光ドラム１、帯電部材である帯電ローラ２、導電粒子ｍによって形成される。
【００２９】
Ｂ．露光工程；前記帯電工程で注入帯電された感光ドラム１周面上に、レーザービームスキャナ６によるレーザー露光Ｌがなされて目的のプリントパターン（画像情報）に対応した静電潜像が形成される。
【００３０】
Ｃ．現像工程；感光ドラム１周面上に形成された静電潜像は、現像装置３により現像されトナー像として顕像化される。本実施例の現像工程は現像剤３ａとして一成分磁性ネガトナーを用いた反転現像である。本実施例の現像剤のトナーの平均粒径は７μｍのものを用いた。
【００３１】
現像装置３は現像剤担持搬送部材たる現像スリーブ３ｂと現像スリーブ３ｂ内に設置されたマグネットローラ３ｃで構成され、現像スリーブ３ｂと感光ドラム１の間に現像バイアス電源Ｓ２から所定の現像バイアスが印加されることによって現像を行う。本実施例において現像バイアス電圧は、以下のＤＣ成分とＡＣ成分の重畳電圧である。
【００３２】
ＤＣ電圧：−５００Ｖ
ＡＣ電圧：矩形波：ピーク問電圧１６００Ｖ、周波数１．８ｋＨｚ
Ｄ．転写工程；感光ドラム１周面上に形成されたトナー像は、転写ローラ４によって、不図示の給紙部から所定の制御タイミングにて給送された記録材Ｐに転写される。
【００３３】
転写ローラ４には、転写バイアス電源Ｓ３から所定の転写バイアス電圧が印加される。本実施例では転写バイアス電圧として＋２ｋＶのＤＣ電圧を加えた。
【００３４】
Ｅ．定着工程；トナー像を転写された記録材Ｐは定着装置５に搬送されて、熱および圧力による定着処理を受け、画像形成物（プリント・コピー）として機外に排紙される。
【００３５】
Ｆ．回収工程；転写工程において転写されず、感光ドラム１周面上に残留しているトナーは、前記の帯電工程の帯電ニップまで運ばれる。残留トナーの一部は帯電ローラに付着し、他の一部は感光体に付着したまま帯電ニップを通過する。
【００３６】
帯電ニップを通過したトナーは、感光体面同様に帯電工程による注入帯電によってトナーの正規の極性（本実施例では−極性）に帯電され、続く現像工程で回収可能とされる。帯電ローラに付着した残留トナーも、何度か帯電ニップを通過するうちに上記のように正規の極性に帯電されて感光体に転移し、続く現像工程で回収可能とされる。
【００３７】
現像工程では、露光部上のトナーはそのまま感光体上にとどまり（すなわち現像され）、非露光部上のトナーは現像装置に回収される。即ち、現像装置によって現像動作が行なわれると同時に回収動作が行なわれる。
【００３８】
本実施例のプリンタは、感光ドラム１、帯電ローラ２、現像装置３を一括してプロセスユニット（プロセスカートリッジ）９とし、プリンタ本体に対して着脱・交換自在に構成した。
【００３９】
（２）感光ドラム
本実施例で用いた感光ドラム１は、抵抗の調整がしやすい電荷注入層１６（図２）を有する感光体である。感光体電荷注入層１６は光硬化型のアクリル樹脂にＳｎＯ_２超微粒子１６ａ（粒子径が約０．０３μｍ）、テフロン（商品名）（登録商標）などの滑剤等を混合分散し、塗工後、光硬化法により膜形成したものである。
【００４０】
図２に本実施例で用いた感光ドラムの層構成模型図を示す。感光ドラム１は、アルミドラム基体１１上に、下引き層１２、正電荷注入防止層１３、電荷発生層１４、電荷輸送層１５の順に重ねて塗工された一般的な有機感光体ドラムに上記の電荷注入層１６を塗布することにより、電荷注入性能を向上した。
【００４１】
電荷の直接注入による帯電方式において重要な点は表層の抵抗にある。被帯電体側の抵抗を下げることでより効率良く電荷の授受が行えるようになる。一方、感光体として用いる場合には静電潜像を一定時間保持する必要があるため、必要以上に抵抗を下げることはできない。本実施例では後述する時間係数αの測定のため、表層の抵抗が異なるものを数種類作成した。
【００４２】
本実施例の表層の抵抗は、作成した感光ドラム上に図３に示すような櫛歯上の電極（電極間のギャップ２００μｍ、延べ電極長５．６ｍｍ）を蒸着し、電極間に１００Ｖの電圧を印加して、電極間を流れる電流値から求めた抵抗率ρ（Ω）を実効表面抵抗率として用いている。
【００４３】
（３）帯電ローラ
帯電ローラ２は芯金２ａ上に中抵抗の弾性発泡体層２ｂを形成することにより作成される。本実施例では、弾性発泡体層２ｂはウレタン樹脂に導電性カーボンブラック粒子を分散して抵抗を調整した上で発泡形成したもので、必要に応じて表面を研磨している。本実施例では、直径１８ｍｍ、長手長さ２００ｍｍ、の帯電ローラ２を標準として用いた。また、後述する時間係数αの測定のため、感光ドラム１とのニップ部Ｗの幅を変られるように弾性発泡体層２ｂの外径の異なるものを数種類用意した。
【００４４】
本実施例の標準帯電ローラ２の抵抗を測定したところ１００ｋΩであった。計測は、帯電ローラ２の芯金２ａに総圧ｌｋｇの加重がかかるように３０ｍｍのアルミドラムに圧着した状態で、芯金２ａとアルミドラムに１００Ｖを印加して行った。
【００４５】
電荷の直接注入による帯電方式において帯電ローラ２は電極として機能することが重要である。弾性を持たせ十分な接触状態を得ると同時に、移動する被帯電体を充電するに十分低い抵抗を有する必要がある。一方では被帯電体にピンホールなどの欠陥部位が存在した場合に電圧のリークを防止する必要がある。上記から、十分な帯電性と耐リーク性を得るには上記測定法で１０^４〜１０^７Ωの抵抗が望ましい。
【００４６】
帯電ローラ２の硬度は、硬度が高すぎても低すぎても、感光体表面へのミクロな接触性が悪くなるので、アスカ−Ｃ硬度で２５度から５０度が好ましい範囲である。本実施例の標準帯電ローラ２のアスカ−Ｃ硬度は３５度である。
【００４７】
（４）導電性粒子
本実施例では、導電性粒子ｍとして、比抵抗が３×１０^３Ωｃｍの導電性酸化スズ粒子を用いた。これを使用前の帯電ローラ２の表面に均一に塗布した。
【００４８】
ここで抵抗測定は、錠剤法により測定し正規化して求めた。底面積２．２６ｍｍ^２の円筒内に凡そ０．５ｇの粉体試料を入れ上下電極に１５ｋｇの加圧を行うと同時に１００Ｖの電圧を印加し抵抗値を計測、その後、正規化して比抵抗を算出した。
【００４９】
粒径の測定には、光学あるいは電子顕微鏡による観察から、１００個以上抽出し、水平方向最大弦長をもって体積粒度分布を算出しその５０％平均粒径をもって決定した。
【００５０】
（５）注入帯電条件
前述したように、帯電ローラ２と感光ドラム１を速度差もって接触させ、帯電ローラ２の表面に存在する導電性粒子ｍを感光ドラム表面に摺擦することで感光ドラム表面への接触性を向上させている。接触性が向上することで電荷の受け渡し機会が増え、注入帯電性能を向上させている。
【００５１】
帯電ローラ２と感光ドラム１とのニップ部Ｎの幅（ｍｍ／以下同じ符号Ｗであらわす）、と帯電ローラ２の表面に存在する導電性粒子ｍの密度Ｎｃ（個／ｍｍ^２）は、導電性粒子ｍの接触機会を左右する重要なファクターである。また、帯電ローラ２の周面速度Ｖｃ（ｍｍ／ｓｅｃ）、感光ドラム１の周面速度Ｖｄ（ｍｍ／ｓｅｃ）とその周面速度比率をｋ（＝Ｖｃ／Ｖｄ）は、導電性粒子ｍの接触時間と単位時間当たりの帯電総量を定めるファクターとなる。
【００５２】
本実施例では、帯電ローラ２と感光ドラム１とのニップ部Ｎの幅Ｗは帯電ローラ２の径を振ることで約０．５〜５．０ｍｍの値をとることができるようにした。また、帯電ローラ２は感光ドラム１の回転駆動装置は各々任意に回転数を変えられるように構成した。
【００５３】
帯電ローラ２の表面の導電性粒子ｍの密度Ｎｃ（個／ｍｍ^２）は、使用時の帯電ローラ２に上記接触を形成するのと同じ条件でスライドガラスを当接させ、その当接面を光学顕微鏡で観察し、１０μｍ平方あたりの導電性粒子ｍのカウント数を１０ポイント程度平均することで求めた。光学的に導電性粒子ｍの数をカウントすることが困難な場合は、ＳＥＭの蛍光Ｘ線分析での元素分析により、先と同様に定められた面積あたりの導電性粒子ｍに固有に含まれる物質量を１０ポイント程度測定し、その平均値を予め求めておいた換算式で、粒子個数を概算して決定した。
【００５４】
（６）注入帯電機構の論理的考察
以下、図４〜６を用いて本発明に従う注入帯電機構の論理的考察を説明する。
【００５５】
感光体表面で１個の導電性粒子ｍによる注入帯電の様子を、図４を用いて説明する。
【００５６】
図４は帯電ローラ２と感光ドラム１とのニップ部Ｎのある瞬間における導電性粒子ｍ近傍の状態を表したもので、導電性粒子ｍから距離ｒ（ｍｍ）離れた微小領域ΔＳに、導電性粒子ｍから感光体表面を伝って電荷Ｑ（Ｃ）が注入されることを想定したものである。
【００５７】
微小領域ΔＳのドラムの静電容量をＣ（Ｆ）とし、距離ｒ（ｍｍ）の間の感光体表面に沿った抵抗値をＲ（Ω）とすると、微小領域ΔＳを目標電位Ｅ（Ｖ）に帯電する場合の図４の等価回路は図５のように表される。この等価回路を、時間を変数として充電電荷Ｑ（Ｃ）にしたがって解くと、よく知られた以下の式で表される。
【００５８】
Ｑ＝ＣＥ｛１−ｅｘｐ（−ｔ／ＲＣ）｝
上式でＲＣは時定数と呼ばれ時間（ｓｅｃ）の単位を持ち、充電時間ｔ（ｓｅｃ）が時定数ＲＣの何倍になるかによって、目標電位Ｅ（Ｖ）の何％の電位になるかがわかる。
【００５９】
ｔ＝α・ＲＣ
として、充電時間ｔ（ｓｅｃ）が時定数ＲＣのα倍のとき充電電位が目標電位に対して何％（Ｘ％）になるかを表１に示す。以下、αを時間係数と呼ぶ。
【００６０】
【表１】

【００６１】
表１に示すように、理論上はαが時定数ＲＣの５倍のとき９９％を超え、時定数ＲＣの９倍のとき約９９．９９％となる。本発明の構成では、導電性粒子ｍが帯電ローラ２上から動かず固定されていると仮定すると、導電性粒子ｍが感光ドラム１に触れている時間ｔの上限は、ニップ部Ｎの幅Ｗ（ｍｍ）を通過する時間、すなわち、Ｔｃ＝Ｗ／Ｖｃ（ｓｅｃ）である。したがって、Ｔｃが時定数ＲＣを大幅に上回る値であれば十分に注入帯電が可能であることが示唆され、ほぼ等しい場合（α≒１）には注入帯電が不十分になる可能性が高くなることがわかる。
【００６２】
上記をさらに詳細に検討する。まず、１つの導電性粒子ｍが充電しなければならない面積を算出する。
【００６３】
帯電ローラ２の長手方向長をＬ（ｍｍ）、延べ帯電時間ｔ０（ｓｅｃ）とすると、
帯電ローラ２の接触延べ面積（ｍｍ^２）：
Ｓｃ＝Ｌ・（Ｖｃ・ｔ０＋Ｗ）・・・式１
感光ドラム１の接触延べ面積（ｍｍ^２）：
Ｓｄ＝Ｌ・（Ｖｄ・ｔ０＋Ｗ）・・・式２
感光ドラム１上の単位面積に接触する導電性粒子ｍの個数：Ｎｄ（個／ｍｍ^２）とすると、
Ｎｄ＝Ｎｃ・Ｓｃ／Ｓｄ
本発明の条件下では、Ｖｃ・ｔ０≫Ｗ，Ｖｄ・ｔ０≫Ｗ、であることから、Ｗは無視できるのでＷ＝０として、式１、２を代入すると、

１つの導電性粒子ｍが充電しなければならない面積は、上記のＮｄの逆数である。この面積の形状が、半径ｒｃ（ｍｍ）の円であると仮定すると、
π（ｒｃ）^２＝１／Ｎｄ
であるので、ｒｃは以下のようになる。
【００６４】
ｒｃ＝（πＮｄ）^−１／２・・・式４
感光体の単位面積あたりの静電容量をＣｄ（Ｆ／ｍｍ^２）とすると、１つの導電性粒子ｍが充電しなければならない面積の静電容量Ｃ（Ｆ）は、以下のようになる。
【００６５】
Ｃ＝Ｃｄ／Ｎｄ＝Ｃｄ／（Ｎｃ・ｋ）・・・式５
次に１つの導電性粒子ｍが充電しなければならないに感光ドラム１の面積の表層の抵抗を考察する。
【００６６】
図６に示すように、一般に半径ｒ（ｍｍ）の外径を持つ円電極と、この円電極と同一中心を持つ半径ｒ＋Δｒ（ｍｍ）の内径を持つ電極の間の抵抗ΔＲ（Ω）はΔｒ（ｍｍ）が小さい場合は、表面抵抗率ρ０（Ω）とすると以下の式で表される。
【００６７】
ΔＲ≒ρ０・Δｒ／（２πｒ）
Δｒ（ｍｍ）→０の極限では、半径ｒ（ｍｍ）の電極の半径方向の抵抗の増分ｄＲ（Ω）は、以下のようになる。
【００６８】
ｄＲ／ｄｒ＝ρ０／（２πｒ）
導電性粒子ｍがある瞬間に感光ドラム１に接触している面積が半径Ａ（ｍｍ）の円であると仮定したとき、上記の１つの導電性粒子ｍが充電しなければならない面積の円換算半径ｒｃ（ｍｍ）の抵抗値Ｒ（Ω）は、上記の式をＡ（ｍｍ）からｒｃ（ｍｍ）まで積分して以下のように示される。
【００６９】
Ｒ＝（ρ０／２π）・｛ｌｎ（ｒｃ）−ｌｎＡ｝・・・式６
（ここでｌｎは、自然対数のことである。）
表面抵抗率ρ０（Ω）を前述の測定法による表層の抵抗率ρ（Ω）で置き換えると、上記から、導電性粒子ｍ１個あたりの充電面積を円換算したモデルでは、式６に式３、式４、式５を代入すると、時定数ＲＣが以下のように求められる。
【００７０】
ＲＣ＝（ρ／２π）・Ｃｄ／（Ｎｃ・ｋ）・［−０．５・ｌｎ（π・ｋ・Ｎｃ）−ｌｎＡ］
よって、時間係数αは、ｔ＝α・ＲＣ、ｔ＝Ｗ／Ｖｃであることより以下の式で示される。
【００７１】
α＝２π・ｋ・Ｎｃ・（Ｗ／Ｖｃ）／（ρ・Ｃｄ・Ｚ）
Ｚ＝−０．５・ｌｎ（π・ｋ・Ｎｃ）−ｌｎＡ
上記の式で、Ａは導電粒子ｍの想定上の接触面積を示すパラメータであって、任意の値を取れる。しかしながら、αを指標として意味のある値にするには、その基準としてふさわしい値をとる必要があり、以下のような制約を満たすことが望ましい。
【００７２】
１．用いる導電粒子ｍの粒径Ｄに相関を持つ値を取る必要がある。
【００７３】
２．現実に接触可能な導電性粒子ｍの個数（上限値）を考慮する必要がある。
【００７４】
３．αを時定数に関連付けているため、想定可能なほとんどの場合にＺが負の値をとらないようにしなければならない。
【００７５】
上記の制約以外は単純化した理想的な状況を想定しても問題ない。ここで、導電性粒子ｍが十分にあり、導電性粒子ｍが接触した部分は、感光ドラム１と帯電ローラ２の相対速度が零でも（ｋ＝１）すべて帯電されるような理想的な状況を想定する。また、粒径Ｄの球体導電粒子ｍが帯電ローラ２上を最密充填している時に、導電粒子ｍの１個が充電しなければならない感光ドラム１の面積（１／Ｎｄ）にすべて接触する（導電粒子ｍが直接電荷を注入可能な）状況を想定した。剛体として帯電ローラ２上を隙間なく埋めた球体導電粒子ｍが、感光ドラム１と帯電ローラ２の間で潰れて、感光ドラム１上に隙間なく接触するような状況を想定すればよい。
【００７６】
上記の想定では、前述の面積（１／Ｎｄ）の円換算面積：π・ｒｃ^２（ｍｍ^２）と、粒径Ｄの球体導電粒子ｍが感光ドラム１に投影する面積：π・（Ｄ／２）^２（ｍｍ^２）とは、最密充填率σ（＝０．８３８７）と以下の関係にある。
【００７７】
π・ｒｃ^２：π・（Ｄ／２）^２＝１：σ
上記の想定では、Ａ＝ｒｃに他ならないため、β＝σ^−１とすると、以下のようになる。
【００７８】
【式７】

【００７９】
注入帯電性が良好な場合は、導電性粒子ｍがニップ部Ｎの幅Ｗを通過する時間よりもはるかに短い時間で注入帯電が完了してしまうため、上記のモデルに良く従う。ここで注意すべきは、注入帯電性が劣り時間がかかるほど、もしくは接触時間が短くなるほど上記モデルから外れていくことである。よって、表１に示した時間係数αとＸ％の相関はαが小さくなるほどズレが生じる。例えば、αの値が５であったとしても９９％の帯電ができている保証はない。しかしながら、逆に上記モデルから外れていくほど注入帯電性が劣り上記の時間係数αの値は０に収束していくので、注入帯電性の指標値としての有効性は保たれる。
【００８０】
上記から、注入帯電性が保たれるか否かのαの閾値を新たに定める必要がある。
【００８１】
（７）時間係数αの閾値の決定
本実施例の画像形成装置で、時間係数αのパラメータを振って注入帯電性を調べ、前述のαの閾値を決定する。注入帯電がしにくい状況として、感光層が薄い場合（感光ドラム１が消耗した場合など）や、導電性粒子ｍの密度Ｎｃが１００〜１０００（個／ｍｍ^２）のオーダーまで少なくなった場合を想定した。この状況下で、以下に示す表題のパラメータを変えて、前述の時間係数αを求め、そのときの注入帯電の成否（主に画像を形成したときの帯電ムラの有無、ドラム上カブリの状況）などを比較した。
【００８２】
Ａ．表層の抵抗率：ρを変化させた結果を以下に示す。
【００８３】
その他のパラメータは以下のとおり、
感光ドラムの膜厚　　　　　：Ｙ　≒１０（μｍ）
単位面積あたりの静電容量　：Ｃｄ≒３．９８×１０^−１２（Ｆ）
導電性粒子ｍの直径　　　　：Ｄ　＝０．５（μｍ）
ニップ幅　　　　　　　　　：Ｗ　＝２．０（ｍｍ）
帯電ローラ２の周面速度　　：Ｖｃ＝８５．５（ｍｍ／ｓｅｃ）
感光ドラム１の周面速度　　：Ｖｄ＝１７１（ｍｍ／ｓｅｃ）
周面速度比率　　　　　　　：ｋ　＝０．５
導電性粒子の密度　　　　　：Ｎｃ≒１０００（個／ｍｍ^２）
【００８４】
【表２】

【００８５】
表２に示すように、感光体表層の抵抗率ρ（Ω）を変化させると、若干のばらつきがあるものの時間係数αがおおよそ７〜９の間に帯電ムラの閾値が存在す
る。おおむねα＞１２では、良好な帯電結果となる。
【００８６】
Ｂ．帯電ローラ上の導電性粒子の粒径：Ｄおよび密度：Ｎｃを変化させた結果を以下に示す。その他のパラメータは以下のとおり、
表層の抵抗率　　　　　　　：ρ　＝３．０１×１０^１１（Ω）
感光ドラムの膜厚　　　　　：Ｙ　≒１０（μｍ）
単位面積あたりの静電容量　：Ｃｄ≒３．９８×１０^−１２（Ｆ）
ニップ幅　　　　　　　　　：Ｗ　＝２．０（ｍｍ）
帯電ローラ２の周面速度　　：Ｖｃ＝８５．５（ｍｍ／ｓｅｃ）
感光ドラム１の周面速度　　：Ｖｄ＝１７１（ｍｍ／ｓｅｃ）
周面速度比率　　　　　　　：ｋ　＝０．５
【００８７】
【表３】

【００８８】
表３は、導電性粒子の粒径Ｄおよび密度Ｎｃを変化させた結果を示したものである。時間係数αが低いほど帯電不良や、帯電ムラが発生しやすくなる。帯電ムラ発生の閾値は、ばらつきがあるが時間係数αで１０前後と認められる。
【００８９】
Ｃ．ニップの周方向幅：Ｗを変化させた結果を以下に示す。
【００９０】
帯電ローラの径に合わせて帯電ローラの周面速度Ｖｃが一定になるよう駆動回転数を調整した。感光ドラム１に対する押し付け圧力もほぼ一定になるように調整している。その他のパラメータは以下のとおり、
表層の抵抗率　　　　　　　：ρ　＝３．０１×１０^１１（Ω）
感光ドラムの膜厚　　　　　：Ｙ　≒１０（μｍ）
単位面積あたりの静電容量　：Ｃｄ≒３．９８×１０^−１２（Ｆ）
帯電ローラの周面速度　　　：Ｖｃ＝８５．５（ｍｍ／ｓｅｃ）
感光ドラムの周面速度　　　：Ｖｄ＝１７１（ｍｍ／ｓｅｃ）
周面速度比率　　　　　　　：ｋ　＝０．５
導電性粒子の密度　　　　　：Ｎｃ≒７７５（個／ｍｍ^２）
導電性粒子ｍの直径　　　　：Ｄ　＝１．３（μｍ）
【００９１】
【表４】

【００９２】
表４は、帯電ローラの径を変えて、感光ドラム１と帯電ローラ２間のニップ幅を変化させたものである。帯電ムラ発生の閾値は、時間係数αで１２前後である。
【００９３】
Ｄ．帯電手段の周面速度：Ｖｃ、像担持体の周面速度：Ｖｄを変化させた結果を以下に示す。その他のパラメータは以下のとおり、

【００９４】
【表５】

【００９５】
表５は、現像装置本体の駆動を制御して、各々の周面速度：Ｖｄ（ｍｍ／ｓｅｃ）、Ｖｃ（ｍｍ／ｓｅｃ）を変化させたものである。表中下２段の［＊］の付いたものは、ニップ幅Ｗが５．０（ｍｍ）の帯電ローラ２を用いた場合を示している。
【００９６】
表中の上から６段までの比較から、帯電ムラ発生の閾値は、時間係数αで１０〜１２の間と認められる。
【００９７】
なお、周面速度Ｖｄが８６ｍｍ／ｓｅｃの場合に示すように、αが２０前後もしくはそれ以上であっても、導電性粒子ｍがニップ部Ｗの中に留まる時間Ｔｃ（＝Ｗ／Ｖｃ）が長くなるような設定をすると、縦スジ状のムラが発生する場合がある。これは、全体として注入帯電性は十分に高いが、注入帯電に寄与する導電性粒子ｍの個数が少なくなるため、導電性粒子ｍの分布の偏りに応じて帯電均一性が損なわれるためと考えられる。
【００９８】
表中の下から５段までの比較から、Ｔｃは０．３（ｓｅｃ）以下が好ましい。
【００９９】
以上Ａ〜Ｄで述べたように、注入帯電不良に関わる画像不良発生の時間係数αの閾値は、パラメータによって若干のばらつきがあるが、αが１０〜１３の間に存在している。よって、本発明の関係式によって求められる時間係数αが余裕を見込んで１５以上になるように各パラメータを調整することで、注入帯電における帯電不良を回
避することができる。
【０１００】
（８）各パラメータの適応範囲
本実施例で用いた感光体は電荷注入層１６を有する感光体であるが、これは表層の抵抗率ρを変化させやすいために導入したに過ぎず、電荷注入層がない通常の感光層を持つ感光体やアモルファスシリコン感光体でもよい。ただし、表層の抵抗率ρの下限は、いずれの感光体によらず、高湿度環境の静電潜像維持の観点から１０^９（Ω）以上が必要である。本発明では、α＞１５を満たせば、表層の抵抗率ρの上限の規定はないが、ρの値が増大するほどρ以外のパラメータの設定条件は狭まることになる。
【０１０１】
表層の抵抗率ρは時間係数αに関わるパラメータの中でも、初期設定で決まる主要なものである。表層の抵抗率ρ以外のパラメータの設定条件を広げるためには、表層の抵抗率ρは以下の範囲であることが望ましい。
【０１０２】
１．０×１０^９＜ρ＜５．０×１０^１１（Ω）
なお、本発明の実施例の像担持体は円筒アルミの周上に感光層を形成した直径３０ｍｍの感光ドラムを用いたが、径の大きさは当然ながらこれに限定されるものではなく、画像形成上必要な任意の大きさを取ることができる。また形状も円筒ドラムに限るものではなくシームレスベルト等の形状のものに代換可能である。
【０１０３】
本発明の導電性粒子を担持する帯電手段は、実施例に示したウレタン樹脂に導電性カーボンブラック粒子を分散した弾性発泡体に限らず、ＥＰＤＭ，ＮＢＲ，ウレタン、シリコーンなどのゴム材料やその発泡体でもよく、導電性粒子たる導電性粒子ｍを表面に保持可能で電荷注入に支障のない程度の抵抗値を有するものであればよい。もちろん外径の大きさは任意に定めてよい。形状もローラ形状が使用しやすいが、本発明の趣旨に沿い、像担持体と速度差を持って一定の接触領域を維持できるならば、これに限るものではない。
【０１０４】
導電性粒子たる導電性粒子は、実施例中に示した酸化スズに限らず、酸化亜鉛，酸化チタン，酸化アンチモン，酸化インジウム，酸化ビスマス，他の金属酸化物をドープした酸化ジルコニウムなど金属酸化物系の導電性粒子や、前記の酸化物微粒子やグラファイトなどを混合して抵抗調整した有機樹脂粒子などが使用可能である。
【０１０５】
時間係数α＞１５を満たしても、帯電不良となる導電性粒子の密度Ｎｃ（個／ｍｍ^２）の下限は存在し、本実施例では、おおよそ５０〜７０（個／ｍｍ^２）以下になると縦スジ状のムラになる。これは、導電性粒子の分布に偏りが生じた場合に全帯電領域をカバーしきれないために生じるもので、時間係数α＞１５を満たした上でほぼ確実に帯電領域をカバーできる下限は１００（個／ｍｍ^２）である。導電性粒子の密度Ｎｃ（個／ｍｍ^２）の上限は、帯電手段全面を導電性粒子が覆った状態であり、導電性粒子を球体としてその最密充填状態を想定すると、５μｍの粒子で２．１４×１０^４（個／ｍｍ^２）、０．５μｍの粒子で５．３４×１０^６（個／ｍｍ^２）となる。これより多い分は理論的には直接感光層に触れていないものであり、本発明の接触モデルの適用範囲外である。
【０１０６】
電荷注入助剤としての働きの上で導電性粒子の大きさは特に規定されないが、０．１μｍ以下では帯電手段の内部に埋もれてしまうものや、現像剤に付着してしまうものが多く、像担持体への接触状況が本発明の接触モデルに適合しない。また、現像剤の粒径より大きくなるほど、帯電手段から脱離しやすく、その一部が像担持体に付着した場合に潜像露光を妨げるなどの弊害が顕著になることが知られている。よって、本発明では、本発明の導電性粒子の接触モデルに適合してかつ他の弊害を起こさない範囲として、導電性粒子の粒径は０．５μｍ以上でかつトナーの粒径以下であることが望ましい。
【０１０７】
本発明において、像担持体と帯電手段のニップ幅Ｗ（ｍｍ）は、接触時間を定める重要なファクターである。必要なニップ幅Ｗ（ｍｍ）の注入帯電性の観点からは上限値は存在しない。しかしながら、本実施例Ｃで５．０ｍｍの幅を確保した時の帯電ローラの直径は２４ｍｍであり、従来の放電に依る帯電で用いられる帯電ローラの直径に比べて非常に大きなものとなる。ニップ幅Ｗを５．０ｍｍ以上に増やす方法として帯電ローラの硬度を下げるか、感光体に押し当てる力を増やすなどが考えられるが、ニップ内部の当接圧力がニップの中央部と端部で大幅に異なることや、駆動負荷がかなり大きくなることが予想され、装置構成上機械的，力学的に過大な負担が掛かり現実的でない。また、実施例のＤの表５で示したように、帯電粒子が必要以上（Ｔｃ＜０．３（ｓｅｃ））にニップ部Ｗの中に留まるとスジ状の画像不良を起こしやすいことが確認された。ニップ幅Ｗの好ましい範囲は２．０〜５．０（ｍｍ）である。
【０１０８】
本実施例では、帯電手段周面を像担持体周面と逆方向に回転させる例を示したが、同一方向に回転させても差し支えない。理論的には同一方向に回転させても本発明の接触モデルが適応可能であり、導電性粒子を用いた接触注入帯電方式において帯電手段と像担持体との駆動方向を限定する意味はない。
【０１０９】
【発明の効果】
以上説明したように本発明によれば、導電性粒子を用いた接触注入帯電方式の帯電条件をあらかじめ検証でき、様々な状況に応じた個々のパラメータの設計上の適応範囲を予測して帯電の環境安定性，耐久安定性の高い帯電システム、画像形成装置およびプロセスユニットを提供することができる。
【０１１０】
本発明は、以上の実施例に限定されるものではなく、本発明の技術思想の範囲内においてあらゆる変形が可能である。
【図面の簡単な説明】
【図１】実施例の画像形成装置の概略構成模型図
【図２】使用した感光ドラムの層構成模型図
【図３】感光層表層の抵抗率の測定方法説明図
【図４】本発明の注入帯電状況のモデル説明図
【図５】図４の等価回路図
【図６】感光層表層の抵抗算定方式の説明図
【符号の説明】
１・・感光体、２・・帯電ローラ、２ａ・・芯金、２ｂ・・弾性体ローラ、ｍ・・導電性粒子、３・・現像装置、３ａ・・現像剤（トナー）、３ｂ・・現像スリーブ、３ｃ・・マグネット、３ｄ・・規制ブレード[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a charging system for performing charging using conductive particles, a process cartridge, and an image forming apparatus.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, for example, in an image forming apparatus (image recording apparatus) such as an electrophotographic apparatus or an electrostatic recording apparatus, an image carrier (a body to be charged) such as an electrophotographic photosensitive member or an electrostatic recording dielectric has a required polarity and potential. A corona charger has been used as a charging device that uniformly performs a charging process (including a static elimination process).
[0003]
Recently, a contact-type charging device (contact charging device) has been put into practical use in which a charging member to which a voltage is applied is brought into contact with a member to be charged to perform charging. However, even in a contact charging device, the harmful effects of a discharge product such as ozone cannot be avoided in principle.
[0004]
In order to solve this problem, there has been proposed an injection charging system in which charges are directly injected from a contact charging member to a member to be charged. For example, it was arbitrarily formed by applying a voltage to a contact charging member such as a charged magnetic brush and providing a charge injection layer including a charge retaining member such as a conductive particle or a trap order originally present on the surface of the member to be charged. This is a mechanism for directly injecting and charging by injecting charges in the trap order (for example, see Patent Document 1).
[0005]
The conventional contact charging is mainly charging by a discharge phenomenon, and it is necessary to apply a voltage equal to or higher than a discharge dark value. It can be charged to a potential.
[0006]
However, in the injection charging method using a magnetic brush or the like used in Patent Document 1 and the like, a mechanism for holding the charged magnetic particles and rubbing the charged magnetic particles against the charged band causes a cost increase. I was
[0007]
Therefore, as a simpler injection charging method, a configuration in which conductive fine particles (conductive particles) for improving the contact charging property are interposed in a flexible roller as a contact member has been proposed (for example, Patent Reference 2). In this configuration, the contact between the contact charging member and the member to be charged can be controlled by the amount and the particle size of the conductive particles to be interposed, and good charging properties can be obtained.
[0008]
In addition, the contact injection charging method using conductive particles is suitably used in a cleaner-less mechanism omitting a mechanism for removing a developer remaining on a member to be charged after a transfer process, and mixing the conductive particles with the developer. A configuration has been proposed in which the amount of conductive particles in the charging contact member is maintained to maintain the charging ability (for example, see Patent Document 3).
[Patent Document 1]
JP-A-06-003921
[Patent Document 2]
JP-A-10-307454
[Patent Document 3]
JP-A-2000-081760
[0009]
[Problems to be solved by the invention]
In the contact injection charging method using the conductive particles described above, it is known that various parameters are related to each other and the charging condition is determined, but the correlation of the individual parameters is unclear, and it is suitable for various situations. The adaptation range of individual parameters was difficult to predict.
[0010]
For this reason, the variation in the environmental stability and the durability stability of the charging state is large, which has hindered the design.
[0011]
In particular, since the injection charging characteristics depend on the electrical characteristics of the member to be charged, it has been required to make the resistivity of the surface of the member to be charged appropriate.
[0012]
SUMMARY OF THE INVENTION An object of the present invention is to provide a charging system, a process cartridge, and an image forming apparatus that optimize various charging conditions in a method of performing charging using conductive particles.
[0013]
Another object of the present invention is to provide a charging system, a process cartridge, and an image forming apparatus suitable for an injection charging method.
[0014]
Another object of the present invention is to provide a charging system, a process cartridge, and an image forming apparatus in which electric characteristics of a member to be charged are optimized.
[0015]
Another object of the present invention is to provide a charging system, a process cartridge and an image forming apparatus having high charging stability.
[0016]
Further objects and features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.
[0017]
[Means for Solving the Problems]
A typical configuration of the image forming apparatus according to the present invention for achieving the above object includes an image carrier, a nip portion formed with the image carrier, and a charging member that charges the image carrier. The nip portion is provided with conductive particles, the resistivity of the surface of the image carrier is ρ (Ω), and the capacitance of the image carrier is Cd (F / mm). ² ), The particle diameter of the conductive particles is D (mm), and the density of the conductive particles on the charging member is Nc (particles / mm). ² ), The surface moving speed of the charging member is Vc (mm / sec), the surface moving speed of the image carrier is Vd (mm / sec), k = Vc / Vd, and the nip portion in the image carrier moving direction is An image forming apparatus characterized in that a time coefficient α represented by the following equation is α> 15, where W is a width (mm).
[0018]
Record
[0019]
(Equation 4)

[0020]
A typical configuration of a process cartridge according to the present invention for achieving the above object is a process cartridge detachable from an image forming apparatus main body, and forms an image carrier, and a nip portion between the image carrier and the image carrier. And a charging member for charging the image carrier. The nip portion is provided with conductive particles, and the resistivity of the surface of the image carrier is ρ (Ω), and the capacitance of the image carrier is To Cd (F / mm ² ), The particle diameter of the conductive particles is D (mm), and the density of the conductive particles on the charging member is Nc (particles / mm). ² ), The surface moving speed of the charging member is Vc (mm / sec), the surface moving speed of the image carrier is Vd (mm / sec), k = Vc / Vd, and the nip portion in the image carrier moving direction is If the width is W (mm), a time coefficient α represented by the following equation is α> 15.
[0021]
Record
[0022]
(Equation 5)

[0023]
In addition, a typical configuration of the charging system according to the present invention for achieving the above object includes a member to be charged, a nip portion formed with the member to be charged, and a charging member that charges the member to be charged, Conductive particles are provided in the nip portion, the resistivity of the surface of the member to be charged is ρ (Ω), and the capacitance of the member to be charged is Cd (F / mm). ² ), The particle diameter of the conductive particles is D (mm), and the density of the conductive particles on the charging member is Nc (particles / mm). ² ), The surface moving speed of the charging member is Vc (mm / sec), the surface moving speed of the member to be charged is Vd (mm / sec), k = Vc / Vd, and the nip portion in the moving direction of the member to be charged. Assuming that the width is W (mm), a time coefficient α expressed by the following equation is α> 15.
[0024]
Record
[0025]
(Equation 6)

[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a charging system, a process cartridge, and an image forming apparatus according to the present invention will be described with reference to FIGS.
[0027]
FIG. 1 is a schematic configuration model diagram of an image forming apparatus according to the present embodiment. The image forming apparatus of this embodiment is a laser beam printer using a transfer type electrophotographic process, a direct injection charging type using conductive particles (charge accelerating particles), a reversal developing type, and a process cartridge type.
[0028]
(1) Overall schematic configuration of printer
In FIG. 1, 1 is a photosensitive drum having a diameter of 30 mm as a member to be charged (image carrier), 2 is a charging roller having a diameter of 18 mm as a contact charging member, 3 is a developing device, 4 is a transfer roller as a transfer charging device, and 5 is a fixing device. An apparatus 6 is a laser beam scanner as an exposure apparatus.
A. Charging step: The photosensitive drum 1 is driven to rotate at a predetermined peripheral surface speed (process speed) Vd (mm / sec) in the direction of arrow A in FIG. The charging roller 2 is coated with the conductive particles m beforehand when the apparatus is not used, and is pressed against the photosensitive drum 1 with a predetermined pressing force. In the (charging nip) N, it is driven to rotate in the opposite direction (arrow B in FIG. 1) at a predetermined peripheral surface speed Vc (mm / sec). The charging roller 2 uniformly injects and charges the outer peripheral surface thereof at a predetermined polarity and potential at a pressure contact portion N with the photosensitive drum 1. In the present embodiment, charging was performed so as to be approximately -700V. The charging system is formed by a photosensitive drum 1 as a member to be charged, a charging roller 2 as a charging member, and conductive particles m.
[0029]
B. Exposure step: A laser exposure L is performed by a laser beam scanner 6 on the peripheral surface of the photosensitive drum 1 charged and charged in the charging step to form an electrostatic latent image corresponding to a target print pattern (image information).
[0030]
C. Developing step: The electrostatic latent image formed on the peripheral surface of the photosensitive drum 1 is developed by the developing device 3 and visualized as a toner image. The development process of this embodiment is a reversal development using a one-component magnetic negative toner as the developer 3a. The average particle diameter of the toner of the developer of this embodiment was 7 μm.
[0031]
The developing device 3 includes a developing sleeve 3b as a developer carrying member and a magnet roller 3c provided in the developing sleeve 3b. A predetermined developing bias is applied between the developing sleeve 3b and the photosensitive drum 1 from a developing bias power source S2. Then, development is performed. In this embodiment, the developing bias voltage is a superimposed voltage of the following DC component and AC component.
[0032]
DC voltage: -500V
AC voltage: rectangular wave: peak interrogation voltage 1600 V, frequency 1.8 kHz
D. Transfer step: The toner image formed on the peripheral surface of the photosensitive drum 1 is transferred by the transfer roller 4 onto a recording material P fed from a paper feed unit (not shown) at a predetermined control timing.
[0033]
A predetermined transfer bias voltage is applied to the transfer roller 4 from a transfer bias power supply S3. In this embodiment, a DC voltage of +2 kV was applied as a transfer bias voltage.
[0034]
E. FIG. Fixing step: The recording material P to which the toner image has been transferred is conveyed to the fixing device 5, subjected to a fixing process by heat and pressure, and discharged outside the apparatus as an image formed product (print / copy).
[0035]
F. Collection step: The toner not transferred in the transfer step and remaining on the peripheral surface of the photosensitive drum 1 is carried to the charging nip in the charging step. Part of the residual toner adheres to the charging roller, and the other part passes through the charging nip while adhering to the photoconductor.
[0036]
The toner that has passed through the charging nip is charged to the normal polarity (− polarity in this embodiment) of the toner by injection charging in the charging step, similarly to the photoconductor surface, and can be recovered in the subsequent developing step. The residual toner adhering to the charging roller is also charged to the normal polarity and transferred to the photoconductor as described above while passing through the charging nip several times, and can be recovered in the subsequent developing process.
[0037]
In the developing step, the toner on the exposed portion remains on the photoreceptor as it is (ie, is developed), and the toner on the non-exposed portion is collected by the developing device. That is, the collecting operation is performed at the same time as the developing operation is performed by the developing device.
[0038]
In the printer of the present embodiment, the photosensitive drum 1, the charging roller 2, and the developing device 3 are collectively formed as a process unit (process cartridge) 9, which is detachably mountable and replaceable with respect to the printer main body.
[0039]
(2) Photosensitive drum
The photosensitive drum 1 used in this embodiment is a photosensitive member having a charge injection layer 16 (FIG. 2) whose resistance can be easily adjusted. The photoreceptor charge injection layer 16 is made of a photocurable acrylic resin made of SnO. ₂ A lubricant and the like such as ultrafine particles 16a (particle diameter: about 0.03 μm) and Teflon (trade name) (registered trademark) are mixed and dispersed, and after coating, a film is formed by a photocuring method.
[0040]
FIG. 2 shows a schematic diagram of the layer structure of the photosensitive drum used in this embodiment. The photosensitive drum 1 is formed by coating a general organic photosensitive drum coated on an aluminum drum substrate 11 in the order of an undercoat layer 12, a positive charge injection preventing layer 13, a charge generation layer 14, and a charge transport layer 15 in the order described above. By applying the charge injection layer 16, the charge injection performance was improved.
[0041]
An important point in the charging method by direct injection of electric charge lies in the resistance of the surface layer. By lowering the resistance of the member to be charged, it is possible to more efficiently transfer charges. On the other hand, when used as a photoreceptor, it is necessary to hold the electrostatic latent image for a certain period of time, so that the resistance cannot be reduced more than necessary. In the present embodiment, several types having different surface resistances were prepared for measuring a time coefficient α described later.
[0042]
The resistance of the surface layer of the present embodiment is determined by depositing an electrode on a comb tooth (a gap between electrodes of 200 μm and a total electrode length of 5.6 mm) as shown in FIG. And the resistivity ρ (Ω) obtained from the value of the current flowing between the electrodes is used as the effective surface resistivity.
[0043]
(3) Charging roller
The charging roller 2 is formed by forming a medium resistance elastic foam layer 2b on a cored bar 2a. In this embodiment, the elastic foam layer 2b is formed by foaming after adjusting the resistance by dispersing conductive carbon black particles in urethane resin, and polishing the surface as necessary. In this embodiment, a charging roller 2 having a diameter of 18 mm and a length of 200 mm was used as a standard. In order to measure the time coefficient α described later, several elastic foam layers 2b having different outer diameters were prepared so that the width of the nip portion W with the photosensitive drum 1 could be changed.
[0044]
When the resistance of the standard charging roller 2 of this embodiment was measured, it was 100 kΩ. The measurement was performed by applying a voltage of 100 V to the core 2a and the aluminum drum in a state where the core 2a of the charging roller 2 was pressed against a 30 mm aluminum drum so that a total pressure of 1 kg was applied.
[0045]
It is important that the charging roller 2 functions as an electrode in a charging method by direct injection of charges. It is necessary to have elasticity to obtain a sufficient contact state, and at the same time, to have a sufficiently low resistance to charge the moving charged object. On the other hand, it is necessary to prevent voltage leakage when a defect site such as a pinhole is present in the member to be charged. From the above, in order to obtain sufficient chargeability and leakage resistance, 10 ⁴ -10 ⁷ A resistance of Ω is desirable.
[0046]
If the hardness of the charging roller 2 is too high or too low, the microscopic contact with the photoreceptor surface is deteriorated. Therefore, the Asker-C hardness is preferably in the range of 25 to 50 degrees. The Asker-C hardness of the standard charging roller 2 of this embodiment is 35 degrees.
[0047]
(4) Conductive particles
In this embodiment, the conductive particles m have a specific resistance of 3 × 10 ³ Ωcm conductive tin oxide particles were used. This was uniformly applied to the surface of the charging roller 2 before use.
[0048]
Here, the resistance was measured by a tablet method and normalized. Bottom area 2.26mm ² A powder sample of about 0.5 g was placed in the cylinder, and a pressure of 15 kg was applied to the upper and lower electrodes, and at the same time, a voltage of 100 V was applied to measure the resistance value. Thereafter, the resistance value was normalized to calculate the specific resistance.
[0049]
For the measurement of the particle size, 100 or more samples were extracted from observation by an optical or electron microscope, the volume particle size distribution was calculated based on the maximum chord length in the horizontal direction, and the 50% average particle size was determined.
[0050]
(5) Injection charging conditions
As described above, the charging roller 2 and the photosensitive drum 1 are brought into contact with a speed difference, and the conductive particles m present on the surface of the charging roller 2 are rubbed against the photosensitive drum surface to improve the contact property with the photosensitive drum surface. Let me. Improving the contact property increases the chance of transferring charges, and improves the injection charging performance.
[0051]
The width of the nip portion N between the charging roller 2 and the photosensitive drum 1 (mm / hereinafter expressed by the same symbol W), and the density Nc of the conductive particles m present on the surface of the charging roller 2 (number / mm) ² ) Is an important factor that determines the chance of contact with the conductive particles m. Further, the peripheral surface speed Vc (mm / sec) of the charging roller 2 and the peripheral surface speed Vd (mm / sec) of the photosensitive drum 1 and the ratio of the peripheral surface speed to k (= Vc / Vd) are determined by the conductive particles m. It is a factor that determines the contact time and the total charge per unit time.
[0052]
In the present embodiment, the width W of the nip portion N between the charging roller 2 and the photosensitive drum 1 can be set to a value of about 0.5 to 5.0 mm by changing the diameter of the charging roller 2. The charging roller 2 is configured such that the rotation driving device of the photosensitive drum 1 can change the number of rotations arbitrarily.
[0053]
Density Nc (particles / mm) of conductive particles m on the surface of charging roller 2 ² ), The slide glass is brought into contact with the charging roller 2 at the time of use under the same conditions as those for forming the above contact, the contact surface is observed with an optical microscope, and the number of conductive particles m per 10 μm square is calculated. It was determined by averaging about 10 points. When it is difficult to optically count the number of the conductive particles m, the conductive particles m are inherently included in the conductive particles m per area determined in the same manner as above by elemental analysis using X-ray fluorescence analysis of SEM. The amount of the substance was measured at about 10 points, and the average value was determined by a conversion formula which was determined in advance, and the number of particles was roughly determined.
[0054]
(6) Logical consideration of injection charging mechanism
Hereinafter, a logical consideration of the injection charging mechanism according to the present invention will be described with reference to FIGS.
[0055]
The state of injection charging by one conductive particle m on the photoconductor surface will be described with reference to FIG.
[0056]
FIG. 4 shows a state near the conductive particles m at a certain moment of the nip portion N between the charging roller 2 and the photosensitive drum 1. It is assumed that charges Q (C) are injected from the conductive particles m along the surface of the photoreceptor.
[0057]
Assuming that the capacitance of the drum in the minute area ΔS is C (F) and the resistance along the photoconductor surface during the distance r (mm) is R (Ω), the minute area ΔS is set to the target potential E (V). The equivalent circuit shown in FIG. 4 when charging is performed as shown in FIG. When this equivalent circuit is solved according to the charge Q (C) using time as a variable, it is represented by the following well-known equation.
[0058]
Q = CE {1-exp (-t / RC)}
In the above equation, RC is called a time constant and has a unit of time (sec). Depending on how many times the charging time t (sec) becomes larger than the time constant RC, the potential becomes a percentage of the target potential E (V). I understand.
[0059]
t = α · RC
Table 1 shows what percentage (X%) the charging potential is relative to the target potential when the charging time t (sec) is α times the time constant RC. Hereinafter, α is referred to as a time coefficient.
[0060]
[Table 1]

[0061]
As shown in Table 1, theoretically, when α is 5 times the time constant RC, it exceeds 99%, and when α is 9 times the time constant RC, it is about 99.99%. In the configuration of the present invention, assuming that the conductive particles m are fixed without moving from the charging roller 2, the upper limit of the time t during which the conductive particles m touch the photosensitive drum 1 is determined by the width W of the nip portion N. (Mm), that is, Tc = W / Vc (sec). Therefore, it is suggested that if Tc is much larger than the time constant RC, it is possible to sufficiently perform the injection charging. If Tc is substantially equal (α ≒ 1), the possibility that the injection charging becomes insufficient increases. You can see that.
[0062]
Consider the above in more detail. First, the area where one conductive particle m must be charged is calculated.
[0063]
Assuming that the length of the charging roller 2 in the longitudinal direction is L (mm) and the total charging time t0 (sec),
Total contact area of charging roller 2 (mm ² ):
Sc = L · (Vc · t0 + W) Equation 1
Total contact area of photosensitive drum 1 (mm ² ):
Sd = L · (Vd · t0 + W) Equation 2
Number of conductive particles m in contact with a unit area on photosensitive drum 1: Nd (pieces / mm ² )
Nd = Nc · Sc / Sd
Under the conditions of the present invention, since Vc · t0≫W and Vd · t0≫W, W can be neglected. Therefore, when W = 0 and Equations 1 and 2 are substituted,

The area where one conductive particle m has to be charged is the reciprocal of Nd described above. Assuming that the shape of this area is a circle having a radius rc (mm),
π (rc) ² = 1 / Nd
Therefore, rc is as follows.
[0064]
rc = (πNd) ^-1/2 ... Equation 4
The capacitance per unit area of the photoconductor is expressed as Cd (F / mm ² ), The capacitance C (F) of the area where one conductive particle m has to be charged is as follows.
[0065]
C = Cd / Nd = Cd / (Nc · k) Equation 5
Next, consider the resistance of the surface layer of the area of the photosensitive drum 1 when one conductive particle m has to be charged.
[0066]
As shown in FIG. 6, the resistance ΔR (Ω) between a circular electrode having an outer diameter of radius r (mm) and an electrode having an inner diameter of radius r + Δr (mm) having the same center as the circular electrode is generally Δr. When (mm) is small, the surface resistivity is expressed by the following equation, assuming that the surface resistivity is ρ0 (Ω).
[0067]
ΔR ≒ ρ0 · Δr / (2πr)
In the limit of Δr (mm) → 0, the resistance increase dR (Ω) in the radial direction of the electrode of radius r (mm) is as follows.
[0068]
dR / dr = ρ0 / (2πr)
Assuming that the area where the conductive particles m are in contact with the photosensitive drum 1 at a certain moment is a circle having a radius A (mm), the area of one conductive particle m required to be charged is converted into a circle. The resistance value R (Ω) of the radius rc (mm) is expressed as follows by integrating the above equation from A (mm) to rc (mm).
[0069]
R = (ρ0 / 2π) · {ln (rc) −lnA} Equation 6
(Where ln is the natural logarithm)
When the surface resistivity ρ0 (Ω) is replaced with the surface resistivity ρ (Ω) obtained by the above-described measurement method, from the above, in the model in which the charged area per m of the conductive particles m is converted into a circle, Equation 3 is expressed by Equation 3, By substituting Equations 4 and 5, the time constant RC is obtained as follows.
[0070]
RC = (ρ / 2π) · Cd / (Nc · k) · [−0.5 · ln (π · k · Nc) -lnA]
Therefore, the time coefficient α is represented by the following equation since t = α · RC and t = W / Vc.
[0071]
α = 2π · k · Nc · (W / Vc) / (ρ · Cd · Z)
Z = −0.5 · ln (π · k · Nc) −lnA
In the above formula, A is a parameter indicating an assumed contact area of the conductive particles m, and can take an arbitrary value. However, in order for α to be a meaningful value as an index, it is necessary to take an appropriate value as the criterion, and it is desirable to satisfy the following constraints.
[0072]
1. It is necessary to take a value having a correlation with the particle diameter D of the conductive particles m used.
[0073]
2. It is necessary to consider the number (upper limit) of the conductive particles m that can actually contact.
[0074]
3. Since α is related to the time constant, Z must not be negative in most conceivable cases.
[0075]
Other than the above restrictions, there is no problem even if a simplified ideal situation is assumed. Here, there is a sufficient amount of the conductive particles m, and the portion where the conductive particles m are in contact with the photosensitive drum 1 and the charging roller 2 is fully charged even when the relative speed between the photosensitive drum 1 and the charging roller 2 is zero (k = 1). Is assumed. Further, when the spherical conductive particles m having the particle diameter D are closest packed on the charging roller 2, one of the conductive particles m contacts all the area (1 / Nd) of the photosensitive drum 1 to be charged. A situation (where the conductive particles m can directly inject charges) was assumed. It is sufficient to assume a situation in which spherical conductive particles m which are rigidly filled on the charging roller 2 without gaps are crushed between the photosensitive drum 1 and the charging roller 2 and come into contact with the photosensitive drum 1 without gaps.
[0076]
In the above assumption, a circle-converted area of the area (1 / Nd): π · rc ² (Mm ² ) And the area projected by the spherical conductive particles m having the particle diameter D onto the photosensitive drum 1: π · (D / 2) ² (Mm ² ) Has the following relationship with the closest packing ratio σ (= 0.8387).
[0077]
π · rc ² ： Π ・ (D / 2) ² = 1: σ
In the above assumption, since A = rc is nothing but β = σ ^-1 Then, it becomes as follows.
[0078]
(Equation 7)

[0079]
When the injection charging property is good, the injection charging is completed in a much shorter time than the time when the conductive particles m pass through the width W of the nip portion N, and thus the above model is well followed. It should be noted here that the more the injection charging property is inferior, or the shorter the contact time, the more the model deviates from the above model. Therefore, the correlation between the time coefficient α and X% shown in Table 1 shifts as α decreases. For example, even if the value of α is 5, there is no guarantee that 99% charging has been achieved. However, on the contrary, as the value deviates from the above-mentioned model, the injection charging property deteriorates and the value of the time coefficient α converges to 0, so that the validity as an index value of the injection charging property is maintained.
[0080]
From the above, it is necessary to newly determine the threshold value of α for determining whether the injection charging property is maintained.
[0081]
(7) Determination of threshold of time coefficient α
In the image forming apparatus of the present embodiment, the charging property is examined by varying the parameter of the time coefficient α, and the threshold value of α is determined. It is difficult to perform injection charging when the photosensitive layer is thin (for example, when the photosensitive drum 1 is worn out) or when the density Nc of the conductive particles m is 100 to 1000 (particles / mm). ² ) Was assumed to be reduced. In this situation, the above-mentioned time coefficient α is obtained by changing the parameters of the following titles, and the success or failure of the injection charging at that time (mainly the presence or absence of uneven charging when forming an image, the situation of fog on the drum) And so on.
[0082]
A. The result of changing the resistivity of the surface layer: ρ is shown below.
[0083]
Other parameters are as follows:
Photosensitive drum film thickness: YＹ10 (μm)
Capacitance per unit area: Cd ≒ 3.98 × 10 ^-12 (F)
Diameter of conductive particles m: D = 0.5 (μm)
Nip width: W = 2.0 (mm)
Peripheral surface speed of charging roller 2: Vc = 85.5 (mm / sec)
Peripheral surface speed of photosensitive drum 1: Vd = 171 (mm / sec)
Peripheral surface speed ratio: k = 0.5
Density of conductive particles: Nc ≒ 1000 (pieces / mm ² )
[0084]
[Table 2]

[0085]
As shown in Table 2, when the resistivity ρ (Ω) of the surface layer of the photoreceptor is changed, there is a slight variation, but the threshold value of the charging unevenness exists when the time coefficient α is approximately 7 to 9.
You. When α> 12, good charging results are obtained.
[0086]
B. The results obtained by changing the particle diameter D and the density Nc of the conductive particles on the charging roller are shown below. Other parameters are as follows:
Surface resistivity: ρ = 3.01 × 10 ¹¹ (Ω)
Photosensitive drum film thickness: YＹ10 (μm)
Capacitance per unit area: Cd ≒ 3.98 × 10 ^-12 (F)
Nip width: W = 2.0 (mm)
Peripheral surface speed of charging roller 2: Vc = 85.5 (mm / sec)
Peripheral surface speed of photosensitive drum 1: Vd = 171 (mm / sec)
Peripheral surface speed ratio: k = 0.5
[0087]
[Table 3]

[0088]
Table 3 shows the results obtained by changing the particle diameter D and the density Nc of the conductive particles. As the time coefficient α is lower, poor charging and uneven charging are more likely to occur. Although the threshold value of the occurrence of the charging unevenness varies, it is recognized that the time coefficient α is around 10.
[0089]
C. The result of changing the circumferential width of the nip: W is shown below.
[0090]
The driving rotation speed was adjusted so that the peripheral surface speed Vc of the charging roller was constant according to the diameter of the charging roller. The pressing pressure on the photosensitive drum 1 is also adjusted to be substantially constant. Other parameters are as follows:
Surface resistivity: ρ = 3.01 × 10 ¹¹ (Ω)
Photosensitive drum film thickness: YＹ10 (μm)
Capacitance per unit area: Cd ≒ 3.98 × 10 ^-12 (F)
Peripheral surface speed of charging roller: Vc = 85.5 (mm / sec)
Peripheral surface speed of photosensitive drum: Vd = 171 (mm / sec)
Peripheral surface speed ratio: k = 0.5
Density of conductive particles: Nc ≒ 775 (pieces / mm ² )
Diameter of conductive particles m: D = 1.3 (μm)
[0091]
[Table 4]

[0092]
Table 4 changes the nip width between the photosensitive drum 1 and the charging roller 2 by changing the diameter of the charging roller. The threshold value of the occurrence of uneven charging is about 12 in terms of the time coefficient α.
[0093]
D. The results obtained by changing the peripheral surface speed of the charging means: Vc and the peripheral surface speed of the image carrier: Vd are shown below. Other parameters are as follows:

[0094]
[Table 5]

[0095]
Table 5 shows changes in the peripheral surface speeds: Vd (mm / sec) and Vc (mm / sec) by controlling the driving of the developing device main body. In the table, the lower two columns marked with [*] indicate the case where the charging roller 2 having a nip width W of 5.0 (mm) is used.
[0096]
From the comparison of the top six rows in the table, it is recognized that the threshold value of the occurrence of uneven charging is between 10 and 12 in terms of the time coefficient α.
[0097]
In addition, as shown in the case where the peripheral surface speed Vd is 86 mm / sec, even when α is about 20 or more, the time Tc (= W / Vc) in which the conductive particles m stay in the nip portion W is small. If the setting is made longer, vertical streak-like unevenness may occur. This is thought to be due to the fact that although the injection chargeability is sufficiently high as a whole, the number of the conductive particles m contributing to the injection charge is reduced, and the charge uniformity is impaired in accordance with the uneven distribution of the conductive particles m. Can be
[0098]
Tc is preferably 0.3 (sec) or less from the comparison from the bottom to the fifth stage in the table.
[0099]
As described above in A to D, the threshold value of the time coefficient α for the occurrence of an image defect related to the injection charging defect slightly varies depending on the parameter, but α exists between 10 and 13. Therefore, by adjusting each parameter so that the time coefficient α obtained by the relational expression of the present invention becomes 15 or more in consideration of a margin, charging failure in injection charging can be prevented.
Can be avoided.
[0100]
(8) Applicable range of each parameter
The photoreceptor used in the present embodiment is a photoreceptor having a charge injection layer 16, but this is only introduced because the resistivity ρ of the surface layer is easily changed. It may be a photoreceptor having the same or an amorphous silicon photoreceptor. However, the lower limit of the resistivity ρ of the surface layer is set to 10 from the viewpoint of maintaining an electrostatic latent image in a high humidity environment regardless of which photoconductor is used. ⁹ (Ω) or more is required. In the present invention, if α> 15 is satisfied, there is no upper limit for the resistivity ρ of the surface layer, but the setting conditions of parameters other than ρ become narrower as the value of ρ increases.
[0101]
The surface resistivity ρ is a main parameter determined by the initial setting, among the parameters related to the time coefficient α. In order to widen the setting conditions of parameters other than the surface resistivity ρ, the surface resistivity ρ is desirably in the following range.
[0102]
1.0 × 10 ⁹ <Ρ <5.0 × 10 ¹¹ (Ω)
Although the image bearing member of the embodiment of the present invention uses a photosensitive drum having a diameter of 30 mm in which a photosensitive layer is formed on the circumference of a cylindrical aluminum, the size of the diameter is not limited to this. Any size required for formation can be taken. Further, the shape is not limited to a cylindrical drum, but can be replaced with a shape such as a seamless belt.
[0103]
The charging means for supporting the conductive particles of the present invention is not limited to the elastic foam in which the conductive carbon black particles are dispersed in the urethane resin shown in Examples, but may be a rubber material such as EPDM, NBR, urethane, silicone, or the like. Any material may be used as long as it can hold the conductive particles m as conductive particles on the surface and has a resistance value that does not hinder charge injection. Of course, the size of the outer diameter may be arbitrarily determined. Although the shape of the roller is easy to use, the shape is not limited to this as long as a constant contact area can be maintained with a speed difference from the image carrier according to the gist of the present invention.
[0104]
The conductive particles, which are conductive particles, are not limited to the tin oxide shown in the examples, but may be metal oxides such as zinc oxide, titanium oxide, antimony oxide, indium oxide, bismuth oxide, and zirconium oxide doped with other metal oxides. It is possible to use organic conductive particles or organic resin particles whose resistance has been adjusted by mixing the above-mentioned oxide fine particles or graphite.
[0105]
Even if the time coefficient α> 15 is satisfied, the density Nc (particles / mm) of the conductive particles that causes charging failure ² ) Is present, and in this embodiment, approximately 50 to 70 (pieces / mm) ² If it is less than the following, vertical streak-like unevenness occurs. This is because when the distribution of the conductive particles is biased, the entire charged area cannot be covered, and the lower limit for covering the charged area almost certainly after satisfying the time coefficient α> 15 is 100. (Pcs / mm ² ). Density Nc of conductive particles (pieces / mm ² The upper limit of ()) is a state in which the entire surface of the charging means is covered with conductive particles. Assuming that the conductive particles are spherical and the closest packing state is 2.14 × 10 5 particles of 5 μm. ⁴ (Pcs / mm ² ), 5.34 × 10 with 0.5 μm particles ⁶ (Pcs / mm ² ). Anything greater than this theoretically does not directly touch the photosensitive layer and is outside the scope of the contact model of the present invention.
[0106]
The size of the conductive particles is not particularly limited in terms of the function as a charge injection aid, but if it is 0.1 μm or less, many of the conductive particles are buried inside the charging means or adhere to the developer, and the The state of contact with the carrier does not fit the contact model of the present invention. It is also known that the larger the particle size of the developer, the more easily the toner is detached from the charging means, and if a part of the developer adheres to the image carrier, adverse effects such as obstruction of latent image exposure become significant. Therefore, in the present invention, the particle size of the conductive particles is not less than 0.5 μm and not more than the particle size of the toner as a range that conforms to the contact model of the conductive particles of the present invention and does not cause other adverse effects. Is desirable.
[0107]
In the present invention, the nip width W (mm) between the image carrier and the charging means is an important factor that determines the contact time. There is no upper limit from the viewpoint of the required chargeability of the nip width W (mm). However, when the width of 5.0 mm is secured in Example C, the diameter of the charging roller is 24 mm, which is much larger than the diameter of the charging roller used for charging by conventional discharge. As a method of increasing the nip width W to 5.0 mm or more, it is conceivable to lower the hardness of the charging roller or increase the pressing force against the photoconductor, but the contact pressure inside the nip is large at the center and the end of the nip. However, it is expected that the driving load will be considerably large, and an excessive load is imposed mechanically and mechanically on the device configuration, which is not realistic. Also, as shown in Table 5 of Example D, it was confirmed that streak-like image defects are likely to occur when the charged particles stay in the nip W more than necessary (Tc <0.3 (sec)). Was done. A preferred range of the nip width W is 2.0 to 5.0 (mm).
[0108]
In the present embodiment, the example in which the peripheral surface of the charging unit is rotated in the opposite direction to the peripheral surface of the image carrier is shown. Theoretically, the contact model of the present invention can be applied even when rotated in the same direction, and there is no point in limiting the driving direction of the charging unit and the image carrier in the contact injection charging method using conductive particles.
[0109]
【The invention's effect】
As described above, according to the present invention, the charging conditions of the contact injection charging method using the conductive particles can be verified in advance, and the design adaptation range of each parameter according to various situations is predicted to perform charging. A charging system, an image forming apparatus, and a process unit having high environmental stability and high durability stability can be provided.
[0110]
The present invention is not limited to the above embodiments, and various modifications are possible within the scope of the technical idea of the present invention.
[Brief description of the drawings]
FIG. 1 is a schematic configuration model diagram of an image forming apparatus according to an embodiment.
FIG. 2 is a schematic diagram of a layer configuration of a photosensitive drum used.
FIG. 3 is a diagram illustrating a method for measuring the resistivity of the surface layer of a photosensitive layer.
FIG. 4 is a model explanatory diagram of an injection charging state of the present invention.
FIG. 5 is an equivalent circuit diagram of FIG.
FIG. 6 is an explanatory diagram of a method for calculating the resistance of the photosensitive layer surface layer.
[Explanation of symbols]
1. photoconductor, 2. charging roller, 2a core metal, 2b elastic roller, m conductive particles, 3 developing device, 3a developer (toner), 3b Developing sleeve, 3c magnet, 3d regulating blade

Claims (21)

An image carrier;
A charging member that forms the nip portion with the image carrier, and charges the image carrier;
Conductive particles are provided in the nip portion, the resistivity of the surface of the image carrier is ρ (Ω), the capacitance of the image carrier is Cd (F / mm ² ), The particle diameter is D (mm), the density of the conductive particles on the charging member is Nc (particles / mm ² ), the surface movement speed of the charging member is Vc (mm / sec), and the surface movement of the image carrier is Assuming that the speed is Vd (mm / sec), k = Vc / Vd, and the width of the nip in the image carrier moving direction is W (mm), the time coefficient α expressed by the following equation is α> 15. An image forming apparatus, comprising:
Note [Equation 1]

The image forming apparatus according to claim 1, wherein the resistivity ρ is 1.0 × 10 ^{9 to} 5.0 × 10 ¹¹ (Ω). The image forming apparatus according to claim 1, wherein the density Nc is 1.0 × 10 ^{2 to} 5.0 × 10 ⁶ (pieces / mm ² ). 2. The image forming apparatus according to claim 1, wherein W / Vc is equal to or less than 0.3 (sec). The image forming apparatus according to claim 1, wherein the width W is 2.0 to 5.0 (mm). 2. The image forming apparatus according to claim 1, further comprising a developing unit configured to develop the electrostatic image formed on the image carrier with a developer, and a transfer unit configured to transfer the image of the developer from the image carrier to an image receiving member. 2. The image forming apparatus according to 1. The image forming apparatus according to claim 6, wherein the developer includes a toner, and an average particle diameter of the conductive particles is 0.5 μm or more and a particle diameter of the toner or less. 7. The image forming apparatus according to claim 6, wherein the developing unit can perform an operation of collecting a residual developer on the image carrier at the same time as performing the developing operation. The developer includes the conductive particles, the developing unit supplies the conductive particles to the image carrier, and the conductive particles carried on the image carrier are transported to the charging member. The image forming apparatus according to claim 8, wherein The image forming apparatus according to claim 1, wherein the image carrier has a charge injection layer on a surface thereof. A process cartridge that is detachable from the image forming apparatus main body,
An image carrier;
A charging member that forms the nip portion with the image carrier, and charges the image carrier;
Conductive particles are provided in the nip portion, the resistivity of the surface of the image carrier is ρ (Ω), the capacitance of the image carrier is Cd (F / mm ² ), The particle diameter is D (mm), the density of the conductive particles on the charging member is Nc (particles / mm ² ), the surface movement speed of the charging member is Vc (mm / sec), and the surface movement of the image carrier is Assuming that the speed is Vd (mm / sec), k = Vc / Vd, and the width of the nip in the image carrier moving direction is W (mm), the time coefficient α expressed by the following equation is α> 15. A process cartridge, comprising:
Note [Equation 2]

The process cartridge according to claim 11, wherein the resistivity ρ is 1.0 × 10 ^{9 to} 5.0 × 10 ¹¹ (Ω). The process cartridge according to claim 11, wherein the density Nc is 1.0 × 10 ^{2 to} 5.0 × 10 ⁶ (pieces / mm ² ). The process cartridge according to claim 11, wherein W / Vc is 0.3 (sec) or less. The process cartridge according to claim 11, wherein the width (W) is 2.0 to 5.0 (mm). 2. The image forming apparatus according to claim 1, further comprising a developing unit configured to develop the electrostatic image formed on the image carrier with a developer, and a transfer unit configured to transfer the image of the developer from the image carrier to an image receiving member. 12. The process cartridge according to item 11. 17. The process cartridge according to claim 16, wherein the developer contains a toner, and an average particle size of the conductive particles is 0.5 μm or more and a particle size of the toner or less. 17. The process cartridge according to claim 16, wherein the developing unit can perform an operation of collecting a residual developer on the image carrier at the same time as performing the developing operation. The developer includes the conductive particles, the developing unit supplies the conductive particles to the image carrier, and the conductive particles carried on the image carrier are transported to the charging member. The process cartridge according to claim 18, wherein The process cartridge according to claim 11, wherein the image carrier has a charge injection layer on a surface thereof. An object to be charged;
A charging member that forms the nip portion with the member to be charged, and charges the member to be charged;
Conductive particles are provided in the nip portion, the resistivity of the surface of the member to be charged is ρ (Ω), the capacitance of the member to be charged is Cd (F / mm ² ), The particle diameter of the particles is D (mm), the density of the conductive particles on the charging member is Nc (particles / mm ² ), the surface moving speed of the charging member is Vc (mm / sec), Assuming that the surface moving speed is Vd (mm / sec), k = Vc / Vd, and the width of the nip in the moving direction of the member to be charged is W (mm), the time coefficient α represented by the following equation is α> 15. The charging system according to item 15, wherein
Note [Equation 3]

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