JP2004077915A - Image forming apparatus, transfer method, and transfer condition setting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing the occurrence of transfer dust with high accuracy and obtaining an image having excellent sharpness. <P>SOLUTION: A pressure of 1-10N/cm is applied between a photoreceptor 1 and a transfer roller 52, an elastic layer 52b is formed on the transfer roller 52, and a transfer bias current between the photoreceptor 1 and the transfer roller 52 is set to be 0.05 to 0.2μA/cm. Besides, the hardness of the elastic layer 52b is set to be 60 to 80°, the CTL layer of the OPC photoreceptor 1 is set to be 10 to 20μm and the electrostatic capacity is set to be ≥ 100PF/cm<SP>2</SP>. <P>COPYRIGHT: (C)2004,JPO

Description

【０００９】
領域ＡとＣでの放電を少なくするためには、トナー周りの電界は現状維持のまま、感光体と転写紙間の電位差を小さくすることが必要となる。そのために、転写構成要素の誘電厚みを小さくすることが挙げられる。
また、本発明ではネガ・ポジ方式で作像している。ネガ・ポジ方式の場合、地肌部の感光体電位が高く（例えば−９００Ｖ）、画像部の電位が低い（例えば−１５０Ｖ）方式である。画像部と非画像部（地肌部）で比較すると、転写紙が一様に帯電しているとすれば、地肌部の方の電位差が７５０Ｖ高いことになる。
この電位差は、転写工程後の分離時に画像部より非画像部で放電が発生しやすいことを意味する。分離時に画像部と非画像部の境界部で放電が発生し、画像部のトナーにも影響を与えてトナーチリの原因となっている。
【００１０】
以上のような原因から、改善策として以下のような手法が考えられる。
（ａ）転写紙と感光体の間にあるトナー周りの電界を一定に保ちながら、感光体と転写紙間のエアーギャップを小さくすると、式（２）から、感光体と転写紙間の電位差を小さくすることができる。実際の手段としてはゴム状の表面層を持った転写ローラと感光体間に圧力を加え、転写紙と感光体の密着を多くし実質の平均エアーギャップを小さくすることで達成できる。
（ｂ）感光体の誘電厚みを小さくすることにより感光体と転写紙間の電位差を小さくする。実質、ＯＰＣ感光体（Ｏｒｇａｎｉｃ　Ｐｈｏｔｏ−Ｃｏｎｄｕｃｔｏｒ：有機光感光体）では導電性支持体上に下引き層（ＵＬ）、電荷発生層（ＣＧＬ）及び電荷輸送層（ＣＴＬ）からなっている。ＵＬ層とＣＧＬ層は薄く数ミクロン以下である。最も層の厚いＣＴＬ層を薄くすることで達成できる。
（ｃ）感光体上のトナー層を均一にすることでも平均的なエアーギャップを小さくすることができる。感光体上のトナー像としては、トナーが２層形成されていれば画像濃度は十分に得られる。ベタ画像、ドット画像の感光体上のトナーの面粗さが小さいことが良好な結果となる。
【００１１】
以上のような手法でトナー周りの電界は十分であり、転写紙上の電荷量を少なくできる。これにより、転写後の剥離時に感光体と転写紙間の電位差が小さくなり、放電が起きにくい条件が得られる。
具体的には、上記目的を達成するために、請求項１記載の発明では、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する画像形成装置において、上記像担持体と上記転写手段との間に圧力を加え、トナー周りの電界を維持したまま上記像担持体と上記記録媒体間の電位差を小さくして転写する、という構成を採っている。
【００１２】
請求項２記載の発明では、請求項１記載の画像形成装置において、上記圧力が、１〜１０Ｎ／ｃｍである、という構成を採っている。
【００１３】
請求項３記載の発明では、請求項１又は２記載の画像形成装置において、上記電位差を小さくすることが、上記像担持体と上記転写手段間に弾性層を介在させることである、という構成を採っている。
【００１４】
請求項４記載の発明では、請求項３記載の画像形成装置において、上記転写手段がローラ又は無端状のベルトであり、上記弾性層が該転写手段に一体に形成されている、という構成を採っている。
【００１５】
請求項５記載の発明では、請求項３又は４記載の画像形成装置において、上記弾性層の硬度が６０〜８０度である、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又は２記載の画像形成装置において、上記電位差を小さくすることが、上記像担持体と上記転写手段との間に印加する転写バイアス電流を小さくすることである、という構成を採っている。
【００１７】
請求項７記載の発明では、請求項６記載の画像形成装置において、上記転写バイアス電流が、０．０５〜０．２μＡ／ｃｍである、という構成を採っている。
【００１８】
請求項８記載の発明では、請求項１又は２記載の画像形成装置において、上記電位差を小さくすることが、上記像担持体としてＯＰＣ感光体を用い、その誘電厚みを小さくして静電容量を大きくすることである、という構成を採っている。
【００１９】
請求項９記載の発明では、請求項８記載の画像形成装置において、上記ＯＰＣ感光体のＣＴＬ層が１０〜２０μｍで、静電容量が１００ＰＦ／ｃｍ^２以上である、という構成を採っている。
【００２０】
請求項１０記載の発明では、請求項９記載の画像形成装置において、上記ＣＴＬ層の表面に保護層を設けた、という構成を採っている。
【００２１】
請求項１１記載の発明では、上記保護層の表面の摩擦係数を低下させる処理がなされている、という構成を採っている。
【００２２】
請求項１２記載の発明では、請求項１又は２記載の画像形成装置において、上記電位差を小さくすることが、上記像担持体上のトナー層厚を平均１０〜２０μｍとし、トナー像の表面粗さを０．１ｍｍ×０．１ｍｍ以上の面積でトナー平均層厚以下の値に設定することである、という構成を採っている。
【００２３】
請求項１３記載の発明では、請求項１２記載の画像形成装置において、粒径４．０〜７．０μｍのトナーを用いる、という構成を採っている。
【００２４】
請求項１４記載の発明では、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する画像形成装置において、上記像担持体と上記転写手段との間に１〜１０Ｎ／ｃｍの圧力を加え、上記像担持体と上記転写手段間に弾性層を設け、上記像担持体と上記転写手段間の転写バイアス電流が０．０５〜０．２μＡ／ｃｍである、という構成を採っている。
【００２５】
請求項１５記載の発明では、請求項１４記載の画像形成装置において、上記弾性層の硬度が６０〜８０度である、という構成を採っている。
【００２６】
請求項１６記載の発明では、請求項１４又は１５記載の画像形成装置において、上記像担持体としてＯＰＣ感光体を用い、該ＯＰＣ感光体のＣＴＬ層が１０〜２０μｍであり、静電容量が１００ＰＦ／ｃｍ^２以上である、という構成を採っている。
【００２７】
請求項１７記載の発明では、請求項１６記載の画像形成装置において、上記ＣＴＬ層の表面に保護層を設けた、という構成を採っている。
【００２８】
請求項１８記載の発明では、請求項１７記載の画像形成装置において、上記保護層の表面の摩擦係数を低下させる処理がなされている、という構成を採っている。
【００２９】
請求項１９記載の発明では、請求項１４又は１５記載の画像形成装置において、上記像担持体上のトナー層厚を平均１０〜２０μｍとし、トナー像の表面粗さを０．１ｍｍ×０．１ｍｍ以上の面積でトナー平均層厚以下の値に設定する、という構成を採っている。
【００３０】
請求項２０記載の発明では、請求項１９記載の画像形成装置において、粒径４．０〜７．０μｍのトナーを用いる、という構成を採っている。
【００３１】
請求項２１記載の発明では、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する画像形成装置において、上記像担持体上のトナー層厚を平均１０〜２０μｍとし、トナー像の表面粗さを０．１ｍｍ×０．１ｍｍ以上の面積でトナー平均層厚以下の値に設定する、という構成を採っている。
【００３２】
請求項２２記載の発明では、請求項２１記載の画像形成装置において、粒径４．０〜７．０μｍのトナーを用いる、という構成を採っている。
【００３３】
請求項２３記載の発明では、請求項１乃至２２の何れか１つに記載の画像形成装置において、上記像担持体の表面摩擦係数が０．７以下である、という構成を採っている。
【００３４】
請求項２４記載の発明では、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する転写方法において、上記像担持体と上記転写手段との間に圧力を加え、トナー周りの電界を維持したまま上記像担持体と上記記録媒体間の電位差を小さくして転写することとした。
【００３５】
請求項２５記載の発明では、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する画像形成装置における転写条件の設定方法において、上記像担持体と上記転写手段との間に加える圧力と、上記像担持体と上記転写手段間の転写バイアス電流をパラメータとし、転写率、転写チリ、転写白抜けの評価実験を行い、これらの実験値から許容レベルを抽出して転写条件とすることとした。
【００３６】
【発明の実施の形態】
以下、本発明の実施形態を図１乃至図７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の全体構成の概要を説明する。画像形成装置は周知の電子写真方式を用いており、内部に像担持体としてのドラム状の感光体を備えている。感光体１の周囲には矢印で示す回転方向に沿って、電子写真複写工程を実施する帯電手段２、露光手段３、現像手段４、転写搬送手段５、クリーニング手段６及び定着手段７が配置されている。
露光手段３は、画像読み取り装置３１により読み取られた原稿の画像信号を基に、ポリゴンミラー３２でレーザー光をスキャンさせミラー３３で反射して感光体１上に静電潜像を形成する。感光体１としては有機感光体の他、アモルファスシリコーン等の既存の感光体を用いることができる。
【００３７】
感光体１上に形成された静電潜像は、現像ローラ４１を有する現像手段４によってトナー像として可視像化される。給紙トレイ１０１、１０６に収容された記録媒体としての転写紙Ｐは給紙ローラ１０２、１０７で給紙され、搬送ローラ対１０３、１０８によりレジストローラ対１０４に向けて搬送される。
転写紙Ｐはレジストローラ対１０４で一旦停止されて斜めずれを修正された後、感光体１上に形成されたトナー像の先端と搬送方向の所定位置とが一致するタイミングでレジストローラ対１０４により転写部位へ向けて搬送される。
転写部位へ送られた転写紙Ｐは、転写搬送手段５における転写手段としての転写ローラ５２と感光体１との間に印加される転写バイアスにより、トナー像を静電転写される。トナー像を転写された転写紙Ｐは転写搬送手段５の搬送ベルト５３により定着手段７へ搬送され、定着される。定着が終了した転写紙Ｐは排紙ローラ対１０５により排紙トレイ１１０へ排出される。
【００３８】
転写後、感光体１上に残留したトナーや汚れはクリーニング手段６により清掃される。クリーニング手段６は、クリーニングブレード６１と、クリーニングブラシ６２と、摩擦係数低減剤６３を有している。クリーニング後の感光体１は図示しない除電ランプにより表面電位を初期化され、次の作像工程に備えられる。転写ローラ５２は、金属ローラ５２ａと、該金属ローラ５２ａの表面に形成された弾性層を有している。搬送ベルト５３は駆動ローラ５４と従動ローラ５５により回転駆動され、その表面をベルトクリーニング手段５６によりクリーニングされる。
定着手段７は、内部にハロゲンランプ等の加熱源７４を備えた定着ローラ７１と、同じく内部にハロゲンランプ等の加熱源７３を備えた加圧ローラ７２を有している。定着ローラ７１と加圧ローラ７２は、面圧が９．３Ｎ／ｃｍ^２の加圧力で圧接されて約１０ｍｍの定着ニップ幅を形成している。定着手段７は図示しない駆動手段により駆動力を受けて転写紙Ｐを挟持搬送する。この際、定着ローラ７１は加熱源７４によって所定の温度に制御されており、転写紙Ｐ上のトナー像は両ローラ間を通過するときに圧力を受けながら溶融し、ニップ部を出て冷却されることによって永久像として転写紙Ｐに定着される。
【００３９】
次に、図２に基づいて転写ローラ５２の構成と加圧構造を説明する。転写ローラ５２は、ステンレス（ＳＵＳ）や鉄（Ｆｅ）等からなる直径２０〜３０ｍｍの芯金としての金属ローラ５２ａと、この金属ローラ５２ａの表面に形成された弾性層５２ｂを有している。弾性層５２ｂは、ＥＰＤＭ、シリコーン、ＮＢＲ、ウレタン等の材料でソリッド状に形成されている。
弾性層５２ｂは、０．１〜１．０ｍｍの厚みで硬度６０〜８０度（ＡｓｋｅｒＣ／１ｋｇ荷重時）、体積抵抗１×１０^８〜１×１０^１１Ωｃｍに設定されている。弾性層５２ｂの表面抵抗は体積抵抗より１〜２桁高い抵抗の範囲が最適である。
転写ローラ５２の加圧は、ローラの両端に設けられた軸受５２ｃとバネ５２ｄによりなされ、感光体１に押圧されている。以降、加圧力の測定は転写ローラ５２前後のバネ押圧力の合計を転写ローラ５２の長さで割った値で表示する。単位はＮ／ｃｍで示す。
【００４０】
次に、図３に基づいて感光体１と転写ローラ５２間の加圧時のトナー周りの空隙について説明する。
図３（ａ）は、従来における圧力１Ｎ／ｃｍ以下の転写部位における加圧状態を示す図である。転写紙Ｐは微小面積では硬く、広い面積では変形（曲がり）する特性を有している。従来のように圧力が弱いと、転写紙Ｐの繊維の凹凸で表面、裏面における数点の凸部で受けて（接触して）搬送される。
その結果、トナーＴや感光体１と転写紙Ｐの接触点が少なく、感光体１と転写ローラ５２の表面の間の空隙が多い状態となる。
感光体とゴム層（弾性層５２ｂ）の無いローラ間に０．５Ｎ／ｃｍの加圧力を加え、転写紙Ｐ（リコー製Ｔｙｐｅ６２００）を挟み、ギャップを測定した。また、１．０Ｎ／ｃｍの加圧で同様に測定した結果、２０μｍの差があった。つまり、加圧によって空隙が２０μｍ減ったことになる。
【００４１】
その様子、すなわち本実施形態における加圧状態を図３（ｂ）に示す。加圧によって転写紙Ｐの凸部と感光体１、転写ローラ５２が多く接触するようになる。その結果、空隙が小さくなり、トナーＴと転写紙Ｐ間の距離が平均的に狭くなり、式（２）の分母項の「ｇ」が小さくなる。さらにトナー層Ｔの厚みが一定でトナー表面の粗さも均一であれば、平均の空隙が小さくなる。
次に、感光体１の感光層の厚みについて説明する。ＯＰＣ感光体１は、一般的に金属支持体上にＵＬ層、ＣＧＬ層、さらにＣＴＬ層が塗布され、感光層全体の厚みは２５〜３０μｍとなっている。これらの層のうちではＣＴＬ層が最も厚く２０〜３０μｍとなっている。理由としては耐久性を考慮したもので、ＣＴＬ層を削りながら常にトナー皮膜などがない感光体表面を使っている。このときの静電容量は約８０ＰＦ／ｃｍ^２である。
【００４２】
本実施形態では、ＣＴＬ層を約１０μｍ薄くする。これにより、感光体１の感光層の誘電厚みは小さくなる。
以上のように、空隙と感光体１の誘電厚みを小さくすることで、転写紙Ｐと感光体１の電位差を小さくしても、式（２）のＥ（トナー周りの電界）は従来と同じ電界が得られる。すなわち、トナー周りの電界を維持したまま分離時の放電による転写チリを減少させることができる。
【００４３】
次に、本実施形態で用いられる感光体１の表面特性について説明する。感光体１の表面摩擦係数は０．７以下であることが望ましい。表面摩擦係数が０．７以上の場合、転写の際、トナーとの離型性が悪くなり、転写効率が悪化する。特に本実施形態では、凝集力の低いトナー（非重合トナー）を用いるが、転写での圧力が大きいため、転写時の転写白抜け（トナーのまとまった量が感光体１から分離せず抜け跡が生じる現象）が発生しやすい。そのため、表面摩擦係数を低くする必要がある。
感光体１の表面摩擦係数を低くする方法としては、例えばステアリン酸亜鉛、ステアリン酸カルシウムなどの脂肪酸金属塩を感光体１の表面に均一に塗布することが挙げられる。
表面摩擦係数の測定には、全自動摩擦摩耗解析装置（協和界面科学（株）社製）を用い、接触子として３ｍｍのステンレス球を用いた。
【００４４】
本実施形態では、図４に示すように、感光体１の表面、すなわち厚みを薄くしたＣＴＬ層１ａ―３の表面に金属酸化物を含有した保護層１ｃを設けている。図４において符号１ａは感光層を、１ａ―１はＵＬ層を、１ａ―２はＣＧＬ層を、１ｂは金属支持体をそれぞれ示している。
保護層１ｃを有する感光体１を用いた場合、機械的強度が強いため、感光体１の膜削れが少なく、経時で安定した画像を得ることができる。保護層１ｃ中に含有する金属酸化物としては、アルミナ、酸化チタン、シリカの中から選ばれる一種であることが望ましい。
【００４５】
次に、本実施形態で用いられるトナーについて説明する。製造方法は、従来公知の方法でよく、結着樹脂、離型剤、着色剤、帯電制御剤等をミキサーで混合し、熱ロール、エクストレーダー等の混練機で混練した後、冷却固化し、これをジェットミル、ターボジェット、クリプトロン等の粉砕装置で粉砕し、その後分級して得られるトナーである。
さらに、上記トナーに無機微粉末を添加し、スーパーミキサー、ヘンシェルミキサー等で混合して得られるトナーでもよい。
感光体１上のトナー層Ｔの層厚が均一で、さらにトナー層Ｔの表面粗さが均一であれば、感光体１と転写紙Ｐ間のトナー周りの電界が均一になり、トナーの移動する力（Ｆ）は均一になる。さらにトナーの帯電量が均一であることが転写チリには良好な条件となる。
【００４６】
次に、本実施形態における評価方法について説明する。
＜転写率、定着性、転写チリ、転写白抜けの評価方法＞
評価機は、リコーＩｍａｇｉｏＭＦ７０７０の転写部を改造して用いた。ユニット構成は図１の装置の概略図と同じである。現像として二成分方式の現像装置を用い、転写はローラ転写方式で行い、ベルト搬送される構成である。定着は面圧が９．３Ｎ／ｃｍの加圧力で、温度が１６５〜１８５℃の条件下で行った。この装置を用いて、６００ｄｐｉのドットから形成されたグレースケールを中心としたテストチャート（図５参照）をプリントアウトしてサンプル画像を得た。
【００４７】
＜転写率の評価＞
現像された感光体１上のチャートを転写し、転写紙Ｐが搬送ベルト５３上にある時に装置を停止する。チャートの黒ベタ部に着目し、感光体１上の黒ベタ部の転写残トナー量を粘着テープで剥がし、感光体１上の残トナー量を求める。一方、転写されたトナーは、黒ベタ部を切り取り、トナーを圧縮エアーで吹き飛ばす。吹き飛ばし前後の重さより転写されたトナー量を求め、下記式
（転写トナー量／（転写トナー＋残トナー量））×１００　　（％）
で転写率（％）を求める。転写率の許容値は一般環境下で７０％以上である。転写率８０％以上は「○」と判定する。同じく７０〜７９％は「△」、６９％以下は「×」と示した。許容レベルは「△」以上である。
【００４８】
＜定着性の評価＞
次に、定着性はスミア法で行う。ＩＤ：０．６〜０．８のハーフトーン部で８．８Ｎ／１５φの重りに付着した布を転写紙Ｐの上に載せ、５往復擦ったときの布上の濃度で評価を行った。
問題無いレベルである０．３以下は「○」、許容できるレベルである０．５以下は「△」で表示し、０．５１以上のレベルは「×」と評価した。
【００４９】
＜転写チリ、転写白抜けの評価＞
転写チリと転写白抜けは、汎用的な評価法が確立していないため、サンプルとランク見本を目視での官能評価法で行った。転写チリのランク見本は図７に、転写白抜けのランク見本は図６に示した。ランク３の画像の「△」が許容レベルであり、それ以上は「ＯＫ」、「△」に満たないものは「ＮＧ」である。
【００５０】
以下、実施例により本発明を説明する。
［実施例１］
図１と同様のテスト機はリコーイマジオＭＦ７０７０の転写部を改造して用いた。感光体１と転写ローラ５２の接触が安定するように、転写ローラ５２の表面に硬度７０度（ＡｓｋｅｒＣ／１ｋｇ荷重時）、１×１０^９〜１×１０^１０Ωｃｍのゴム層（弾性層）を設けた。図２で示したバネ５２ｄによる転写圧力（パラメータ）を０．５、１．０、５．０、１０．０Ｎ／ｃｍの４水準、転写ローラ５２と感光体１間に印加する電圧を制御して、流れる電流（パラメータ）を０．０３、０．０５、０．２、０．３μＡ／ｃｍの４水準でプリンタにより図５に示すテストチャートをプリントした。
評価は転写率、転写チリ（図７参照）、転写白抜け（図６参照）で確認した。それぞれの結果を表１、表２、表３に示す。なお、現状値、すなわち従来値は、転写電流が０．３〜０．４μＡ／ｃｍで、圧力は１Ｎ／ｃｍ前後の条件である。
【００５１】
【表１】

【００５２】
【表２】

【００５３】
【表３】

【００５４】
表１、２、３から、転写率は０．０５μＡ未満では電流不足で不良である。圧力が小さく、電流も小さいと転写率が低下する。転写チリでは、転写電流が０．３μＡ以上で放電によるチリが大きくなる。また、転写圧力が１０Ｎ／ｃｍでは転写白抜けが発生しかかっており、その結果転写率でも低下している。
以上の結果、最適条件として、圧力で１〜１０Ｎ／ｃｍ、転写バイアス電流で０．０５〜０．２μＡ／ｃｍの組み合わせを選択できる。好ましくは圧力で１．０〜５．０Ｎ／ｃｍ、電流で０．１〜０．２μＡ／ｃｍであった。
【００５５】
［実施例２］
実施例１と同じ方法で感光体１のＣＴＬ層１ａ―３の厚みを５μｍ、１０μｍ、１５μｍ、２０μｍ、２５μｍの５水準で変化させ、転写圧力は３．０Ｎ／ｃｍ、転写電流は０．３μＡ／ｃｍで図５のテストチャートをプリントして転写チリについて評価を行った。その結果を表４に示す。
【００５６】
【表４】

【００５７】
表４から、ＣＴＬ層１ａ―３の厚さが薄くなると、リークによる黒ポチが発生することが判る。また、厚みが２０μｍ以上になると、感光体１の静電容量が小さくなり（８０ＰＦ／ｃｍ^２以下）、感光体１の帯電量が大きくなって転写チリが悪くなる。
以上の結果より、感光体１のＣＴＬ層１ａ―３の厚みは１０〜２０μｍで、静電容量は１００ＰＦ／ｃｍ^２以上が良好な条件となる。
実施例１と２の組み合わせで感光体１と転写紙Ｐの電位差を求めると以下のようになる。式（２）から誘電厚みの変動分を計算すると、従来例では転写紙ｔ：８０μｍ、ε：４、トナー層ｔ：１０μｍ、ε：３、感光体ｔ：３０μｍ、ε：３、空隙ｔ：４０μｍ、となる。
これに対し、本実施形態の最適条件では、加圧と感光体厚みを感光体ｔ：２０μｍ、ε：３、空隙２０μｍとすると、誘電厚みは６８％になり、トナー周りの電界を同じ値に維持した場合、感光体１と転写ローラ５２間の電位差を３２％小さくできることになる。その結果、分離時の放電が少なくなり、転写チリを減少できた。
【００５８】
［実施例３］
実施例１と同じ方法で転写圧力は３．０Ｎ／ｃｍ、転写電流は０．１５μＡ／ｃｍで図５のテストチャートをプリントし、サンプルを得た。感光体１のＣＴＬ層１ａ―３を２０μｍとし、表面に保護層１ｃを設けた。
具体的には、溶剤に溶かしたポリカーボネート樹脂に粒径０．３μｍの酸化アルミニウムを２５ｗ％加え、分散してスプレー塗布し、１５０℃で加熱乾燥して５μｍの保護層１ｃを形成した。
保護層有無の２種類の感光体で連続コピーテストを行った。その結果を表５に示す。
【００５９】
【表５】

【００６０】
表５より、保護層１ｃが無い場合、２００Ｋ（Ｋ＝１０００枚）から黒ポチが発生した。原因としてクリーニング工程でのブラシとウレタンゴムのクリーニングブレードで削れ、１５μｍ以下になると電界強度が強くなり、部分的にリークが発生し、黒ポチ画像なる。
保護層１ｃを設けることにより、感光体１の削れが少なく、３００〜５００Ｋ枚の耐久性が得られる。保護層１ｃの無い場合は、厚みが２５μｍのＣＴＬ層１ａ―３を塗布した感光体で、２００〜３００Ｋの寿命で画像劣化が発生する。テストの結果、３００Ｋで１０μｍの削れが発生した。
【００６１】
［実施例４］
実施例１と同じ方法で転写圧力は３．０Ｎ／ｃｍ、転写電流は０．１５μＡ／ｃｍで図５のテストチャートをプリントし、サンプルを得た。現像に用いる現像剤を次のように作成した。
トナー処方
ポリエステル樹脂（重量平均分子量：１８５０００、Ｔｇ：６７℃）　　８０重量部
カルナウバワックス（平均粒径：３００μｍ）　　　　　　　　　　　４重量部
カーボンブラック（♯４４：三菱化学）　　　　　　　　　　　　１５重量部
荷電制御剤（スピロンブラックＴＲ−Ｈ：保土ヶ谷化学）　　　　　１重量部
以上の処方で２軸エクストルーダーを用いて１６０℃で混練後、機械式粉砕機により粉砕条件を変化させて粉砕し、分級して重量平均粒径４．２μｍ、６．８μｍ、９．０μｍのトナーを得た。さらにヘンシェルミキサーを用い、シリカ（Ｒ−９７２　日本アエロジル）１．０重量％を混合し、３種類の粒径の異なるトナーを得た。
【００６２】
キャリアは重量平均粒径５０μｍの球形フェライトを用い、表面にシリコーン樹脂をコートし、熱乾燥して得た。
４．２μｍトナーはキャリアに対し５．０ｗ％、６．８μｍトナーはキャリアに対し４．０ｗ％、９．０μｍトナーはキャリアに対し３．０ｗ％を混合し、３種の現像剤を得た。
以上の現像剤を用いて、画像濃度と転写チリを確認した。同時に感光体１上に形成されたトナー像のＺ軸方向の平均高さ、０．１×０．１ｍｍの面積の表面粗さを測定して併記した。感光体１上のトナー像のＺ軸のデータは、キーエンス社製の超深度形状測定顕微鏡ＶＫ８５００を用い、対物レンズ５０倍（１５インチのＣＲＴ上倍率１０００倍）で、断面高さの平均値と面粗さ測定値を用いた。その結果を表６に示す。
【００６３】
【表６】

【００６４】
表６より、トナーの平均高さはほぼ同じであるが、表面粗さが均一であると転写チリが良いことが判る。これはトナーの層構成が不均一である場合、転写時、転写分離時にトナーが異方向に転移する確率が高くなるからであると考えられる。
画像濃度は付着量がほぼ同じであるため、定着後の画像濃度はほぼ同じになっている。
【００６５】
上記実施形態では転写手段として転写ローラを例示したが、無端状のベルトであっても同様の機能を得ることができる。
また、上記実施形態では、全ての条件を同時に備えた構成で説明したが、像担持体と記録媒体間の電位差を小さくするための各条件等を単独に設定した構成においても従来に比べて転写チリを低減できる。
【００６６】
【発明の効果】
請求項１記載の発明によれば、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する画像形成装置において、上記像担持体と上記転写手段との間に圧力を加え、トナー周りの電界を維持したまま上記像担持体と上記記録媒体間の電位差を小さくして転写する構成としたので、像担持体と記録媒体との接触を多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００６７】
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、上記圧力が、１〜１０Ｎ／ｃｍである構成としたので、像担持体と記録媒体との接触を多くして転写効率を確実に上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００６８】
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、上記電位差を小さくすることが、上記像担持体と上記転写手段間に弾性層を介在させることである構成としたので、像担持体と記録媒体との接触を弾性層の変形により多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００６９】
請求項４記載の発明によれば、請求項３記載の画像形成装置において、上記転写手段がローラ又は無端状のベルトであり、上記弾性層が該転写手段に一体に形成されている構成としたので、構成の簡易化を実現できる。
【００７０】
請求項５記載の発明によれば、請求項３又は４記載の画像形成装置において、上記弾性層の硬度が６０〜８０度である構成としたので、像担持体と記録媒体との接触を弾性層の最適な変形により多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００７１】
請求項６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、上記電位差を小さくすることが、上記像担持体と上記転写手段との間に印加する転写バイアス電流を小さくすることである構成としたので、記録媒体の電荷を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００７２】
請求項７記載の発明によれば、請求項６記載の画像形成装置において、上記転写バイアス電流が、０．０５〜０．２μＡ／ｃｍである構成としたので、転写チリを高精度に低減することができる。
【００７３】
請求項８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、上記電位差を小さくすることが、上記像担持体としてＯＰＣ感光体を用い、その誘電厚みを小さくして静電容量を大きくすることである構成としたので、像担持体の帯電電位を下げることができ、像担持体と転写手段間の電位差を減少させることによって転写チリを低減することができる。
【００７４】
請求項９記載の発明によれば、請求項８記載の画像形成装置において、上記ＯＰＣ感光体のＣＴＬ層が１０〜２０μｍで、静電容量が１００ＰＦ／ｃｍ^２以上である構成としたので、転写チリを高精度に低減することができる。
【００７５】
請求項１０記載の発明によれば、請求項９記載の画像形成装置において、上記ＣＴＬ層の表面に保護層を設けた構成としたので、転写チリを低減できるとともに像担持体の長寿命化を図ることができる。
【００７６】
請求項１１記載の発明によれば、上記保護層の表面の摩擦係数を低下させる処理がなされている構成としたので、転写圧力が大きくても転写白抜けを防止することができる。
【００７７】
請求項１２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、上記電位差を小さくすることが、上記像担持体上のトナー層厚を平均１０〜２０μｍとし、トナー像の表面粗さを０．１ｍｍ×０．１ｍｍ以上の面積でトナー平均層厚以下の値に設定することである構成としたので、エアーギャップが小さくなることによって像担持体と記録媒体との接触を多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００７８】
請求項１３記載の発明によれば、請求項１２記載の画像形成装置において、粒径４．０〜７．０μｍのトナーを用いる構成としたので、トナー層の厚みのさらなる均一化によって転写チリを高精度に低減することができる。
【００７９】
請求項１４記載の発明によれば、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する画像形成装置において、上記像担持体と上記転写手段との間に１〜１０Ｎ／ｃｍの圧力を加え、上記像担持体と上記転写手段間に弾性層を設け、上記像担持体と上記転写手段間の転写バイアス電流が０．０５〜０．２μＡ／ｃｍである構成としたので、像担持体と記録媒体との接触を多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００８０】
請求項１５記載の発明によれば、請求項１４記載の画像形成装置において、上記弾性層の硬度が６０〜８０度である構成としたので、像担持体と記録媒体との接触を弾性層の最適な変形により多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００８１】
請求項１６記載の発明によれば、請求項１４又は１５記載の画像形成装置において、上記像担持体としてＯＰＣ感光体を用い、該ＯＰＣ感光体のＣＴＬ層が１０〜２０μｍであり、静電容量が１００ＰＦ／ｃｍ^２以上である構成としたので、転写チリを高精度に低減することができる。
【００８２】
請求項１７記載の発明によれば、請求項１６記載の画像形成装置において、上記ＣＴＬ層の表面に保護層を設けた構成としたので、転写チリを低減できるとともに像担持体の長寿命化を図ることができる。
【００８３】
請求項１８記載の発明によれば、請求項１７記載の画像形成装置において、上記保護層の表面の摩擦係数を低下させる処理がなされている構成としたので、転写圧力が大きくても転写白抜けを防止することができる。
【００８４】
請求項１９記載の発明によれば、請求項１４又は１５記載の画像形成装置において、上記像担持体上のトナー層厚を平均１０〜２０μｍとし、トナー像の表面粗さを０．１ｍｍ×０．１ｍｍ以上の面積でトナー平均層厚以下の値に設定する構成としたので、エアーギャップが小さくなることによって像担持体と記録媒体との接触を多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００８５】
請求項２０記載の発明によれば、請求項１９記載の画像形成装置において、粒径４．０〜７．０μｍのトナーを用いる構成としたので、トナー層の厚みのさらなる均一化によって転写チリを高精度に低減することができる。
【００８６】
請求項２１記載の発明によれば、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する画像形成装置において、上記像担持体上のトナー層厚を平均１０〜２０μｍとし、トナー像の表面粗さを０．１ｍｍ×０．１ｍｍ以上の面積でトナー平均層厚以下の値に設定する構成としたので、エアーギャップが小さくなることによって像担持体と記録媒体との接触を多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００８７】
請求項２２記載の発明によれば、請求項２１記載の画像形成装置において、粒径４．０〜７．０μｍのトナーを用いる構成としたので、トナー層の厚みのさらなる均一化によって転写チリを高精度に低減することができる。
【００８８】
請求項２３記載の発明によれば、請求項１乃至２２の何れか１つに記載の画像形成装置において、上記像担持体の表面摩擦係数が０．７以下である構成としたので、転写圧力が大きくても転写白抜けを防止することができる。
【００８９】
請求項２４記載の発明によれば、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する転写方法において、上記像担持体と上記転写手段との間に圧力を加え、トナー周りの電界を維持したまま上記像担持体と上記記録媒体間の電位差を小さくして転写することとしたので、像担持体と記録媒体との接触を多くして転写効率を上げることができ、結果として転写電流を小さくでき、分離時の放電を減少させることによって転写チリを低減することができる。
【００９０】
請求項２５記載の発明によれば、像担持体上のトナー像を転写手段により記録媒体上に静電的に転写する画像形成装置における転写条件の設定方法において、上記像担持体と上記転写手段との間に加える圧力と、上記像担持体と上記転写手段間の転写バイアス電流をパラメータとし、転写率、転写チリ、転写白抜けの評価実験を行い、これらの実験値から許容レベルを抽出して転写条件とすることとしたので、転写条件を最適化することができ、転写チリを高精度に低減することができる。
【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の概要正面図である。
【図２】転写加圧構成を示す要部正面図である。
【図３】（ａ）は従来における転写加圧状態を示す模式図、（ｂ）は本発明における転写加圧状態を示す模式図である。
【図４】感光体の感光層の模式断面図である。
【図５】テストチャートを示す図である。
【図６】転写白抜けランクの見本を示す図である。
【図７】転写チリランクの見本を示す図である。
【図８】転写チリの発生メカニズムを示す模式図である。
【符号の説明】
１　像担持体としての感光体
１ｃ　保護層
５２　転写手段としての転写ローラ
５２ｂ　弾性層
Ｐ　記録媒体としての転写紙
Ｔ　トナー[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, etc., a transfer method in the image forming apparatus, and a method of setting transfer conditions.
[0002]
[Prior art]
In recent years, high image quality has also been required in an electrophotographic color image forming method, and improvements have been made by reducing the diameter of toner and improving development. However, when observed in each image forming process, it is confirmed that the image is deteriorated in each of the latent image, development, intermediate transfer, and fixing. In particular, the deterioration in the transfer step is large.
As a phenomenon, so-called transfer dust due to discharge at the time of pre-transfer at the time of transfer entry and at the time of peeling after transfer has become a major problem. Transfer dust means that at the time of primary transfer, the toner image formed on the photoreceptor is not transferred to the position where it should be originally transferred, but is diffused and transferred to its periphery, resulting in blurred image This is a phenomenon that impairs the sharpness of an image particularly at a thin line portion.
[0003]
Many proposals have been made for these issues. For example, in Japanese Patent Application Laid-Open No. 2000-22188, a pressing roller that presses the intermediate transfer belt from the inner peripheral surface toward the photosensitive drum is provided at least in a contact area of the intermediate transfer belt with the photosensitive drum in the lateral direction of the belt. , Improves the adhesion between the photosensitive drum and the intermediate transfer belt, increases the cohesive force of the toner at the contact nip, and prevents partial transfer failure and transfer dust due to insufficient adhesion between the photosensitive drum and the intermediate transfer belt It has been proposed to.
Japanese Patent Application Laid-Open No. 2000-209255 discloses a transfer process in which a contact pressure of 15 g / cm or more is used as a transfer process in order to solve the transfer dust and bug-like transfer failure in the transfer from the photosensitive drum to the intermediate transfer belt in the color image forming method. , The linear velocity ratio between the intermediate transfer belt and the photosensitive drum is set to 0.85 to 1.10, the surface / volume resistance and the like are defined as the characteristics of the intermediate transfer belt, and the agglomeration degree and bulk are defined as the toner characteristics. It has been proposed to regulate the density and the particle size and optimize the adhesion between toners, the transfer gap, the charge retention of the intermediate transfer belt, and the like.
[0004]
[Problems to be solved by the invention]
In recent years, image quality has been further improved, and polymerized toner has been used as toner. This makes it easier to obtain toner characteristics such as a smaller diameter, a spherical shape, a uniform shape, and a uniform charge amount, leading to an improvement in image quality.
The polymerized toner is referred to as an emulsion polymerization method toner or an EA (Emulsion Aggregation) toner, and is obtained by a method of chemically aggregating and coalescing a fine particle resin of an emulsion polymerized product, a pigment and a wax to produce a toner. Toner. The feature of this production method is that a highly uniform spherical toner having a small particle size of several microns can be formed.
In the above-mentioned prior art, when the polymerized toner is used, the transfer dust tends to be large, and although the appearance is smooth at first glance, the image has poor sharpness and lacks sharpness.
[0005]
Therefore, the present invention has an object to provide an image forming apparatus, a transfer method, and a method for setting transfer conditions, which can prevent transfer dust with high accuracy and can obtain an image with excellent sharpness. I do.
[0006]
[Means for Solving the Problems]
First, the generation mechanism of transfer dust as a problem in the transfer step will be described with reference to FIG. In the transfer step, transfer dust occurs in areas A and C where the transfer paper P as a recording medium and the photoconductor 1 as an image carrier are not in contact with each other before and after the transfer area B. In FIG. 8, reference numeral 1a denotes a photosensitive layer, T denotes a toner or a toner layer, and 80 denotes a transfer roller as transfer means.
In the area A, the toner T flies from the photoreceptor 1 onto the transfer paper P due to the electric charge and the electric field on the transfer paper P before the optimal transfer area, causing dust. In the area C, the transfer paper P charged by the transfer is electrostatically attached to the photoconductor 1. Discharge occurs when separating after transfer.
[0007]
As described above, it is considered that the main cause of the transfer dust is a discharge phenomenon when approached.
The toner between the image carrier and the transfer paper is transferred (transferred) by the force represented by the equation (1). The electric field around the toner is expressed by equation (2), and the potential difference between the transfer paper and the photoconductor is divided by the dielectric thickness of the transfer paper, the dielectric thickness of the toner layer, the dielectric thickness of the photoconductor layer, and the gap around the toner. Is represented by
Transferring force of toner F = qE (1)
q: charge amount of toner
E: Electric field between photoconductor and transfer paper (around toner)
[0008]
(Equation 1)

[0009]
In order to reduce the discharge in the areas A and C, it is necessary to reduce the potential difference between the photoconductor and the transfer paper while maintaining the electric field around the toner as it is. For this purpose, the dielectric thickness of the transfer component may be reduced.
In the present invention, an image is formed by a negative / positive method. In the case of the negative-positive method, the potential of the photoconductor in the background is high (for example, -900 V) and the potential of the image portion is low (for example, -150 V). Comparing the image portion and the non-image portion (background portion), if the transfer paper is uniformly charged, the potential difference in the background portion is 750 V higher.
This potential difference means that discharge is more likely to occur in the non-image area than in the image area during separation after the transfer step. At the time of separation, discharge occurs at the boundary between the image portion and the non-image portion, which also affects the toner in the image portion and causes toner dust.
[0010]
From the above reasons, the following methods can be considered as improvement measures.
(A) When the air gap between the photoconductor and the transfer paper is reduced while the electric field around the toner between the transfer paper and the photoconductor is kept constant, the potential difference between the photoconductor and the transfer paper can be calculated from the equation (2). Can be smaller. As an actual means, it can be achieved by applying pressure between the transfer roller having a rubber-like surface layer and the photoreceptor to increase the close contact between the transfer paper and the photoreceptor and to reduce the actual average air gap.
(B) The potential difference between the photoconductor and the transfer paper is reduced by reducing the dielectric thickness of the photoconductor. The OPC photoreceptor (Organic Photo-Conductor: organic photoreceptor) substantially comprises an undercoat layer (UL), a charge generation layer (CGL), and a charge transport layer (CTL) on a conductive support. The UL and CGL layers are thin and less than a few microns. This can be achieved by thinning the thickest CTL layer.
(C) Even if the toner layer on the photoreceptor is made uniform, the average air gap can be reduced. As the toner image on the photoreceptor, sufficient image density can be obtained if two layers of toner are formed. Good results are obtained when the surface roughness of the toner on the photoconductor of the solid image and the dot image is small.
[0011]
With the above method, the electric field around the toner is sufficient, and the charge amount on the transfer paper can be reduced. As a result, the potential difference between the photoconductor and the transfer paper at the time of peeling after transfer is reduced, and conditions are obtained in which discharge is less likely to occur.
Specifically, in order to achieve the above object, according to the first aspect of the present invention, there is provided an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit. Pressure is applied between the image bearing member and the transfer means, and the transfer is performed with a small potential difference between the image carrier and the recording medium while maintaining the electric field around the toner.
[0012]
According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the pressure is 1 to 10 N / cm.
[0013]
According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, reducing the potential difference includes interposing an elastic layer between the image carrier and the transfer unit. I am taking it.
[0014]
According to a fourth aspect of the present invention, in the image forming apparatus of the third aspect, the transfer unit is a roller or an endless belt, and the elastic layer is formed integrally with the transfer unit. ing.
[0015]
According to a fifth aspect of the present invention, in the image forming apparatus of the third or fourth aspect, the elastic layer has a hardness of 60 to 80 degrees.
[0016]
According to a sixth aspect of the present invention, in the image forming apparatus according to the first or second aspect, the potential difference is reduced by reducing a transfer bias current applied between the image carrier and the transfer unit. There is a configuration that there is.
[0017]
According to a seventh aspect of the present invention, in the image forming apparatus of the sixth aspect, the transfer bias current is 0.05 to 0.2 μA / cm.
[0018]
According to an eighth aspect of the present invention, in the image forming apparatus according to the first or second aspect, the potential difference is reduced by using an OPC photosensitive member as the image carrier, reducing a dielectric thickness of the OPC photosensitive member, and reducing a capacitance. This is to increase the size.
[0019]
According to a ninth aspect of the present invention, in the image forming apparatus of the eighth aspect, the CTL layer of the OPC photoreceptor has a thickness of 10 to 20 μm and a capacitance of 100 PF / cm. ² That is the configuration of the above.
[0020]
According to a tenth aspect of the present invention, in the image forming apparatus according to the ninth aspect, a configuration is adopted in which a protective layer is provided on a surface of the CTL layer.
[0021]
An eleventh aspect of the present invention employs a configuration in which a treatment is performed to reduce the friction coefficient of the surface of the protective layer.
[0022]
In the twelfth aspect of the present invention, in the image forming apparatus according to the first or second aspect, reducing the potential difference is performed by setting an average thickness of the toner layer on the image carrier to 10 to 20 μm and a surface roughness of the toner image. Is set to a value equal to or less than the average toner layer thickness in an area of 0.1 mm × 0.1 mm or more.
[0023]
According to a thirteenth aspect of the present invention, in the image forming apparatus according to the twelfth aspect, a configuration is adopted in which a toner having a particle size of 4.0 to 7.0 μm is used.
[0024]
According to a fourteenth aspect of the present invention, in the image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, 1 to 10 N / cm is provided between the image carrier and the transfer unit. cm pressure, an elastic layer is provided between the image carrier and the transfer means, and a transfer bias current between the image carrier and the transfer means is 0.05 to 0.2 μA / cm. I am taking it.
[0025]
According to a fifteenth aspect of the present invention, in the image forming apparatus of the fourteenth aspect, the elastic layer has a hardness of 60 to 80 degrees.
[0026]
According to a sixteenth aspect of the present invention, in the image forming apparatus according to the fourteenth or fifteenth aspect, an OPC photoconductor is used as the image carrier, the CTL layer of the OPC photoconductor has a thickness of 10 to 20 μm, and a capacitance of 100 PF. / Cm ² That is the configuration of the above.
[0027]
According to a seventeenth aspect of the present invention, in the image forming apparatus according to the sixteenth aspect, a protective layer is provided on a surface of the CTL layer.
[0028]
According to an eighteenth aspect of the present invention, in the image forming apparatus according to the seventeenth aspect, a configuration is adopted in which a process for reducing a friction coefficient of a surface of the protective layer is performed.
[0029]
According to a nineteenth aspect of the present invention, in the image forming apparatus according to the fourteenth or fifteenth aspect, an average thickness of the toner layer on the image carrier is 10 to 20 μm, and a surface roughness of the toner image is 0.1 mm × 0.1 mm. A configuration is adopted in which the above area is set to a value equal to or less than the average toner layer thickness.
[0030]
According to a twentieth aspect, in the image forming apparatus according to the nineteenth aspect, a configuration is employed in which a toner having a particle size of 4.0 to 7.0 μm is used.
[0031]
In an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, an average thickness of the toner layer on the image carrier is 10 to 20 μm, The surface roughness of the toner image is set to a value equal to or less than the average toner layer thickness in an area of 0.1 mm × 0.1 mm or more.
[0032]
According to a twenty-second aspect of the present invention, in the image forming apparatus according to the twenty-first aspect, a configuration is employed in which a toner having a particle size of 4.0 to 7.0 μm is used.
[0033]
According to a twenty-third aspect of the present invention, in the image forming apparatus according to any one of the first to twenty-second aspects, a configuration is adopted in which a surface friction coefficient of the image carrier is 0.7 or less.
[0034]
According to a twenty-fourth aspect of the present invention, in the transfer method of electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, a pressure is applied between the image carrier and the transfer unit, The transfer was performed with a small potential difference between the image carrier and the recording medium while maintaining the surrounding electric field.
[0035]
According to a twenty-fifth aspect of the present invention, in the method for setting a transfer condition in an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, the transfer between the image carrier and the transfer unit is performed. Using the pressure applied in the middle and the transfer bias current between the image carrier and the transfer means as parameters, an evaluation experiment of the transfer rate, transfer dust, and transfer whiteout was performed, and an allowable level was extracted from these experimental values to transfer. It was decided to be a condition.
[0036]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to FIGS.
First, an outline of the overall configuration of the image forming apparatus according to the present embodiment will be described with reference to FIG. The image forming apparatus uses a well-known electrophotographic system, and includes a drum-shaped photoconductor as an image carrier inside. Around the photoreceptor 1, a charging unit 2, an exposing unit 3, a developing unit 4, a transfer / conveying unit 5, a cleaning unit 6, and a fixing unit 7 for performing an electrophotographic copying process are arranged along a rotation direction indicated by an arrow. ing.
Exposure means 3 forms an electrostatic latent image on photoreceptor 1 by scanning laser light with polygon mirror 32 and reflecting it on mirror 33 based on the image signal of the document read by image reading device 31. As the photoconductor 1, an existing photoconductor such as amorphous silicone can be used in addition to the organic photoconductor.
[0037]
The electrostatic latent image formed on the photoconductor 1 is visualized as a toner image by a developing unit 4 having a developing roller 41. The transfer paper P as a recording medium stored in the paper feed trays 101 and 106 is fed by paper feed rollers 102 and 107 and is conveyed by a pair of conveyance rollers 103 and 108 to a pair of registration rollers 104.
The transfer paper P is temporarily stopped by the pair of registration rollers 104 to correct the oblique displacement, and then the pair of registration rollers 104 at the timing when the leading end of the toner image formed on the photoconductor 1 coincides with the predetermined position in the conveyance direction. It is transported to the transfer site.
The transfer sheet P sent to the transfer portion is electrostatically transferred with a toner image by a transfer bias applied between the transfer roller 52 as a transfer unit in the transfer conveyance unit 5 and the photoconductor 1. The transfer paper P on which the toner image has been transferred is conveyed to the fixing means 7 by the conveyance belt 53 of the transfer conveyance means 5 and is fixed. The transfer paper P on which the fixing has been completed is discharged to the discharge tray 110 by the discharge roller pair 105.
[0038]
After the transfer, the toner and stains remaining on the photoconductor 1 are cleaned by the cleaning unit 6. The cleaning means 6 has a cleaning blade 61, a cleaning brush 62, and a friction coefficient reducing agent 63. The surface potential of the photoreceptor 1 after cleaning is initialized by an unillustrated discharging lamp, and is prepared for the next image forming process. The transfer roller 52 has a metal roller 52a and an elastic layer formed on the surface of the metal roller 52a. The transport belt 53 is driven to rotate by a driving roller 54 and a driven roller 55, and the surface thereof is cleaned by a belt cleaning unit 56.
The fixing unit 7 includes a fixing roller 71 having a heating source 74 such as a halogen lamp therein, and a pressure roller 72 having a heating source 73 such as a halogen lamp therein. The surface pressure of the fixing roller 71 and the pressure roller 72 is 9.3 N / cm. ² To form a fixing nip width of about 10 mm. The fixing unit 7 receives the driving force from a driving unit (not shown) and nips and conveys the transfer paper P. At this time, the fixing roller 71 is controlled to a predetermined temperature by a heating source 74, and the toner image on the transfer paper P is melted while receiving pressure when passing between the rollers, and is cooled out of the nip portion. As a result, the image is fixed on the transfer paper P as a permanent image.
[0039]
Next, a configuration and a pressing structure of the transfer roller 52 will be described with reference to FIG. The transfer roller 52 has a metal roller 52a made of stainless steel (SUS), iron (Fe), or the like and having a diameter of 20 to 30 mm as a metal core, and an elastic layer 52b formed on the surface of the metal roller 52a. The elastic layer 52b is formed of a material such as EPDM, silicone, NBR, urethane, etc. in a solid state.
The elastic layer 52b has a thickness of 0.1 to 1.0 mm, a hardness of 60 to 80 degrees (Asker C / 1 kg load), and a volume resistance of 1 × 10 ⁸ ~ 1 × 10 ¹¹ Ωcm is set. The surface resistance of the elastic layer 52b is optimally in the range of a resistance one to two orders of magnitude higher than the volume resistance.
The transfer roller 52 is pressurized by bearings 52c and springs 52d provided at both ends of the roller, and is pressed against the photoconductor 1. Hereinafter, the measurement of the pressing force is indicated by a value obtained by dividing the sum of the spring pressing forces before and after the transfer roller 52 by the length of the transfer roller 52. The unit is shown in N / cm.
[0040]
Next, the gap around the toner when the pressure is applied between the photosensitive member 1 and the transfer roller 52 will be described with reference to FIG.
FIG. 3A is a view showing a conventional pressurized state at a transfer portion having a pressure of 1 N / cm or less. The transfer paper P has a characteristic of being hard in a small area and deforming (bending) in a large area. When the pressure is weak as in the related art, the transfer paper P is received (contacted) by several projections on the front and back surfaces of the fibers of the transfer paper P and conveyed.
As a result, the number of contact points between the toner T or the photosensitive member 1 and the transfer paper P is small, and the gap between the photosensitive member 1 and the surface of the transfer roller 52 is large.
A pressure of 0.5 N / cm was applied between the photoreceptor and the roller without the rubber layer (elastic layer 52b), the transfer paper P (Type 6200 manufactured by Ricoh) was sandwiched, and the gap was measured. Further, as a result of the same measurement under a pressure of 1.0 N / cm, there was a difference of 20 μm. That is, the gap is reduced by 20 μm by the pressurization.
[0041]
FIG. 3B shows the state, that is, the pressurized state in the present embodiment. By pressing, the convex portion of the transfer paper P comes into contact with the photosensitive member 1 and the transfer roller 52 in a large amount. As a result, the gap becomes smaller, the distance between the toner T and the transfer paper P becomes narrower on average, and “g” in the denominator term of the equation (2) becomes smaller. Furthermore, if the thickness of the toner layer T is constant and the surface roughness of the toner is uniform, the average gap becomes small.
Next, the thickness of the photosensitive layer of the photosensitive member 1 will be described. The OPC photoreceptor 1 generally has a UL layer, a CGL layer, and a CTL layer applied on a metal support, and the entire photosensitive layer has a thickness of 25 to 30 μm. Among these layers, the CTL layer has the largest thickness of 20 to 30 μm. The reason is that durability is taken into consideration, and the photoconductor surface without a toner film or the like is always used while shaving the CTL layer. The capacitance at this time is about 80 PF / cm ² It is.
[0042]
In the present embodiment, the CTL layer is thinned by about 10 μm. Thereby, the dielectric thickness of the photosensitive layer of the photosensitive member 1 is reduced.
As described above, even if the potential difference between the transfer paper P and the photoconductor 1 is reduced by reducing the gap and the dielectric thickness of the photoconductor 1, E (Electric field around the toner) of the equation (2) is the same as that of the related art. An electric field is obtained. That is, transfer dust due to discharge at the time of separation can be reduced while maintaining the electric field around the toner.
[0043]
Next, the surface characteristics of the photoconductor 1 used in the present embodiment will be described. It is desirable that the surface friction coefficient of the photoconductor 1 is 0.7 or less. If the surface friction coefficient is 0.7 or more, the releasability from the toner during transfer deteriorates, and transfer efficiency deteriorates. Particularly, in this embodiment, a toner having a low cohesive force (non-polymerized toner) is used. However, since the pressure at the time of transfer is large, a transfer white spot at the time of transfer (a large amount of toner is not separated from the photoconductor 1 without being separated from the photoreceptor 1) Phenomenon). Therefore, it is necessary to lower the surface friction coefficient.
As a method of reducing the surface friction coefficient of the photoconductor 1, for example, a fatty acid metal salt such as zinc stearate and calcium stearate is uniformly applied to the surface of the photoconductor 1.
For the measurement of the surface friction coefficient, a fully automatic friction and wear analyzer (manufactured by Kyowa Interface Science Co., Ltd.) was used, and a 3 mm stainless steel ball was used as a contact.
[0044]
In the present embodiment, as shown in FIG. 4, a protective layer 1c containing a metal oxide is provided on the surface of the photoreceptor 1, that is, on the surface of the CTL layer 1a-3 having a reduced thickness. In FIG. 4, reference numeral 1a denotes a photosensitive layer, 1a-1 denotes an UL layer, 1a-2 denotes a CGL layer, and 1b denotes a metal support.
When the photoreceptor 1 having the protective layer 1c is used, since the mechanical strength is strong, the film of the photoreceptor 1 is less scraped, and a stable image can be obtained over time. The metal oxide contained in the protective layer 1c is desirably a kind selected from alumina, titanium oxide, and silica.
[0045]
Next, the toner used in the exemplary embodiment will be described. The production method may be a conventionally known method, a binder resin, a release agent, a colorant, a charge control agent and the like are mixed by a mixer, heated rolls, kneaded by a kneader such as an extractor, and then cooled and solidified, This is a toner obtained by pulverizing it with a pulverizing device such as a jet mill, a turbojet, or a kryptron, and then classifying it.
Further, a toner obtained by adding an inorganic fine powder to the above toner and mixing with a super mixer, a Henschel mixer or the like may be used.
If the thickness of the toner layer T on the photoconductor 1 is uniform and the surface roughness of the toner layer T is uniform, the electric field around the toner between the photoconductor 1 and the transfer paper P becomes uniform, and the toner moves. The applied force (F) becomes uniform. Further, the uniform charge amount of the toner is a favorable condition for transfer dust.
[0046]
Next, an evaluation method according to the present embodiment will be described.
<Evaluation method of transfer rate, fixing property, transfer dust, and transfer white spot>
The evaluator was used by modifying the transfer portion of Ricoh Imageio MF7070. The unit configuration is the same as the schematic diagram of the apparatus in FIG. A two-component developing device is used for development, transfer is performed by a roller transfer method, and the belt is conveyed. The fixing was performed under the condition that the surface pressure was 9.3 N / cm and the temperature was 165 to 185 ° C. Using this apparatus, a test chart (see FIG. 5) centered on a gray scale formed from 600 dpi dots was printed out to obtain a sample image.
[0047]
<Evaluation of transfer rate>
The developed chart on the photoconductor 1 is transferred, and the apparatus is stopped when the transfer paper P is on the transport belt 53. Paying attention to the black solid portion of the chart, the transfer residual toner amount of the black solid portion on the photoconductor 1 is peeled off with an adhesive tape, and the residual toner amount on the photoconductor 1 is obtained. On the other hand, the transferred toner cuts off the solid black portion and blows off the toner with compressed air. Calculate the amount of transferred toner from the weight before and after blowing off,
(Transfer toner amount / (transfer toner + remaining toner amount)) × 100 (%)
To determine the transfer rate (%). The allowable value of the transfer rate is 70% or more under a general environment. If the transfer rate is 80% or more, it is judged as “○”. Similarly, 70 to 79% was indicated by “Δ”, and 69% or less was indicated by “x”. The allowable level is “△” or more.
[0048]
<Evaluation of fixability>
Next, fixation is performed by a smear method. The cloth adhered to the weight of 8.8 N / 15φ in the halftone portion of ID: 0.6 to 0.8 was placed on the transfer paper P, and the evaluation was performed based on the density on the cloth when the cloth was rubbed five times.
A level of 0.3 or less, which is no problem, was indicated by "○", an acceptable level of 0.5 or less was indicated by "で", and a level of 0.51 or more was evaluated by "x".
[0049]
<Evaluation of transfer dust and transfer void>
Since a general-purpose evaluation method has not been established for the transfer dust and the transfer white spot, the sample and the rank sample were visually evaluated by a sensory evaluation method. FIG. 7 shows a sample of the rank of the transferred chile, and FIG. 6 shows a rank of the transferred white spot. “△” of the image of rank 3 is an allowable level, and “OK” is higher than that, and “NG” is less than “△”.
[0050]
Hereinafter, the present invention will be described with reference to examples.
[Example 1]
A test machine similar to that shown in FIG. 1 was used by modifying the transfer portion of Ricoh Imagio MF7070. The surface of the transfer roller 52 has a hardness of 70 degrees (Asker C / 1 kg load) so that the contact between the photosensitive member 1 and the transfer roller 52 is stable. ⁹ ~ 1 × 10 ¹⁰ A rubber layer (elastic layer) of Ωcm was provided. The transfer pressure (parameter) by the spring 52d shown in FIG. 2 is controlled at four levels of 0.5, 1.0, 5.0 and 10.0 N / cm, and the voltage applied between the transfer roller 52 and the photosensitive member 1 is controlled. The test chart shown in FIG. 5 was printed by a printer at four levels of flowing currents (parameters) of 0.03, 0.05, 0.2, and 0.3 μA / cm.
The evaluation was confirmed by the transfer rate, the transfer dust (see FIG. 7), and the transfer white spot (see FIG. 6). Tables 1, 2 and 3 show the results. The current value, that is, the conventional value, is a condition where the transfer current is 0.3 to 0.4 μA / cm and the pressure is about 1 N / cm.
[0051]
[Table 1]

[0052]
[Table 2]

[0053]
[Table 3]

[0054]
According to Tables 1, 2, and 3, when the transfer rate is less than 0.05 μA, the current is insufficient and the transfer rate is poor. When the pressure is small and the current is small, the transfer rate is reduced. In the transfer dust, when the transfer current is 0.3 μA or more, the dust due to the discharge increases. Further, when the transfer pressure is 10 N / cm, transfer white spots are about to occur, and as a result, the transfer rate is also reduced.
As a result, a combination of a pressure of 1 to 10 N / cm and a transfer bias current of 0.05 to 0.2 μA / cm can be selected as optimum conditions. Preferably, the pressure was 1.0 to 5.0 N / cm and the current was 0.1 to 0.2 μA / cm.
[0055]
[Example 2]
In the same manner as in Example 1, the thickness of the CTL layer 1a-3 of the photosensitive member 1 was changed at five levels of 5 μm, 10 μm, 15 μm, 20 μm, and 25 μm, the transfer pressure was 3.0 N / cm, and the transfer current was 0.3 μA. The test chart in FIG. 5 was printed at a rate of / cm to evaluate the transferred dust. Table 4 shows the results.
[0056]
[Table 4]

[0057]
From Table 4, it can be seen that when the thickness of the CTL layer 1a-3 decreases, black spots occur due to leakage. When the thickness is 20 μm or more, the capacitance of the photoconductor 1 becomes small (80 PF / cm). ² Below), the charge amount of the photoreceptor 1 becomes large and transfer dust becomes worse.
From the above results, the thickness of the CTL layer 1a-3 of the photoconductor 1 is 10 to 20 μm, and the capacitance is 100 PF / cm. ² The above are favorable conditions.
The potential difference between the photoconductor 1 and the transfer paper P obtained by combining the first and second embodiments is as follows. When the variation of the dielectric thickness is calculated from Expression (2), in the conventional example, the transfer paper t: 80 μm, ε: 4, the toner layer t: 10 μm, ε: 3, the photoconductor t: 30 μm, ε: 3, the gap t: 40 μm.
On the other hand, under the optimal conditions of the present embodiment, when the pressure and the thickness of the photosensitive member are set to 20 μm, ε: 3, and the gap 20 μm, the dielectric thickness becomes 68%, and the electric field around the toner becomes the same value. If maintained, the potential difference between the photoconductor 1 and the transfer roller 52 can be reduced by 32%. As a result, the discharge at the time of separation was reduced, and transfer dust was reduced.
[0058]
[Example 3]
The test chart of FIG. 5 was printed by the same method as in Example 1 at a transfer pressure of 3.0 N / cm and a transfer current of 0.15 μA / cm to obtain a sample. The CTL layer 1a-3 of the photoreceptor 1 was 20 μm, and a protective layer 1c was provided on the surface.
Specifically, 25 w% of aluminum oxide having a particle diameter of 0.3 μm was added to a polycarbonate resin dissolved in a solvent, dispersed, spray-coated, and heated and dried at 150 ° C. to form a 5 μm protective layer 1c.
A continuous copy test was performed using two types of photoconductors with and without a protective layer. Table 5 shows the results.
[0059]
[Table 5]

[0060]
According to Table 5, when the protective layer 1c was not provided, black spots occurred from 200K (K = 1000 sheets). The reason for this is that in the cleaning step, the brush and the urethane rubber are scraped by a cleaning blade, and when the thickness is 15 μm or less, the electric field intensity increases, a partial leak occurs, and a black spot image is formed.
By providing the protective layer 1c, abrasion of the photoconductor 1 is small, and a durability of 300 to 500K sheets can be obtained. In the case where the protective layer 1c is not provided, the photoreceptor coated with the CTL layer 1a-3 having a thickness of 25 μm causes image deterioration with a life of 200 to 300K. As a result of the test, shaving of 10 μm occurred at 300K.
[0061]
[Example 4]
The test chart of FIG. 5 was printed by the same method as in Example 1 at a transfer pressure of 3.0 N / cm and a transfer current of 0.15 μA / cm to obtain a sample. A developer used for development was prepared as follows.
Toner prescription
80 parts by weight of polyester resin (weight average molecular weight: 185000, Tg: 67 ° C.)
4 parts by weight of carnauba wax (average particle size: 300 μm)
15 parts by weight of carbon black ($ 44: Mitsubishi Chemical)
1 part by weight of charge control agent (Spiron Black TR-H: Hodogaya Chemical)
After kneading at 160 ° C. using a twin-screw extruder with the above formulation, pulverizing with a mechanical pulverizer while changing the pulverizing conditions, and classifying the particles, the particles having a weight average particle size of 4.2 μm, 6.8 μm, 9.0 μm. A toner was obtained. Further, using a Henschel mixer, 1.0% by weight of silica (R-972 Nippon Aerosil) was mixed to obtain three kinds of toners having different particle diameters.
[0062]
The carrier was obtained by using a spherical ferrite having a weight average particle diameter of 50 μm, coating the surface with a silicone resin, and heat drying.
The 4.2 μm toner was mixed with 5.0 w% of the carrier, the 6.8 μm toner was mixed with 4.0 w% of the carrier, and the 9.0 μm toner was mixed with 3.0 w% of the carrier to obtain three kinds of developers. .
Using the above developer, image density and transfer dust were confirmed. At the same time, the average height in the Z-axis direction of the toner image formed on the photoreceptor 1 and the surface roughness of an area of 0.1 × 0.1 mm were measured and indicated together. The Z-axis data of the toner image on the photoreceptor 1 is obtained by using an ultra-depth shape measuring microscope VK8500 manufactured by KEYENCE CORPORATION with an objective lens 50 times (magnification 1000 times on a 15-inch CRT) and the average value of the cross-sectional height. Surface roughness measurements were used. Table 6 shows the results.
[0063]
[Table 6]

[0064]
From Table 6, it can be seen that although the average height of the toner is almost the same, the transfer dust is good if the surface roughness is uniform. It is considered that this is because when the layer structure of the toner is non-uniform, the probability of the toner transferring in a different direction during transfer and transfer separation increases.
Since the image densities are almost the same, the image densities after fixing are almost the same.
[0065]
In the above embodiment, the transfer roller is exemplified as the transfer unit. However, a similar function can be obtained even with an endless belt.
Further, in the above-described embodiment, the configuration in which all the conditions are simultaneously provided has been described. Chile can be reduced.
[0066]
【The invention's effect】
According to the first aspect of the present invention, in an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, a pressure is applied between the image carrier and the transfer unit. In addition, since the transfer is performed by reducing the potential difference between the image carrier and the recording medium while maintaining the electric field around the toner, the transfer efficiency is improved by increasing the contact between the image carrier and the recording medium. As a result, transfer current can be reduced, and transfer dust can be reduced by reducing discharge during separation.
[0067]
According to the second aspect of the present invention, in the image forming apparatus according to the first aspect, the pressure is set to 1 to 10 N / cm, so that the transfer between the image carrier and the recording medium is increased. The efficiency can be reliably increased, and as a result, the transfer current can be reduced, and the transfer dust during separation can be reduced to reduce transfer dust.
[0068]
According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, reducing the potential difference includes interposing an elastic layer between the image carrier and the transfer unit. As a result, the transfer between the image carrier and the recording medium can be increased by deforming the elastic layer to increase the transfer efficiency. As a result, the transfer current can be reduced and the discharge at the time of separation can be reduced to reduce transfer dust. can do.
[0069]
According to a fourth aspect of the present invention, in the image forming apparatus of the third aspect, the transfer unit is a roller or an endless belt, and the elastic layer is formed integrally with the transfer unit. Therefore, the configuration can be simplified.
[0070]
According to the fifth aspect of the present invention, in the image forming apparatus according to the third or fourth aspect, the elastic layer has a hardness of 60 to 80 degrees, so that the contact between the image carrier and the recording medium is elastic. The transfer efficiency can be increased by increasing the optimum deformation of the layer, and as a result, the transfer current can be reduced, and the transfer dust can be reduced by reducing the discharge at the time of separation.
[0071]
According to a sixth aspect of the present invention, in the image forming apparatus according to the first or second aspect, reducing the potential difference reduces a transfer bias current applied between the image carrier and the transfer unit. With this configuration, the charge on the recording medium can be reduced, and the transfer discharge can be reduced by reducing the discharge at the time of separation.
[0072]
According to the seventh aspect of the present invention, in the image forming apparatus of the sixth aspect, the transfer bias current is configured to be 0.05 to 0.2 μA / cm, so that transfer dust is reduced with high accuracy. be able to.
[0073]
According to an eighth aspect of the present invention, in the image forming apparatus according to the first or second aspect, the potential difference is reduced by using an OPC photosensitive member as the image bearing member, reducing the dielectric thickness of the OPC photosensitive member, and reducing the electrostatic thickness. Since the configuration is to increase the capacity, the charging potential of the image carrier can be reduced, and the transfer dust can be reduced by reducing the potential difference between the image carrier and the transfer unit.
[0074]
According to the ninth aspect of the present invention, in the image forming apparatus of the eighth aspect, the CTL layer of the OPC photosensitive member has a capacitance of 10 PF / cm to 20 袖 m and a capacitance of 100 PF / cm. ² With the above configuration, transfer dust can be reduced with high accuracy.
[0075]
According to the tenth aspect of the present invention, in the image forming apparatus according to the ninth aspect, since the protective layer is provided on the surface of the CTL layer, transfer dust can be reduced and the life of the image carrier can be extended. Can be planned.
[0076]
According to the eleventh aspect of the present invention, since the processing for reducing the friction coefficient of the surface of the protective layer is performed, it is possible to prevent the transfer white spot even when the transfer pressure is large.
[0077]
According to the twelfth aspect of the present invention, in the image forming apparatus according to the first or second aspect, the potential difference is reduced by setting an average thickness of the toner layer on the image carrier to 10 to 20 μm, Since the roughness is set to a value equal to or less than the toner average layer thickness in an area of 0.1 mm × 0.1 mm or more, the contact between the image carrier and the recording medium is increased due to a small air gap. As a result, the transfer efficiency can be increased, the transfer current can be reduced as a result, and the transfer dust during separation can be reduced, thereby reducing transfer dust.
[0078]
According to a thirteenth aspect of the present invention, in the image forming apparatus according to the twelfth aspect, a toner having a particle diameter of 4.0 to 7.0 μm is used, so that the transfer dust is reduced by further uniformizing the thickness of the toner layer. It can be reduced with high accuracy.
[0079]
According to the invention described in claim 14, in an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, between the image carrier and the transfer unit, A pressure of 10 N / cm is applied, an elastic layer is provided between the image carrier and the transfer means, and a transfer bias current between the image carrier and the transfer means is 0.05 to 0.2 μA / cm. Therefore, the transfer efficiency can be increased by increasing the contact between the image bearing member and the recording medium, and as a result, the transfer current can be reduced, and the discharge at the time of separation can be reduced, thereby reducing the transfer dust.
[0080]
According to a fifteenth aspect of the present invention, in the image forming apparatus according to the fourteenth aspect, the elastic layer has a hardness of 60 to 80 degrees, so that the contact between the image carrier and the recording medium is reduced by the elastic layer. The transfer efficiency can be increased by increasing the number of the optimum deformations. As a result, the transfer current can be reduced, and the transfer dust can be reduced by reducing the discharge at the time of separation.
[0081]
According to a sixteenth aspect of the present invention, in the image forming apparatus of the fourteenth or fifteenth aspect, an OPC photoconductor is used as the image carrier, and the CTL layer of the OPC photoconductor has a capacitance of 10 to 20 μm. Is 100PF / cm ² With the above configuration, transfer dust can be reduced with high accuracy.
[0082]
According to the seventeenth aspect of the invention, in the image forming apparatus according to the sixteenth aspect, since the protective layer is provided on the surface of the CTL layer, transfer dust can be reduced and the life of the image carrier can be extended. Can be planned.
[0083]
According to the eighteenth aspect of the invention, in the image forming apparatus according to the seventeenth aspect, since the processing for reducing the friction coefficient of the surface of the protective layer is performed, the transfer white spot is generated even when the transfer pressure is large. Can be prevented.
[0084]
According to the nineteenth aspect of the present invention, in the image forming apparatus according to the fourteenth or fifteenth aspect, the average thickness of the toner layer on the image carrier is 10 to 20 μm, and the surface roughness of the toner image is 0.1 mm × 0. .1 mm or more and the thickness is set to be equal to or less than the average toner layer thickness, so that the air gap is reduced, so that the contact between the image carrier and the recording medium is increased, and the transfer efficiency can be increased. As a result, the transfer current can be reduced, and the transfer dust can be reduced by reducing the discharge at the time of separation.
[0085]
According to the twentieth aspect of the present invention, in the image forming apparatus of the nineteenth aspect, since the toner having a particle size of 4.0 to 7.0 μm is used, transfer dust can be reduced by further uniformizing the thickness of the toner layer. It can be reduced with high accuracy.
[0086]
According to the twenty-first aspect, in an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, an average thickness of the toner layer on the image carrier is 10 to 20 μm. The surface roughness of the toner image is set to a value equal to or less than the average toner layer thickness in an area of 0.1 mm × 0.1 mm or more. Transfer efficiency can be increased by increasing the number of contacts. As a result, transfer current can be reduced, and transfer dust can be reduced by reducing discharge during separation.
[0087]
According to the twenty-second aspect of the present invention, in the image forming apparatus according to the twenty-first aspect, a configuration is employed in which a toner having a particle size of 4.0 to 7.0 μm is used. It can be reduced with high accuracy.
[0088]
According to a twenty-third aspect of the present invention, in the image forming apparatus according to any one of the first to twenty-second aspects, the surface friction coefficient of the image carrier is 0.7 or less. Is large, transfer white spots can be prevented.
[0089]
According to a twenty-fourth aspect of the present invention, in the transfer method for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, a pressure is applied between the image carrier and the transfer unit. Since the transfer is performed with a small potential difference between the image carrier and the recording medium while maintaining the electric field around the toner, the transfer efficiency can be increased by increasing the contact between the image carrier and the recording medium. As a result, the transfer current can be reduced, and the transfer dust can be reduced by reducing the discharge at the time of separation.
[0090]
According to a twenty-fifth aspect of the present invention, in the method of setting transfer conditions in an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit, the image carrier and the transfer unit And the transfer bias current between the image bearing member and the transfer unit as a parameter, perform an evaluation experiment of the transfer rate, the transfer dust, and the transfer white spot, and extract an allowable level from these experimental values. Since the transfer conditions are set as the transfer conditions, the transfer conditions can be optimized, and the transfer dust can be reduced with high accuracy.
[Brief description of the drawings]
FIG. 1 is a schematic front view of an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a front view of a main part showing a transfer pressure configuration.
FIG. 3A is a schematic diagram showing a conventional transfer pressurized state, and FIG. 3B is a schematic diagram showing a transfer pressurized state in the present invention.
FIG. 4 is a schematic sectional view of a photosensitive layer of the photosensitive member.
FIG. 5 is a diagram showing a test chart.
FIG. 6 is a diagram showing an example of a transfer white spot rank.
FIG. 7 is a diagram showing a sample of a transfer chile rank.
FIG. 8 is a schematic diagram illustrating a mechanism of generation of transfer dust.
[Explanation of symbols]
1. Photoreceptor as image carrier
1c Protective layer
52 Transfer Roller as Transfer Means
52b elastic layer
P Transfer paper as recording medium
T toner

Claims (25)

In an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit,
An image forming apparatus, wherein a pressure is applied between the image carrier and the transfer unit to transfer the image while reducing the potential difference between the image carrier and the recording medium while maintaining the electric field around the toner. The image forming apparatus according to claim 1,
The image forming apparatus, wherein the pressure is 1 to 10 N / cm. The image forming apparatus according to claim 1, wherein
The image forming apparatus according to claim 1, wherein reducing the potential difference includes interposing an elastic layer between the image carrier and the transfer unit. The image forming apparatus according to claim 3,
An image forming apparatus, wherein the transfer unit is a roller or an endless belt, and the elastic layer is formed integrally with the transfer unit. The image forming apparatus according to claim 3, wherein
An image forming apparatus, wherein the elastic layer has a hardness of 60 to 80 degrees. The image forming apparatus according to claim 1, wherein
The image forming apparatus according to claim 1, wherein reducing the potential difference is reducing a transfer bias current applied between the image carrier and the transfer unit. The image forming apparatus according to claim 6,
An image forming apparatus, wherein the transfer bias current is 0.05 to 0.2 μA / cm. The image forming apparatus according to claim 1, wherein
The image forming apparatus according to claim 1, wherein reducing the potential difference is to use an OPC photoconductor as the image carrier, reduce the dielectric thickness thereof, and increase the capacitance. The image forming apparatus according to claim 8,
An image forming apparatus wherein the CTL layer of the OPC photoreceptor has a thickness of 10 to 20 μm and a capacitance of 100 PF / cm ² or more. The image forming apparatus according to claim 9,
An image forming apparatus comprising a protective layer provided on a surface of the CTL layer. An image forming apparatus, wherein a process for reducing a friction coefficient of a surface of the protective layer is performed. The image forming apparatus according to claim 1, wherein
To reduce the potential difference, the average thickness of the toner layer on the image carrier is 10 to 20 μm, and the surface roughness of the toner image is set to a value equal to or less than the average toner layer thickness in an area of 0.1 mm × 0.1 mm or more. An image forming apparatus characterized by setting. The image forming apparatus according to claim 12,
An image forming apparatus using a toner having a particle size of 4.0 to 7.0 μm. In an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit,
A pressure of 1 to 10 N / cm is applied between the image carrier and the transfer unit, an elastic layer is provided between the image carrier and the transfer unit, and a transfer bias current between the image carrier and the transfer unit is provided. Is 0.05 to 0.2 μA / cm. The image forming apparatus according to claim 14,
An image forming apparatus, wherein the elastic layer has a hardness of 60 to 80 degrees. The image forming apparatus according to claim 14, wherein
An image forming apparatus using an OPC photoconductor as the image carrier, wherein the CTL layer of the OPC photoconductor has a thickness of 10 to 20 μm and a capacitance of 100 PF / cm ² or more. The image forming apparatus according to claim 16,
An image forming apparatus comprising a protective layer provided on a surface of the CTL layer. The image forming apparatus according to claim 17,
An image forming apparatus, wherein a process for reducing a friction coefficient of a surface of the protective layer is performed. The image forming apparatus according to claim 14, wherein
An image characterized in that an average thickness of the toner layer on the image carrier is 10 to 20 μm, and a surface roughness of the toner image is set to a value not more than an average toner layer thickness in an area of 0.1 mm × 0.1 mm or more. Forming equipment. The image forming apparatus according to claim 19,
An image forming apparatus using a toner having a particle size of 4.0 to 7.0 μm. In an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit,
An image characterized in that an average thickness of the toner layer on the image carrier is 10 to 20 μm, and a surface roughness of the toner image is set to a value not more than an average toner layer thickness in an area of 0.1 mm × 0.1 mm or more. Forming equipment. The image forming apparatus according to claim 21,
An image forming apparatus using a toner having a particle size of 4.0 to 7.0 μm. The image forming apparatus according to any one of claims 1 to 22,
An image forming apparatus, wherein the surface friction coefficient of the image carrier is 0.7 or less. In a transfer method of electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit,
A transfer method, wherein a pressure is applied between the image carrier and the transfer means, and transfer is performed with a small potential difference between the image carrier and the recording medium while maintaining an electric field around the toner. In a method for setting transfer conditions in an image forming apparatus for electrostatically transferring a toner image on an image carrier onto a recording medium by a transfer unit,
The pressure applied between the image carrier and the transfer unit, and the transfer bias current between the image carrier and the transfer unit were used as parameters, and transfer rate, transfer dust, and transfer whiteout were evaluated. A method for setting a transfer condition, wherein an allowable level is extracted from an experimental value and used as a transfer condition.

Images