JP2005024682A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which does not cause the irregularity of image concentration period, allows the pitch irregularity inconspicuous and is capable of forming a high quality image even when a gap between a latent carrier and a developer carrier is narrowed. <P>SOLUTION: The image forming apparatus comprises an image carrier 1 whose surface is movable, a developing device 4 which carries a developer on the surface and is provided with a developer carrier 43 for developing a latent image formed on the image carrier and a electrifying device 2 for electrifying image carrier surface. Therein, the image carrier surface is provided with a plurality of grooves extended in the longitudinal direction, the pitch of a plurality of the grooves is ≥0.75 mm and the spatial frequency of the grooves is ≥1.5 cycle/mm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

本発明者らは、現像剤担持体と像担持体との間のギャップ（以下、「現像ギャップ」という）を狭小化した場合における、所望の現像剤の汲み上げ量に設定すべきドクタギャップと溝ピッチの関係について鋭意検討した。複数の溝ピッチ幅を変えた現像剤担持体（具体的には、溝ピッチが０．３５［ｍｍ］、０．５［ｍｍ］、０．７５［ｍｍ］、及び１．０［ｍｍ］の現像剤担持体）を用いて、ドクタギャップに対する現像剤の汲み上げ量を測定したところ、図９に示すような結果が得られた。この結果より以下の知見を得た。ドクタギャップを一定とした場合、現像剤担持体に形成した溝のピッチが小さいときには、それが大きい場合に比して現像剤の汲み上げ量が多くなる。また、現像剤の汲み上げ量を一定とした場合、溝ピッチが大きい方がそれが小さい場合に比してドクタギャップを広くすることが可能である。また、現像剤の汲み上げ量を一定とした場合、溝ピッチが大きい方がそれが小さい場合に比して所望の現像剤汲み上げ量Ｓ（図９の縦軸参照）に設定するためのドクターギャップの調整可能範囲である交差幅Ｇｄ（図９の横軸参照）も広く取ることができる。具体的には、現像剤の汲み上げ量Ｓにするためのドクタギャップの調整可能範囲である交差幅Ｇｄは、溝ピッチが０．３５のときには交差幅Ｇｄ１となり、溝ピッチが０．５のときには交差幅Ｇｄ２となる。すなわち、溝ピッチが大きくなるに従って交差幅Ｇｄが広くなる。ドクタギャップの交差幅Ｇｄが広くなることにより、現像剤を狙いの汲み上げ量Ｓに設定することが容易となり、現像剤の汲み上げ量のバラツキによる画像濃度周期ムラのない良好な画像形成が可能となる。また、ドクタギャップの交差幅Ｇｄが広い場合は、交差幅Ｇｄが狭い場合に比して、画像濃度周期ムラを発生させないための現像スリーブ溝深さ偏差、現像スリーブ揺れ等の許容範囲が広くなる。これらの結果、像担持体と現像剤担持体との間のギャップを狭小化した場合においても、画像濃度周期ムラの発生しない良好な画像を形成することが可能となる。実際、溝ピッチが０．７５［ｍｍ］以上の現像スリーブを用いて画像形成を行ったところ、画像濃度周期ムラのない良好な画像を形成することができた（図１０参照）。
【００１０】
上記画像形成装置では、現像剤担持体表面に形成した複数の溝の空間周波数が１．５［ｃｙｃｌｅ／ｍｍ］である。ここで、感光体ドラム上の画像は、略そのままの形で記録紙に転写される。よって、空間周波数ｆは、感光体ドラム表面の表面移動方向で１［ｍｍ］あたりに現像スリーブの溝が通過する回数となり、次の数２により算出することができる。
【数２】

実験によれば、図７に示すように、現像剤担持体に形成した溝の空間周波数が１．５［ｃｙｃｌｅ／ｍｍ］以下では、ピッチムラが目立ちやすくなってしまう。空間周波数が１．５［ｃｙｃｌｅ／ｍｍ］以上の場合には、像担持体から記録紙に転写された画像上でのピッチは、０．４１［ｍｍ］以下に相当する。一般に肉眼では１［ｍｍ］程度のピッチに最も強い感度を持つといわれ、それより高い空間周波数のピッチムラは目立ちづらくなる。
【００１１】
【発明の実施の形態】
以下、本発明を画像形成装置であるプリンタに適用した一の実施形態について説明する。図１は本実施形態に係るプリンタの要部概略構成図である。
像担持体である感光体ドラム１の周囲には、帯電ローラー等で感光体ドラム１の表面を帯電する帯電装置２、レーザー光線等で感光体ドラム１の一様帯電処理面に潜像を形成する露光装置３が配設されている。また、感光体ドラム１上において潜像に対し帯電したトナーを付着させることでトナー像を形成させる現像装置４が配設されている。また、転写ベルトまたは転写ローラー、チャージャー等で感光体ドラム１上に形成されたトナー像を記録紙６に転写する転写装置５、転写後に感光体ドラム１上に残ったトナーを除去するクリーニング装置７が配設されている。さらに、感光体ドラム１上の残留電位を除去する除電装置８が順に配設されている。
【００１２】
上記構成のプリンタにおいて、帯電装置２の帯電ローラによって表面を一様に帯電された感光体ドラム１は、露光装置３によって静電潜像を形成され、現像装置４によってトナー像を形成される。現像条件としては−７００［Ｖ］に帯電された感光体ドラム１を露光して画像部は−１５０［Ｖ］に減衰させる。この静電潜像に−５５０［Ｖ］の現像バイアスが印加されて現像を行っている。
そして、上記トナー像は転写装置５によって感光体ドラム１表面から、不図示の給紙トレイから搬送された記録紙６へ転写される。その後記録紙上のトナー像は不図示の定着装置によって記録紙に定着される。一方、転写されずに感光体ドラム上に残ったトナーはクリーニング装置７によって回収される。残留トナーを除去された感光体ドラム１は除電ランプ８で初期化され、次回の画像形成プロセスに供される。
【００１３】
上記プリンタでは、環境変動や経時変動による画質変動を抑えるために、プロセスコントロールを行なっている。具体的には、まず現像装置４における現像能力を検出する。例えば、あるトナーパターンの画像を現像バイアス電圧を一定にした条件下で感光体ドラム１上に形成し、その画像濃度を光学センサ９で検出し、濃度変化から現像能力を把握する。そして、この現像能力が所定の目標現像能力になるように、トナー濃度の目標値を変更することで、画質を一定に保つことができる。例えば、光センサ９で検出したトナーパターンの画像濃度が、目標現像濃度よりも薄い場合には、トナー濃度を高くするように、ＣＰＵ１０がモータ駆動回路１２を制御する。一方、光センサ９で検出したトナーパターンの画像濃度が、目標現像濃度よりも薄い場合には、トナー濃度を低くするように、ＣＰＵ１０がモータ駆動回路１２を制御する。ここで、上記トナー濃度はトナー濃度センサ４８（図２参照）で検知される。なお、感光体ドラム１上に形成されるトナーパターンの画像濃度は、現像スリーブ４３による前記画像濃度周期ムラの影響で多少変動することがある。
【００１４】
次に、上記現像装置４の構成を図２に基づき説明する。現像装置４は、現像容器４Ａとトナー補給部４Ｂとで構成されている。そのうち現像容器４Ａ内には現像剤担持体としての現像ローラ４１が、像担持体としての感光体ドラム１に近接するようにして配置されており、両者の対向部分に現像領域Ｄが形成されるようになっている。
【００１５】
上記現像ローラ４１には、アルミニウム、真鍮、ステンレス、導電性樹脂などの非磁性体を円筒状に形成してなる現像スリーブ４３が備えられている。この現像スリーブ４３は、図示を省略した回転駆動機構によって矢印方向すなわち反時計回り方向に回転する。現像スリーブ４３内には該現像スリーブ４３の表面上に現像剤を穂立ちさせるように磁界を形成する磁石ローラ体４４が固定状態で備えられている。このとき現像剤を構成するキャリアは、上記磁石ローラ体４４から発せられる磁力線に沿うようにして現像スリーブ４３上にチェーン状に穂立ちされる。そして、このチェーン状に穂立ちされたキャリアに対して帯電トナーが付着されて磁気ブラシが形成されるようになっている。形成された磁気ブラシは、現像スリーブ４３の回転移送にともなって現像スリーブ４３と同方向、すなわち反時計回り方向に移送されることとなる。現像剤の搬送方向すなわち反時計回り方向における前記現像領域Ｄの上流側部分には、現像剤チェーン穂の穂高さすなわち現像剤の量を規制するドクターブレード４５が設置されている。
【００１６】
さらに上記現像ローラ４１の後方領域には、撹拌ローラ４６およびパドルホィール４７が設けられており、撹拌ローラ４６により撹拌混合されて現像剤がパドルホィール４７によって汲み上げられるようになっている。現像ローラ４１、パドルホイール４７、撹拌ローラ４６を包み込むように現像剤収納部材としての現像剤ケース５１が下側に配置されている。
【００１７】
また、上記トナー補給部４Ｂは、例えば感光体ドラム１に供給されるトナーの濃度が低下したことをトナー濃度センサ４８が検知すると、トナー補給ローラ４Ｂ１の回転によりトナーＴを撹拌ローラ４６に向け繰り出すようになっている。そして、前述したドクターブレード４５の近傍には、延長方向一端をドクターブレード４５の近傍に位置させ、延長方向他端を撹拌ローラ４６の上に位置させたセパレータ４９が配置されている。また、このセパレータ４９における延長方向他端には、回転可能な搬送スクリュー５０が配置されている。
【００１８】
上記構成の現像容器４Ａにおいては、パドルホィール４７の回転によって現像剤が汲み上げられ、現像ローラ４１に向け放出され、磁石ローラ体４４の磁力により現像ローラ４１の表面に担持される。そして、現像ローラ４１に担持された現像剤は、現像スリーブ４３の回転に伴って表面を移動し、ドクターブレード４５によって層厚を規制された上で、現像ローラ４１と感光体ドラム１とが対向する現像領域Ｄを通過する。その後、現像剤ケース５１との間隙を通過し、磁石ローラ体４４の磁力が作用しなくなる位置で現像ケース５１の底部に落下し、再度、パドルホィール４７により撹拌される。また、ドクターブレード４５によって掻き取られた余剰の現像剤は、セパレータ４９上に配置された複数の傾斜するフィン４９ａによって画像形成装置奥側に順次搬送される。セパレータ４９の最奥端には現像剤案内路が配置され、その延長方向他端に位置する搬送スクリュー５０に向け案内される。この搬送スクリュー５０により前述とは反対に画像形成装置の手前側に搬送され、最手前端に配置された撹拌ローラ４６と対向する図示しないスリットを通して落下する。この前後それぞれの現像剤の搬送により、画像形成装置前後方向においてトナー濃度が均一になるよう撹拌されると同時に前後それぞれの現像剤の搬送量を等しく設定することにより現像剤の水準を一定に維持可能である。
【００１９】
次に、本実施形態の特徴部である現像スリーブについて説明する。
図３は、現像スリーブ４３を軸方向からみた部分断面の拡大図である。現像スリーブ４３の表面には長手方向に延びる複数の溝が等間隔に形成されている。一般に溝が深いほど現像剤搬送性能は向上するが、図４に示すような１［ｍｍ］周期程度のピッチムラが発生しやすくなる。一方、溝が浅いほどピッチムラは発生しにくくなるが現像剤搬送性能が低下する。特に近年では、小粒径トナーやキャリアの画像形成技術の進歩、及び、近接現像の画像形成技術の進歩等により画像再現性が向上しているため、ピッチムラが発生しやすくなっている。そこで、本実施形態の現像スリーブ４３では、溝深さを従来よりも浅めに設定して、現像剤搬送性能とピッチムラ防止との両立を図っている。具体的には、現像スリーブ４３の溝深さを０．０５〜０．１５［ｍｍ］の範囲で設定している。
【００２０】
ここで装置の具体的な条件は、以下の通りである。
メカ条件は以下の通りである。
感光体ドラム１線速 ３６０ｍｍ／ｓｅｃ（１００〜５００ｍｍ／ｓｅｃで設定可能）
現像スリーブ４３と感光体ドラム１との間のギャップ ０．３〜０．６ｍｍ
現像スリーブ４３とドクタ４５との間のギャップ ０．３〜０．６ｍｍ
現像スリーブ４３の外径 直径１８ｍｍ（Φ１６〜Φ４０で設定可能）
感光体線速に対する現像スリーブ４３の線速比 １．８（１．５〜３で設定可能）
現像スリーブ４３の溝本数 ７５本（現像スリーブ外径により可変）現像スリーブ４３の抵抗値 １００Ω以下
磁石ローラ体４４の磁力の強さ ６０〜１４０ｍＴ
また、現像剤条件は以下の通りである。
キャリア（マグネタイト、鉄又はフェライト） ３０〜８０μｍ
トナー 磁性体量 １５〜５０ｗｔ％
シリカ量 ０．１〜１．０ｗｔ％
体積平均粒径 ５〜９．５μｍ
キャリアに対するトナー被覆率 ５０〜１２０％
トナー帯電量（Ｑ／Ｍ） １５〜５０μｃ／ｇ
【００２１】
図５は、溝深さに対するピッチムラと現像剤搬送性能との実験結果を示すグラフである。図５からわかるように、溝深さが０．１５［ｍｍ］よりも深いと現像剤搬送性能は向上するが、ピッチムラが発生してしまう。このピッチムラの発生原因としては、次のことが考えられる。現像スリーブ４３の溝の深さが０．１５［ｍｍ］よりも深いと、感光体ドラム１と現像ローラ４１との現像領域Ｄにおいて、感光体ドラム１と現像スリーブ４３の溝とが対向したときに、感光体ドラム１と溝との間の現像電界が弱くなる。この結果、現像能力が低下し、溝に対向した部分の感光体ドラム１上で現像濃度が薄くなってしまうためと考えられる。一方、溝深さが０．０５［ｍｍ］よりも浅いとピッチムラは発生しないが、現像剤搬送性能が低下してしまう。この現像剤搬送性能が低下する原因としては、現像スリーブ４３の溝の深さが０．０５［ｍｍ］よりも浅いと、現像スリーブ４３上で現像剤がスリップしたり、溝による現像剤の搬送量が低下したりするためと考えられる。そこで、本実施形態に係るプリンタの現像スリーブ４３では、溝の深さを０．０５［ｍｍ］以上、０．１５［ｍｍ］以下の範囲内として、従来構成よりも浅めに設定し、ピッチムラ防止と現像剤搬送性能との両立を図っている。
また、例えばサンドブラスト加工で現像スリーブ表面に形成した凹凸に比べ、凹部としての溝の深さが深い。よって、サンドブラスト加工で形成した凹凸に比べ溝は摩耗しにくく、経時においても現像剤の搬送性を維持し、安定した画像濃度を維持することができる。また、高速で使用した場合でも、現像剤搬送性能を維持することができる。さらに、プロセスコントロール時に感光体ドラム１上に形成されるトナーパターンの画像濃度が安定するため、より適切なプロセスコントロールが行なえる。
【００２２】
上述したように、現像スリーブ４３の溝深さを０．０５［ｍｍ］以上、０．１５［ｍｍ］以下の範囲内とすることで、ピッチムラ防止と現像剤搬送性能との両立を図ることができた。
ところが、上記現像スリーブ４３を用いて画像形成を行なったところ、図６に示すように、記録紙６に３０〜５０［ｍｍ］の比較的長い周期の画像濃度周期ムラが発生してしまった。ここで、現像スリーブ４３は直径が１８［ｍｍ］であるため、外周長は５６．５［ｍｍ］となる。感光体ドラム１に対する線速比が１．８なので、画像上では約３１［ｍｍ］が現像スリーブ１周に相当する。よって、上記記録紙６上で生じた画像濃度周期ムラの周期は、現像スリーブ４３の１回転の周期に略相当する。このような画像濃度周期ムラは現像スリーブの偏心で生じることが多いため、現像スリーブ４３の偏心量を測定したが、現像スリーブ４３の偏心量は画像濃度周期ムラを生じるほど大きくはなかった。
【００２３】
そこで、本出願人は、現像スリーブ４３に形成した複数の溝の深さをレーザ測定器で測定したところ、図７のグラフに示すように、溝の深さにスリーブ周方向のムラが見られた。この結果、溝の浅い所では汲み上げ量が少なくて画像濃度が低く、溝の深いところでは反対に汲み上げ量が多くて画像濃度が高くなっていることがわかった。図８は、現像スリーブ４３の溝深さのばらつきを模式的に示した側面図である。現像スリーブ４３に形成した溝の深さにバラツキが生じる原因としては、次のことが考えられる。現像スリーブの溝はダイスで加工するが、本実施形態のように、従来に比べて浅い溝を形成するときに、従来の深い溝の場合と同程度の加工精度で加工すると、溝深さのバラツキが相対的に大きくなってしまうためと考えられる。
【００２４】
図１０は、溝ピッチと画像濃度周期ムラとの関係の実験結果を示すグラフである。図１０からわかるように、溝ピッチが０．７５［ｍｍ］よりも狭いと画像濃度周期ムラが発生してしまう。上述したように溝ピッチを小さくすると、所望の現像剤の汲み上げ量Ｓに設定するためには、ドクターギャップを狭くする必要がある（図９参照）。また、所望の現像剤の汲み上げ量Ｓに設定するためのドクタギャップの調整可能な範囲としての交差幅Ｇｄも狭くなる。交差幅Ｇｄが狭くなれば、狙いの汲み上げ量Ｓを維持することが現像剤の汲み上げ量のバラツキによって困難となり、画像濃度周期ムラが発生しやすくなってしまうのである。また、交差幅Ｇｄが狭くなれば、画像濃度周期ムラを発生させないための現像スリーブの溝深さ偏差、現像スリーブ揺れ等の許容範囲が狭くなってしまうのである。本実施形態では溝ピッチを０．７５［ｍｍ］以上として、画像濃度周期ムラの防止を図っている。
【００２５】
図１１は、溝の空間周波数ｆとピッチムラとの関係の実験結果を示すグラフである。ここで、感光体ドラム上の画像は、略そのままの形で記録紙に転写される。よって、空間周波数ｆは、感光体ドラム表面の表面移動方向で１［ｍｍ］あたりに現像スリーブの溝が通過する回数となり、上記数２により算出することができる。
図１１からわかるように、現像スリーブ４３の溝による空間周波数は、１．５［ｃｙｃｌｅ／ｍｍ］より小さいとピッチムラが発生してしまう。ピッチムラが１．５［ｃｙｃｌｅ／ｍｍ］以上の場合、感光体ドラム１から記録紙６に転写された画像上でのピッチは０．４１［ｍｍ］以下に相当する。一般に肉眼では１［ｍｍ］程度のピッチに最も強い感度を持つと言われ、それより高い空間周波数のピッチムラは目立ちづらくなる。
本実施形態に係るプリンタの現像スリーブ４３では、空間周波数を１．５［ｃｙｃｌｅ／ｍｍ］以上として、ピッチムラを防いでいる。より具体的には、現像スリーブ４３の現像スリーブ径を直径１８［ｍｍ］、現像スリーブの対感光体線速比を１．８倍、溝数を７５本に設定している。これらの数値を上記数２に代入して空間周波数を算出すると、ｆ＝２．４［ｃｙｃｌｅ／ｍｍ］が得られ、ピッチムラの発生が有効に抑えられる。なお、感光体の線速比は、１．５から３．０の間で設定可能であるが、高画質化のため２以下であることが好ましい。
【００２６】
また、一般的に体積平均粒径が８．５［μｍ］以下のトナーを使用して画像形成を行なうと、図１３のグラフに示すように、画像の再現性が格段に向上するためピッチムラも目立ちやすくなる。本実施形態のプリンタでは、上記構成の現像スリーブ４３を用いることで、体積平均粒径が８．５［μｍ］以下のトナーを使用した場合であっても、ピッチムラを防ぎつつ再現性のよい高品質な画像を形成することができる。なお、体積平均粒径が４［μｍ］よりも小さいと、感光体ドラム上に残ったトナーのクリーニング性が低下するので、トナーの体積平均粒径は４［μｍ］以上が望ましい。
【００２７】
更に、本実施形態のプリンタに用いる現像剤はその構成成分である磁性粒子に粒径が６０［μｍ］以下のものを使用している。従来の二成分現像剤には一般的に粒径７０［μｍ］程度の磁性粒子が使用されることがあったが、本実施形態においては、粒径が６０μｍ以下のものを使用することによって、画像の高画質化に有効である。明度７０〜９０領域のハーフトーンドット画像において、磁性粒子の粒径が６０μｍ以上の場合には粒状度０．３程度であっても、粒径を４０［μｍ］程度まで下げると粒状度が０．１と３倍近くもドット再現性が向上する。図１４は、磁性粒子の粒径を４０［μｍ］，６０［μｍ］，８０［μｍ］の３種類に変化させて形成した画像の粒状性を比較したグラフである。この粒状性は０〜５までのランクで示したもので、ランクが大きいほど画像上でのドット再現性が良く、ランクが小さいほどドット再現性が悪いことを示している。この図から、ａ（粒径８０μｍ）がランク２、ｂ（粒径６０μｍ）がランク３、ｃ（粒径４０μｍ）がランク４となり、粒径が小さいほどドット再現性が良好であることが分かる。
このことから、本実施形態のように磁性粒子に粒径が６０［μｍ］以下のものを使用することは、高画質化に有効である。
【００２８】
以上説明したように、本実施形態では、現像剤担持体としての現像スリーブにおいて、現像スリーブ表面の溝ピッチは、０．７５［ｍｍ］以上であり、溝の空間周波数は１．５［ｃｙｃｌｅ／ｍｍ］以上である。実験によれば、溝ピッチを大きくすることにより、所望の現像剤を汲み上げるためのドクタギャップを広くすることができる。また、所望の現像剤を汲み上げるためのドクタギャップの公差幅（調整幅）も広くすることが可能となる。ドクタギャップ公差幅を広く設定できると、現像剤を狙いの汲み上げ量に設定することが容易となり、現像剤の汲み上げ量のバラツキに起因する画像濃度周期ムラの発生を抑えることができる。
また、ドクタギャップ公差幅を広く設定できると、画像濃度周期ムラを発生させないための現像スリーブ溝深さ偏差、現像スリーブ振れの許容範囲が広くなる。実際、溝ピッチが０．７５［ｍｍ］以上の現像スリーブを用いた画像評価実験では、現像剤の汲み上げ量のばらつきが抑えられ、画像濃度周期ムラが発生しない良好な画像を形成することができた。また、溝の空間周波数を１．５［ｃｙｃｌｅ／ｍｍ］以上とすることにより、ピッチムラを目立ち難くすることができる。
一般に肉眼では１［ｍｍ］程度のピッチに最も強い感度を持つと言われている。
実験によれば、現像スリーブの溝による空間周波数は、１．５［ｃｙｃｌｅ／ｍｍ］以上がピッチムラの防止に有効（図１２参照）であることがわかった。
また、上記現像装置において、上記現像剤として体積平均粒径が８．５μｍ以下のトナーを用いる。よって、ピッチムラを防ぎつつ再現性のよい高品質な画像を形成することができる。一般的に体積平均粒径が８．５［μｍ］以下のトナーを使用して画像形成を行なうと、画像の再現性が格段に向上するためピッチムラも目立ちやすくなる。しかし、上記現像スリーブを用いることで、ピッチムラの防止を図ることができる。
また、上記現像装置において、上記現像剤がトナーと磁性粒子とからなる二成分現像剤であり、該磁性粒子の体積平均粒径を６０［μｍ］以下とした。図１４を用いて上述したように、実験によれば、画像上のドット再現性がよくなるため、粒径がそれより大きいものを用いた場合に比してより良好な画像を形成することができた。
【００２９】
【発明の効果】
請求項１乃至３の発明によれば、像担持体と現像剤担持体との間のギャップを狭小化した場合においても、画像濃度周期ムラが発生せず、ピッチムラが目立ち難い高品質な画像を形成することが可能となるという優れた効果がある。
【図面の簡単な説明】
【図１】実施形態に係るプリンタの要部概略構成図。
【図２】現像装置の構成図。
【図３】現像スリーブを軸方向からみた部分断面拡大図。
【図４】記録紙上のピッチムラの説明図。
【図５】溝深さに対するピッチムラと現像剤搬送性能との実験結果を示すグラフ。
【図６】記録紙上の画像濃度周期ムラの説明図。
【図７】現像スリーブの溝深さ、現像剤汲み上げ量及び画像濃度の関係を示すグラフ。
【図８】現像スリーブの溝深さのばらつきを模式的に示した説明図。
【図９】現像スリーブの溝ピッチに対する、ドクタギャップと現像剤汲み上げ量の関係を示すグラフ。
【図１０】溝ピッチと画像濃度周期ムラとの関係を示すグラフ。
【図１１】空間周波数とピッチムラとの関係を示すグラフ。
【図１２】トナーの体積平均粒径とピッチムラとの関係を示すグラフ。
【符号の説明】
１ 感光体ドラム
２ 帯電装置
４ 現像装置
４３ 現像スリーブ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus used for a copying machine, a facsimile, a printer, and the like. Specifically, an image carrier having a movable surface, a developing device having a developer carried on the surface and developing a latent image formed on the image carrier, and charging the surface of the image carrier. The present invention relates to an image forming apparatus having a charging device.
[0002]
[Prior art]
Conventionally, for example, the surface of the developing sleeve as the developer carrying member has been subjected to roughening processing such as sandblasting or grooving except in the case of a low speed machine. This is to prevent the occurrence of a decrease in image density due to the developer slipping on the developing sleeve rotating at a high speed and the developer not being pumped up and stagnating mainly at the doctor as a developer regulating member.
[0003]
In the sandblasting process, aluminum, brass, stainless steel, or conductive resin can be used as the developing sleeve material. Aluminum is common in view of cost and accuracy. When sandblasting an aluminum developing sleeve, for example, abrasive grains are sprayed cold on an aluminum tube extruded into a sleeve shape at a high temperature to create irregularities on the surface. A surface roughness of about Rz 5 to 15 [μm] is often used. In the developing sleeve subjected to sandblasting in this way, even when the developing sleeve is rotated at a high speed, the developer is caught by the unevenness and the occurrence of slip can be prevented.
However, the development sleeve subjected to the sandblasting has a problem in durability that the unevenness of the surface is worn over time and the developer conveying ability is lowered. This can be improved by making the developing sleeve material stainless steel or by subjecting the surface to a hardening treatment, but this increases the cost.
[0004]
On the other hand, in the above groove processing, it is possible to use aluminum, brass, stainless steel, or conductive resin as the image sleeve material. Aluminum is common in view of cost and accuracy. When groove processing is performed on an aluminum developing sleeve, for example, an aluminum tube extruded into a sleeve shape at a high temperature is pulled out cold, and a groove is formed by a die. The shape of the groove is generally a trapezoidal shape, a V shape, a U shape, or the like. The depth of the groove is generally about 0.2 [mm] from the surface of the developing sleeve, and the number of grooves is generally about 50 for a developing sleeve having a diameter of 25 [mm], for example. In the developing sleeve subjected to the groove processing as described above, even when the developing sleeve is rotated at a high speed, the developer is caught in the groove portion and the occurrence of slip can be prevented. In addition, there is an advantage that the developer can be stably transported with less wear even when used for a long period of time as compared with a developing sleeve subjected to sandblasting.
[0005]
However, in the developing sleeve subjected to the groove processing, periodic fluctuations in the image density due to the groove, so-called pitch unevenness, is observed. In general, the deeper the groove, the better the developer conveyance performance, but the more likely the pitch unevenness occurs. On the other hand, as the groove is shallower, pitch unevenness is less likely to occur, but the developer conveying performance is lowered. In particular, in recent years, image reproducibility has improved due to advances in image formation technology for small particle size toners and carriers, image development technology for proximity development, and the like.
Therefore, in Japanese Patent Application No. 2001-401155, the present applicant can maintain the developer conveyance performance while preventing the occurrence of pitch unevenness by setting the depth of the groove of the developer carrying member within the optimum range. Proposed developer carrier was proposed. In this developer carrier, the groove depth was set shallower than in the prior art. Specifically, it has been proposed to set the groove depth to 0.05 [mm] or more and 1.5 [mm] or less.
However, when image formation was performed using the developer carrier proposed in Japanese Patent Application No. 2001-401155, an image density cycle unevenness (hereinafter referred to as “image density cycle”) of a relatively long period corresponding to one round of the developer carrier. The problem of “unevenness” occurs. Conventionally, it has been known that such image density cycle unevenness is caused by the eccentricity of the developer carrier, and thus the eccentric amount of the developer carrier was measured. However, the amount of eccentricity of the developer carrying member is an amount that does not cause image density periodic unevenness, and it has been found that image density periodic unevenness occurs due to a cause different from the eccentricity of the developer carrying member.
For this reason, the present applicant conducted extensive studies on the cause of image density cycle unevenness, and found that there were variations in a plurality of groove depths on the surface of the developer carrying member. And in Japanese Patent Application No. 2002-026842, the developer carrying which can solve the unevenness of the image density cycle by suppressing the variation in the depth of the plurality of grooves extending in the longitudinal direction on the surface of the developer carrying body to ± 30% or less. Suggested body.
[0006]
[Problems to be solved by the invention]
However, when the gap between the image carrier and the developer carrier is narrowed, the image density cycle unevenness cannot be solved even by the proposal of Japanese Patent Application No. 2002-026842. In recent years, the demand for higher image quality has become stricter, and there has been a demand for a high granularity (a low roughness of the image). As a result, the gap between the developer carrier and the image carrier tends to be narrowed. Along with the narrowing of the gap, the amount of developer pumped per unit time when the developer on the developer carrier is detached and transferred onto the electrostatic latent image on the image carrier must also be reduced. It has become. Therefore, the gap between the developer carrying member and the doctor (hereinafter referred to as “doctor gap”) is narrowed to reduce the amount of developer pumped up. Along with this, the adjustment width (tolerance width) of the doctor gap is also narrowed.
As a result, not only the yield in assembling and manufacturing is deteriorated, but also a problem that image density cycle unevenness and pitch unevenness become conspicuous has occurred.
[0007]
The present invention has been made in view of the above problems, and an object of the present invention is to prevent image density cycle unevenness even when the gap between the image carrier and the developer carrier is narrowed. It is another object of the present invention to provide a high-quality image forming apparatus in which pitch unevenness is not noticeable.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, an invention according to claim 1 is directed to an image carrier having a movable surface and a developer carrier for carrying a developer on the surface and developing a latent image formed on the image carrier. And a charging device for charging the surface of the image carrier, the developer carrier surface has a plurality of grooves extending in a longitudinal direction, and the plurality of grooves The groove pitch is 0.75 [mm] or more, and the spatial frequency by the groove is 1.5 [cycle / mm] or more.
According to a second aspect of the present invention, in the image forming apparatus of the first aspect, a toner having a volume average particle size of 8.5 [μm] or less is used as the developer.
According to a third aspect of the present invention, in the image forming apparatus of the first or second aspect, the developer is a two-component developer composed of toner and magnetic particles, and the magnetic particles have a volume average particle diameter of 60 [μm. ] It is characterized by the following.
[0009]
In the first to third aspects of the invention, even when the gap between the image carrier and the developer carrier is narrowed for the reasons described below, the image density period unevenness does not occur and the pitch unevenness is not noticeable. A good image can be provided. In the image forming apparatus, the groove pitch of the plurality of grooves formed on the surface of the developer carrying member is 0.75 [mm] or more, and the spatial frequency of the grooves is 1.5 [cycle / mm]. Here, the groove pitch is obtained by the following equation (1).
[Expression 1]

The present inventors have provided a doctor gap and a groove to be set to a desired developer pumping amount when the gap between the developer carrier and the image carrier (hereinafter referred to as “development gap”) is narrowed. We have intensively studied the relationship between pitches. Developer carriers having different groove pitch widths (specifically, groove pitches of 0.35 [mm], 0.5 [mm], 0.75 [mm], and 1.0 [mm] Using the developer carrier, the amount of developer pumped up with respect to the doctor gap was measured, and the results shown in FIG. 9 were obtained. The following knowledge was obtained from this result. When the doctor gap is constant, when the pitch of the grooves formed in the developer carrier is small, the amount of developer pumped is larger than when the pitch is large. In addition, when the developer pumping amount is constant, the doctor gap can be made wider when the groove pitch is larger than when the groove pitch is smaller. In addition, when the developer pumping amount is constant, the doctor gap for setting the desired developer pumping amount S (see the vertical axis in FIG. 9) is larger when the groove pitch is larger than when the groove pitch is smaller. The crossing width Gd (see the horizontal axis in FIG. 9), which is an adjustable range, can be widened. Specifically, the intersecting width Gd, which is the adjustable range of the doctor gap for setting the developer pumping amount S, becomes the intersecting width Gd1 when the groove pitch is 0.35, and intersects when the groove pitch is 0.5. The width is Gd2. That is, as the groove pitch increases, the intersection width Gd increases. By widening the doctor gap intersection width Gd, it becomes easy to set the developer to the target pumping amount S, and good image formation without unevenness in the image density due to variations in the pumping amount of the developer becomes possible. . When the intersecting width Gd of the doctor gap is wide, the allowable range for developing sleeve groove depth deviation, developing sleeve fluctuation, etc. to prevent image density cycle unevenness is wider than when the intersecting width Gd is narrow. . As a result, even when the gap between the image bearing member and the developer bearing member is narrowed, it is possible to form a good image without occurrence of uneven image density period. Actually, when an image was formed using a developing sleeve having a groove pitch of 0.75 [mm] or more, a good image having no unevenness in the image density cycle could be formed (see FIG. 10).
[0010]
In the image forming apparatus, the spatial frequency of the plurality of grooves formed on the surface of the developer carrying member is 1.5 [cycle / mm]. Here, the image on the photoconductive drum is transferred onto the recording paper in a substantially intact form. Therefore, the spatial frequency f is the number of passages of the groove of the developing sleeve per 1 mm in the surface movement direction of the surface of the photosensitive drum, and can be calculated by the following formula 2.
[Expression 2]

According to the experiment, as shown in FIG. 7, when the spatial frequency of the groove formed on the developer carrying member is 1.5 [cycle / mm] or less, the pitch unevenness is easily noticeable. When the spatial frequency is 1.5 [cycle / mm] or more, the pitch on the image transferred from the image carrier to the recording paper corresponds to 0.41 [mm] or less. In general, it is said that the naked eye has the strongest sensitivity at a pitch of about 1 [mm], and pitch irregularities with higher spatial frequencies are less noticeable.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment in which the present invention is applied to a printer which is an image forming apparatus will be described. FIG. 1 is a schematic configuration diagram of a main part of the printer according to the present embodiment.
A latent image is formed on the uniformly charged surface of the photosensitive drum 1 with a charging device 2 for charging the surface of the photosensitive drum 1 with a charging roller or the like, a laser beam or the like around the photosensitive drum 1 as an image carrier. An exposure device 3 is provided. Further, a developing device 4 is provided on the photosensitive drum 1 to form a toner image by attaching charged toner to the latent image. Further, a transfer device 5 that transfers the toner image formed on the photosensitive drum 1 to the recording paper 6 by a transfer belt, a transfer roller, a charger, or the like, and a cleaning device 7 that removes the toner remaining on the photosensitive drum 1 after the transfer. Is arranged. Further, a static eliminating device 8 for removing the residual potential on the photosensitive drum 1 is disposed in order.
[0012]
In the printer having the above configuration, the photosensitive drum 1 whose surface is uniformly charged by the charging roller of the charging device 2 forms an electrostatic latent image by the exposure device 3 and forms a toner image by the developing device 4. As a developing condition, the photosensitive drum 1 charged to −700 [V] is exposed to attenuate the image portion to −150 [V]. Development is performed by applying a developing bias of −550 [V] to the electrostatic latent image.
The toner image is transferred by the transfer device 5 from the surface of the photosensitive drum 1 to the recording paper 6 conveyed from a paper supply tray (not shown). Thereafter, the toner image on the recording paper is fixed on the recording paper by a fixing device (not shown). On the other hand, the toner remaining on the photosensitive drum without being transferred is collected by the cleaning device 7. The photosensitive drum 1 from which the residual toner has been removed is initialized by the charge eliminating lamp 8 and used for the next image forming process.
[0013]
In the above printer, process control is performed in order to suppress image quality fluctuations due to environmental fluctuations and temporal fluctuations. Specifically, first, the developing ability in the developing device 4 is detected. For example, an image of a certain toner pattern is formed on the photosensitive drum 1 under a condition where the developing bias voltage is constant, the image density is detected by the optical sensor 9, and the developing ability is grasped from the density change. Then, the image quality can be kept constant by changing the target value of the toner density so that the developing ability becomes a predetermined target developing ability. For example, when the image density of the toner pattern detected by the optical sensor 9 is lower than the target development density, the CPU 10 controls the motor drive circuit 12 so as to increase the toner density. On the other hand, when the image density of the toner pattern detected by the optical sensor 9 is lower than the target development density, the CPU 10 controls the motor drive circuit 12 so as to lower the toner density. Here, the toner density is detected by a toner density sensor 48 (see FIG. 2). Note that the image density of the toner pattern formed on the photosensitive drum 1 may vary somewhat due to the influence of the uneven image density period caused by the developing sleeve 43.
[0014]
Next, the configuration of the developing device 4 will be described with reference to FIG. The developing device 4 includes a developing container 4A and a toner supply unit 4B. Among them, a developing roller 41 as a developer carrying member is disposed in the developing container 4A so as to be close to the photosensitive drum 1 as an image carrying member, and a developing region D is formed in a portion facing both of them. It is like that.
[0015]
The developing roller 41 is provided with a developing sleeve 43 formed of a nonmagnetic material such as aluminum, brass, stainless steel, or conductive resin in a cylindrical shape. The developing sleeve 43 is rotated in the direction of the arrow, that is, counterclockwise by a rotation driving mechanism (not shown). In the developing sleeve 43, a magnet roller body 44 that forms a magnetic field is provided in a fixed state so as to cause the developer to stand on the surface of the developing sleeve 43. At this time, the carrier constituting the developer is spiked in the form of a chain on the developing sleeve 43 along the magnetic lines of force generated from the magnet roller body 44. The charged toner is attached to the carrier spiked in a chain shape to form a magnetic brush. The formed magnetic brush is transferred in the same direction as the developing sleeve 43, that is, in the counterclockwise direction as the developing sleeve 43 rotates. A doctor blade 45 that regulates the height of the spikes of the developer chain, that is, the amount of the developer, is installed on the upstream side of the development region D in the developer conveyance direction, that is, in the counterclockwise direction.
[0016]
Further, a stirring roller 46 and a paddle wheel 47 are provided in the rear region of the developing roller 41, and the developer is pumped up by the paddle wheel 47 by being stirred and mixed by the stirring roller 46. A developer case 51 as a developer storage member is disposed on the lower side so as to enclose the developing roller 41, the paddle wheel 47, and the stirring roller 46.
[0017]
Further, for example, when the toner concentration sensor 48 detects that the concentration of the toner supplied to the photosensitive drum 1 has decreased, the toner replenishing unit 4B feeds the toner T toward the stirring roller 46 by the rotation of the toner replenishing roller 4B1. It is like that. In the vicinity of the doctor blade 45 described above, a separator 49 is disposed in which one end in the extending direction is positioned in the vicinity of the doctor blade 45 and the other end in the extending direction is positioned on the stirring roller 46. A rotatable conveying screw 50 is disposed at the other end of the separator 49 in the extending direction.
[0018]
In the developing container 4A having the above configuration, the developer is pumped up by the rotation of the paddle wheel 47, discharged toward the developing roller 41, and carried on the surface of the developing roller 41 by the magnetic force of the magnet roller body 44. The developer carried on the developing roller 41 moves on the surface along with the rotation of the developing sleeve 43, the layer thickness is regulated by the doctor blade 45, and the developing roller 41 and the photosensitive drum 1 face each other. It passes through the developing area D to be processed. Thereafter, it passes through the gap with the developer case 51, falls to the bottom of the developer case 51 at a position where the magnetic force of the magnet roller body 44 does not act, and is again stirred by the paddle wheel 47. Further, excess developer scraped off by the doctor blade 45 is sequentially conveyed to the back side of the image forming apparatus by a plurality of inclined fins 49 a arranged on the separator 49. A developer guide path is disposed at the innermost end of the separator 49 and is guided toward the conveying screw 50 located at the other end in the extension direction. Contrary to the above, the conveying screw 50 conveys the ink to the front side of the image forming apparatus and falls through a slit (not shown) facing the stirring roller 46 disposed at the frontmost end. By this developer transport before and after, the developer is stirred so that the toner density becomes uniform in the front-rear direction of the image forming apparatus, and at the same time the developer transport amount of each front and rear is set equal to keep the developer level constant. Is possible.
[0019]
Next, the developing sleeve which is a characteristic part of the present embodiment will be described.
FIG. 3 is an enlarged view of a partial cross section of the developing sleeve 43 as seen from the axial direction. A plurality of grooves extending in the longitudinal direction are formed at equal intervals on the surface of the developing sleeve 43. In general, as the groove is deeper, the developer conveying performance is improved, but pitch unevenness of about 1 [mm] period as shown in FIG. 4 is likely to occur. On the other hand, as the groove is shallower, pitch unevenness is less likely to occur, but the developer conveying performance is lowered. In particular, in recent years, the image reproducibility has been improved due to the progress of image forming technology for small-diameter toner and carrier and the image forming technology for proximity development. Therefore, in the developing sleeve 43 of the present embodiment, the groove depth is set shallower than the conventional one to achieve both the developer conveying performance and the prevention of pitch unevenness. Specifically, the groove depth of the developing sleeve 43 is set in the range of 0.05 to 0.15 [mm].
[0020]
Here, the specific conditions of the apparatus are as follows.
The mechanical conditions are as follows.
Photosensitive drum 1 linear speed 360 mm / sec (can be set from 100 to 500 mm / sec)
Gap between developing sleeve 43 and photosensitive drum 1 0.3 to 0.6 mm
Gap between developing sleeve 43 and doctor 45 0.3 to 0.6 mm
The outer diameter of the developing sleeve 43 is 18 mm (can be set with Φ16 to Φ40).
Ratio of the developing sleeve 43 to the photosensitive member linear velocity 1.8 (can be set from 1.5 to 3)
Number of grooves in developing sleeve 43 75 (variable depending on outer diameter of developing sleeve) Resistance value of developing sleeve 43 100Ω or less
Magnetic strength of magnet roller body 44 60-140 mT
The developer conditions are as follows.
Carrier (magnetite, iron or ferrite) 30-80μm
Toner Magnetic amount 15-50wt%
Silica amount 0.1-1.0wt%
Volume average particle size 5 to 9.5 μm
Toner coverage on carrier 50-120%
Toner charge amount (Q / M) 15-50 μc / g
[0021]
FIG. 5 is a graph showing experimental results of pitch unevenness and developer conveyance performance with respect to the groove depth. As can be seen from FIG. 5, when the groove depth is deeper than 0.15 [mm], the developer conveying performance is improved, but pitch unevenness occurs. The following can be considered as a cause of the occurrence of this pitch unevenness. When the depth of the groove of the developing sleeve 43 is deeper than 0.15 [mm], when the groove of the photosensitive drum 1 and the developing sleeve 43 face each other in the developing region D of the photosensitive drum 1 and the developing roller 41. In addition, the developing electric field between the photosensitive drum 1 and the groove is weakened. As a result, it is considered that the developing ability is lowered, and the developing density is reduced on the photosensitive drum 1 in the portion facing the groove. On the other hand, if the groove depth is shallower than 0.05 [mm], pitch unevenness does not occur, but the developer conveying performance is deteriorated. As a cause of the deterioration in the developer conveying performance, if the groove depth of the developing sleeve 43 is shallower than 0.05 [mm], the developer slips on the developing sleeve 43 or the developer is conveyed by the groove. This is thought to be due to a decrease in the amount. Therefore, in the developing sleeve 43 of the printer according to the present embodiment, the depth of the groove is set in a range of 0.05 [mm] or more and 0.15 [mm] or less, which is set shallower than the conventional configuration to prevent pitch unevenness. And developer transport performance.
Further, for example, the depth of the groove as the recess is deeper than the unevenness formed on the surface of the developing sleeve by sandblasting. Therefore, the grooves are less likely to be worn compared to the unevenness formed by sandblasting, and the developer transportability can be maintained over time and a stable image density can be maintained. Further, even when used at a high speed, the developer conveying performance can be maintained. Furthermore, since the image density of the toner pattern formed on the photosensitive drum 1 during process control is stabilized, more appropriate process control can be performed.
[0022]
As described above, when the groove depth of the developing sleeve 43 is in the range of 0.05 [mm] or more and 0.15 [mm] or less, it is possible to achieve both prevention of pitch unevenness and developer conveyance performance. did it.
However, when image formation was performed using the developing sleeve 43, as shown in FIG. 6, image density periodic unevenness having a relatively long period of 30 to 50 [mm] occurred on the recording paper 6. Here, since the diameter of the developing sleeve 43 is 18 [mm], the outer peripheral length is 56.5 [mm]. Since the linear velocity ratio with respect to the photosensitive drum 1 is 1.8, about 31 [mm] on the image corresponds to one circumference of the developing sleeve. Therefore, the cycle of the image density cycle unevenness generated on the recording paper 6 substantially corresponds to the cycle of one rotation of the developing sleeve 43. Since such image density periodic unevenness often occurs due to the eccentricity of the developing sleeve, the eccentricity amount of the developing sleeve 43 was measured, but the eccentricity amount of the developing sleeve 43 was not so large as to cause the image density periodic unevenness.
[0023]
Therefore, the present applicant measured the depth of the plurality of grooves formed in the developing sleeve 43 with a laser measuring instrument, and as shown in the graph of FIG. 7, unevenness in the sleeve circumferential direction was seen in the groove depth. It was. As a result, it was found that the amount of pumping is small and the image density is low in the shallow groove, and the image density is high and the pumping amount is deep in the deep groove. FIG. 8 is a side view schematically showing the variation in the groove depth of the developing sleeve 43. The following is considered as a cause of the variation in the depth of the groove formed in the developing sleeve 43. The groove of the developing sleeve is processed with a die. However, when forming a shallow groove as compared with the conventional case as in this embodiment, if the processing is performed with the same processing accuracy as that of the conventional deep groove, the groove depth is reduced. This is thought to be due to the relatively large variation.
[0024]
FIG. 10 is a graph showing the experimental results of the relationship between the groove pitch and the image density cycle unevenness. As can be seen from FIG. 10, when the groove pitch is narrower than 0.75 [mm], the image density cycle unevenness occurs. When the groove pitch is reduced as described above, the doctor gap needs to be narrowed in order to set the desired developer pumping amount S (see FIG. 9). Further, the crossing width Gd as an adjustable range of the doctor gap for setting the desired developer pumping amount S is also narrowed. If the crossing width Gd becomes narrow, it becomes difficult to maintain the target pumping amount S due to variations in the developer pumping amount, and image density cycle unevenness is likely to occur. In addition, if the intersection width Gd is narrowed, the allowable range for developing sleeve groove depth deviation, developing sleeve fluctuation, etc., in order to prevent image density cycle unevenness from occurring, becomes narrow. In this embodiment, the groove pitch is set to 0.75 [mm] or more to prevent image density cycle unevenness.
[0025]
FIG. 11 is a graph showing an experimental result of the relationship between the spatial frequency f of the groove and the pitch unevenness. Here, the image on the photoconductive drum is transferred onto the recording paper in a substantially intact form. Therefore, the spatial frequency f is the number of passages of the groove of the developing sleeve per 1 [mm] in the surface movement direction of the surface of the photosensitive drum, and can be calculated by the above formula 2.
As can be seen from FIG. 11, when the spatial frequency by the groove of the developing sleeve 43 is smaller than 1.5 [cycle / mm], pitch unevenness occurs. When the pitch unevenness is 1.5 [cycle / mm] or more, the pitch on the image transferred from the photosensitive drum 1 to the recording paper 6 corresponds to 0.41 [mm] or less. In general, it is said that the naked eye has the strongest sensitivity at a pitch of about 1 [mm], and pitch irregularities with higher spatial frequencies are less noticeable.
In the developing sleeve 43 of the printer according to this embodiment, the spatial frequency is set to 1.5 [cycle / mm] or more to prevent pitch unevenness. More specifically, the developing sleeve diameter of the developing sleeve 43 is set to 18 [mm], the developing sleeve to photosensitive member linear velocity ratio is set to 1.8 times, and the number of grooves is set to 75. By substituting these numerical values into the above equation 2 and calculating the spatial frequency, f = 2.4 [cycle / mm] is obtained, and the occurrence of pitch unevenness is effectively suppressed. The linear velocity ratio of the photoconductor can be set between 1.5 and 3.0, but is preferably 2 or less for high image quality.
[0026]
In general, when an image is formed using a toner having a volume average particle size of 8.5 [μm] or less, the reproducibility of the image is remarkably improved as shown in the graph of FIG. It becomes easy to stand out. In the printer of this embodiment, by using the developing sleeve 43 having the above-described configuration, even when a toner having a volume average particle size of 8.5 [μm] or less is used, high reproducibility is achieved while preventing pitch unevenness. A quality image can be formed. If the volume average particle diameter is smaller than 4 [μm], the cleaning property of the toner remaining on the photosensitive drum is deteriorated. Therefore, the volume average particle diameter of the toner is desirably 4 [μm] or more.
[0027]
Furthermore, the developer used in the printer of the present embodiment uses magnetic particles, which are constituent components, having a particle size of 60 [μm] or less. In the conventional two-component developer, magnetic particles having a particle size of about 70 [μm] are generally used, but in this embodiment, by using particles having a particle size of 60 μm or less, It is effective for improving image quality. In the halftone dot image of the brightness 70-90 region, when the particle size of the magnetic particles is 60 μm or more, the granularity is 0 when the particle size is reduced to about 40 [μm] even if the particle size is about 0.3. .1 and 3 times the dot reproducibility is improved. FIG. 14 is a graph comparing the graininess of images formed by changing the particle size of magnetic particles to three types of 40 [μm], 60 [μm], and 80 [μm]. This graininess is shown by ranks from 0 to 5, and the higher the rank, the better the dot reproducibility on the image, and the lower the rank, the worse the dot reproducibility. From this figure, a (particle size 80 μm) is rank 2, b (particle size 60 μm) is rank 3, c (particle size 40 μm) is rank 4, and it can be seen that the smaller the particle size, the better the dot reproducibility. .
For this reason, using magnetic particles having a particle size of 60 [μm] or less as in this embodiment is effective for improving image quality.
[0028]
As described above, in this embodiment, in the developing sleeve as the developer carrying member, the groove pitch on the surface of the developing sleeve is 0.75 [mm] or more, and the spatial frequency of the groove is 1.5 [cycle / cycle]. mm] or more. According to experiments, by increasing the groove pitch, the doctor gap for drawing up the desired developer can be widened. Further, the tolerance width (adjustment width) of the doctor gap for pumping up the desired developer can be increased. If the doctor gap tolerance width can be set wide, it becomes easy to set the developer to the target pumping amount, and the occurrence of uneven image density due to variations in the pumping amount of the developer can be suppressed.
Also, if the doctor gap tolerance width can be set wide, the developing sleeve groove depth deviation and the developing sleeve runout tolerance for preventing the occurrence of image density cycle unevenness are widened. Actually, in an image evaluation experiment using a developing sleeve having a groove pitch of 0.75 [mm] or more, it is possible to form a good image in which variation in the amount of developer pumped up is suppressed and image density cycle unevenness does not occur. It was. Further, by setting the spatial frequency of the groove to 1.5 [cycle / mm] or more, the pitch unevenness can be made inconspicuous.
In general, it is said that the naked eye has the strongest sensitivity at a pitch of about 1 [mm].
According to experiments, it was found that a spatial frequency of 1.5 [cycle / mm] or more due to the groove of the developing sleeve is effective in preventing pitch unevenness (see FIG. 12).
In the developing device, a toner having a volume average particle size of 8.5 μm or less is used as the developer. Therefore, it is possible to form a high-quality image with good reproducibility while preventing pitch unevenness. In general, when an image is formed using a toner having a volume average particle size of 8.5 [μm] or less, the reproducibility of the image is remarkably improved, so that the pitch unevenness becomes conspicuous. However, the use of the developing sleeve can prevent pitch unevenness.
In the developing device, the developer is a two-component developer composed of toner and magnetic particles, and the volume average particle size of the magnetic particles is 60 [μm] or less. As described above with reference to FIG. 14, according to the experiment, the dot reproducibility on the image is improved, so that a better image can be formed as compared with the case of using a larger particle size. It was.
[0029]
【The invention's effect】
According to the first to third aspects of the present invention, even when the gap between the image carrier and the developer carrier is narrowed, a high-quality image in which unevenness in the image density does not occur and pitch unevenness is not noticeable. There is an excellent effect that it can be formed.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a main part of a printer according to an embodiment.
FIG. 2 is a configuration diagram of a developing device.
FIG. 3 is an enlarged partial cross-sectional view of the developing sleeve as viewed from the axial direction.
FIG. 4 is an explanatory diagram of pitch unevenness on recording paper.
FIG. 5 is a graph showing experimental results of pitch unevenness and developer conveyance performance with respect to groove depth.
FIG. 6 is an explanatory diagram of image density cycle unevenness on recording paper.
FIG. 7 is a graph showing the relationship between the groove depth of the developing sleeve, the developer pumping amount, and the image density.
FIG. 8 is an explanatory view schematically showing variations in the groove depth of the developing sleeve.
FIG. 9 is a graph showing the relationship between the doctor gap and the developer pumping amount with respect to the groove pitch of the developing sleeve.
FIG. 10 is a graph showing the relationship between the groove pitch and the image density periodic unevenness.
FIG. 11 is a graph showing the relationship between spatial frequency and pitch unevenness.
FIG. 12 is a graph showing the relationship between the volume average particle diameter of toner and pitch unevenness.
[Explanation of symbols]
1 Photosensitive drum
2 Charging device
4 Development device
43 Development Sleeve

Claims (3)

An image carrier having a movable surface, a developing device having a developer carried on the surface and developing a latent image formed on the image carrier, and charging the surface of the image carrier In an image forming apparatus having a charging device for
The developer carrier surface has a plurality of grooves extending in the longitudinal direction,
The groove pitch of the plurality of grooves is 0.75 [mm] or more,
An image forming apparatus, wherein a spatial frequency by the grooves is 1.5 [cycle / mm] or more. The image forming apparatus according to claim 1.
An image forming apparatus using a toner having a volume average particle diameter of 8.5 [μm] or less as the developer. The image forming apparatus according to claim 1 or 2,
The developer is a two-component developer composed of toner and magnetic particles,
An image forming apparatus, wherein the magnetic particles have a volume average particle diameter of 60 [μm] or less.

Images