JP2004287241A - Image forming method and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrophotographic image forming apparatus capable of reliably forming an image with density gradation, one which does not have a low-quality appearance. <P>SOLUTION: The estimated average halftone granularity of the toner image after developing but before electrostatic transfer was set at ≤0.15 and the average halftone granularity of the toner image after fixing was set at ≤0.25. The average halftone granularity was found by measuring the granularity of a toner image at locations of different average brightness of 40-80 by means of a newly proposed specified formula, and averaging the measured values for which the average brightness is 40-80. The estimated average halftone granularity was obtained by estimating the average halftone granularity of the toner image on a photoreceptor 1 by means of a specified method. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００１８】
表１においても、平均明度４０〜８０の箇所の粒状度が他の箇所の粒状度よりも大きくなっていることがわかる。表１において、粒状度の平均値は０．３２になる。これに対し、平均明度Ｌが４０〜８０の範囲になる６つのデータ（太字で示したもの）だけの平均を算出すると０．４３になる。０．１以上もの差が出てしまうのである。
【００１９】
次に、画像形成装置の様々な試験機によってプリントアウトされた上記グレースケール画像について、同様にして平均明度Ｌと、上記数３の式による粒状度とを測定した。そして、１５の面積率階調の全てについて粒状度を平均した結果と、平均明度が４０〜８０になる箇所（以下、中間調部という）だけの粒状度を平均した結果との関係を調べてみた。更に、各グレースケール画像について、複数の試験者によるザラツキ感の主観評価を行った。この結果を図５に示す。なお、この図においては、ザラツキ感のランク（１〜５）の数値が大きくなるほど、ザラツキ感のない良好な画像であることを示している。図示のように、１５の面積率階調の全てについて粒状度を平均した評価法では、ザラツキ感のランクとその平均値との相関が悪く（相関係数＝０．７５２７）、ザラツキ感の指標として適さないことがわかる。これに対し、中間調部だけについて粒状度を平均した評価法では、ザラツキ感のランクとその平均値との相関が極めて良好であり（相関係数＝０．９１２４）、ザラツキ感の指標として優れていることがわかる。本明細書では、この平均値を中間調部平均粒状度として定義することにする。本発明者らの鋭意研究により、この中間調部平均粒状度が０．２５以下になると、ザラツキ感を与えなくなることが判明した。よって、転写紙等の記録体上に定着された後における中間調部平均粒状度が０．２５以下である画像であれば、人に低画質感を与えることがなくなる。
【００２０】
一方、一般に、電子写真方式の画像形成装置において、現像直後のトナー像は、転写紙等の記録体に転写される際に少量のトナー粒子を記録体の画像部周囲に不規則に付着させて、画質を劣化させる。また、記録体上に転写されたトナー像も、加熱部材との密着によってそこに定着せしめられる際に、トナー粒子のつぶれ、光沢、付着領域の広がりなどといった現象を起こして画質を劣化させる。従って、人に低画質感を与えることのない定着済みのトナー像を得るためには、基本的には、現像した時点で中間調部平均粒状度が０．２５以下であるトナー像を得る必要がある。
【００２１】
現像直後のトナー像について、かかるものであるか否かを評価するためには、その中間調部平均粒状度を求める必要がある。そして、そのためには、感光体等の潜像担持体上でそのトナー像をスキャナー等の読み取り手段で読み取ってその画像情報を電子化しなければならない。しかし、潜像担持体上のトナー像を読み取るのは非常に困難である。潜像担持体の表面の湾曲によって所望の読み取り精度が得られなかったり、未定着のトナー像を擦って乱してしまったりするからである。そこで、本発明者らは、次のようにして、現像直後のトナー像の中間調部平均粒状度を推定してみることにした。即ち、まず、図６に示す２×２（＝４）のドットからなるパターンがマトリクス状に７０個並べられたパターン画像を電子写真方式のプリンタの試験機によって転写紙上にプリントアウト（転写及び定着）した。そして、得られたプリントアウト紙を上述のスキャナーで読み取った後、その電子データに基づいて上述の中間調部平均粒状度を測定した。次いで、電子データ上のマトリクスを図６に示したように格子状に等間隔に分割し、それぞれの分割区域データ（７０個）について、図７に示すように２値化してからトナー付着部の面積を解析して、画像部面積の標準偏差σを算出した。かかる算出を、様々な種類の試験機によるプリントアウト紙についてそれぞれ実施して、標準偏差σと中間調部平均粒状度との関係を調べた。次に、同じパターン画像をそれぞれの試験機で現像した後、感光体から転写紙上に転写する前に機械を止めて数時間放置してから、感光体を試験機本体から取り外した。この転写紙の未定着像を乱さないように、スキャナーのコンタクトガラス上に読み取り箇所に対応する穴の空いた０．１［ｍｍ］厚のフィルムを置き、未定着像とコンタクトガラスとを接触させないように、そのフィルムの上に転写紙を置いてスキャナー読込した。そして、その画像部面積の標準偏差σと中間調部平均粒状度とを調べた後に、先に調べた定着後のデータとともに、全てのデータにおける上記標準偏差σと、中間調部平均粒状度とを２次元平面上にプロットして両者の近似直線を得た。
【００２２】
なお、現像直後のパターン画像を感光体上で数時間放置してからそのσや平均粒状度を測定するのは、次の理由からである。即ち、潜像担持体として感光体を用いる場合には、現像直後のトナー像を担持している感光体を機械内部から外部の明るい場所に移してしまうと、感光体の地肌部（非露光部）の急激な電位変化に伴ってトナーを散らせてしまうことがある。そこで、数時間放置して地肌部の電荷を十分に減衰させてから、感光体を明るい外部に取り出すのである。
【００２３】
上述の近似直線を図８に示す。同図に示すように、定着後の画像に基づく画像部面積の標準偏差σから推定される粒状度と、未定着の画像に基づく画像部面積の標準偏差σから推定される粒状度とは、良好に相関することがわかる。よって、感光体上の現像画像を顕微鏡撮影してその画像部面積の標準偏差σを算出し、算出結果を同図に示したグラフにあてはめることで、感光体上における現像後且つ転写前の画像における中間調部平均粒状度を推定することができる。本明細書では、この推定値を、現像後且つ転写前の画像における中間調部平均粒状度推定値と定義する。
【００２４】
従って、現像後で且つ転写前のトナー像における中間調部平均粒状度推定値を０．１５以下にし、且つ定着後のトナー像における中間調部平均粒状度推定値を０．２５以下にする請求項１乃至６の発明では、定着後の濃度階調のある画像として、低画質感を与えることのない高画質のものを確実に形成することができる。
【００２５】
【発明の実施の形態】
以下、本発明を画像形成装置であるカラー電子写真プリンタ（以下、プリンタという）に適用した一実施形態について説明する。
まず、このプリンタの基本的な構成について説明する。図９は本実施形態に係るプリンタの概略構成図である。図において、潜像を担持する潜像担持体たる感光体１は、アモルファス等からなる有機感光層を表面に有する直径１００［ｍｍ］のドラム状のもので、３３０［ｍｍ／ｓｅｃ］の線速で図中時計回りに回転駆動せしめられる。そして、その表面が帯電チャージャ２によって一様に帯電せしめられた後、レーザ光学装置１６によって画像情報に基づいた走査露光処理によって静電潜像を担持する。この画像情報は、図示しないパーソナルコンピュータ等から送られている。感光体１上に形成された静電潜像は、現像手段たる現像装置２０によって現像されてトナー像となった後、後述の転写ニップにて記録体たる転写紙Ｐ上に静電転写される。
【００２６】
図１０は、感光体１と現像装置２０とを示す拡大構成図である。同図において、感光体１の側方に配設された現像装置２０は、互いに着脱可能に構成されたトナー補給器２１と現像器２５とを備えている。トナー補給器２１は、内部にトナーを収容するトナー収容手段として機能しており、アジテータ２２、歯車状のトナー補給ローラ２３、補給規制板２４等を有している。そして、内部に収容しているトナーをアジテータ２２の回転駆動によってほぐしながら、トナー補給ローラ２３に送る。送られたトナーは、図示しない駆動系によって回転せしめられるトナー補給ローラ２３に連れ回りながら補給規制板２４によってローラ上での厚みが規制された後、現像器２５内に補給される。
【００２７】
上記現像器２５は、現像ロール２６、攪拌パドル２７、攪拌ローラ２８、規制ブレード２９、搬送スクリュー３０、トナー濃度センサ（以下、Ｔセンサという）３１等を有している。また、現像ロール２６の側方に配設されたセパレータ３２も有している。現像器２５内には、トナーと、直径５０［μｍ］の球形フェライトからなる磁性キャリアとを含有する二成分現像剤が収容されている。上記トナー補給器２１から現像器２５内に補給されたトナーは、図示しない駆動系によって回転駆動される攪拌ローラ２８上に落下する。攪拌ローラ２８は、落下してくるトナーを二成分現像剤と混合攪拌しながら、攪拌パドル２７に向けて送る。この際、新たに補給されたトナーが磁性キャリアや攪拌ローラ２８などとの摺擦によって摩擦帯電せしめられる。図示しない駆動系によって回転駆動される攪拌パドル２７は、器内の二成分現像剤を攪拌しながら、現像ロール２６に向けて送る。現像ロール２６は、その表面を上記対向位置にてドラム表面と同方向に６６０［ｍｍ／ｓｅｃ］の線速で移動させるように図示しない駆動系によって回転駆動される直径２５［ｍｍ］の非磁性パイプ２６ａを有している。また、これに連れ回らないようにパイプ内部に固定され、且つ周方向に別れる複数の磁極が形成されたマグネットローラ２６ｂも有している。これら複数の磁極のうち、後述の現像領域との対向位置に存在する現像主磁極のピーク磁力は１２０［ｍＴ］に調整されている。
【００２８】
現像部材たる上記現像ロール２６は、そのケーシングに設けられた開口から周面の一部を露出させて感光体１に対面させるように配設されている。そして、攪拌パドル２７から送られてくる二成分現像剤を、マグネットローラ２６ｂの発する磁力の影響によって非磁性パイプ２６ａの表面に担持する。担持された二成分現像剤は、非磁性パイプ２６ａとともに連れ回りながら、現像ロール２６と所定の間隙を保持するように配設された規制ブレード２９によってパイプ上の層厚が規制される。そして、感光体１との対面位置にである現像領域まで搬送される。非磁性パイプ２６ａには図示しない電源によって現像バイアスが印加されている。この印加により、現像領域では、非磁性パイプ２６ａと感光体１の静電潜像との間において、トナーをパイプ側からドラム側に静電移動させる現像ポテンシャルが作用する。また、非磁性パイプ２６ａと感光体１の非画像部（非潜像部分）との間において、トナーをドラム側からパイプ側に静電移動させる非現像ポテンシャルが作用する。よって、現像領域まで搬送された二成分現像剤は、トナーを感光体１の静電潜像だけに付着させて、静電潜像をトナー像に現像する。現像ロール２６の非磁性パイプ２６ａとの連れ回りに伴って現像領域を通過した二成分現像剤は、非磁性パイプ２６ａとの連れ回りに伴って現像器１０１内に回収される。
【００２９】
上述のように、現像ロール２６の非磁性パイプ２６ａに担持された二成分現像剤は、規制ブレード２９によってパイプ上の層厚が規制される。この規制により、規制ブレード２９よりもパイプ回転方向上流側には、非磁性パイプ２６ａとの連れ回りを阻止された二成分現像剤が滞留する。そして、後続の二成分現像剤に押されることで、現像ロール２６の側方に配設されたセパレータ３２の上に溢れる。溢れた二成分現像剤は、セパレータ３２上面の傾斜に沿って移動することで、搬送スクリュー３０に向けて案内される。搬送スクリュー３０は、案内されてくる二成分現像剤をその軸線方向（図中奥行き方向）に向けて攪拌搬送する。これにより、二成分現像剤のいわゆる横攪拌が行われる。この横攪拌に対し、攪拌ロール２６や、攪拌パドル２７は、二成分現像剤をその回転周方向に搬送しながらかき回すいわゆる縦攪拌を行う。搬送スクリュー３０は、二成分現像剤を横攪拌しながら、攪拌ローラ２８上に落下させる。この落下により、現像器内における二成分現像剤の縦循環が実現している。
【００３０】
上記攪拌ローラ２８の下側には、Ｔセンサ３１が配設されており、攪拌ローラ２８によって攪拌搬送される二成分現像剤の透磁率に応じた信号を図示しない制御部に出力する。二成分現像剤のトナー濃度は、透磁率と相関するため、Ｔセンサ３１は結果として二成分現像剤のトナー濃度を検知していることになる。上記制御部は、Ｔセンサ３１からの出力信号を、所定の目標値に近づけるようにトナー補給器２１を適宜動作させることで、現像に伴ってトナー濃度を低下させた二成分現像剤のトナー濃度を回復させる。但し、二成分現像剤の透磁率が湿度等の環境変化や二成分現像剤の嵩変化などによって変動するため、制御部は上記目標値を適宜補正する。具体的には、所定のタイミングで感光体１上に形成せしめた基準トナー像の画像濃度に応じて、上記目標値を補正する。この画像濃度については、例えば基準トナー像の光反射率を検知する反射型フォトセンサからの出力などによって把握される。
【００３１】
上記感光体１の図中下方には、転写ローラ４等を有する転写手段が配設されている。この転写手段は、図示されている転写ローラ４の他、これを回転駆動させる駆動手段や、転写ローラ４に転写バイアスを印加する図示しない電源などを有している。転写ローラ４は、感光体１に所定の圧力で当接して転写ニップを形成しながら、当接部にてその表面を感光体１表面と同方向に移動させるように回転駆動される。この転写ニップにおいては、上記転写バイアスの影響によって転写電界が形成される。感光体１上で現像されたトナー像は、感光体１の回転に伴って転写ニップに進入する。
【００３２】
上記転写手段の下方には、記録体たる転写紙Ｐを複数枚重ねて収容する複数の給紙カセット１０が互いに鉛直方向に重なるように配設されている。これら給紙カセット１０は、一番上の転写紙Ｐに押し当てている給紙ローラ１０ａを所定のタイミングで回転駆動させて、その転写紙Ｐを給紙搬送路に給紙する。給紙搬送路内では、送り出された転写紙Ｐが複数の搬送ローラ対１１を経た後、レジストローラ対１２のローラ間に挟まれて止まる。レジストローラ対１２は、挟み込んだ転写紙Ｐを、上述のようにして感光体１上に形成されたトナー像に重ね合わせ得るタイミングで上記転写ニップに向けて送り出す。これにより、感光体１上のトナー像と、レジストローラ対１２によって送り出された転写紙Ｐとが転写ニップで同期して密着せしめられる。そして、上述の転写電界やニップ圧（転写圧）の影響によって被押圧体たる転写紙Ｐ上に静電転写される。
【００３３】
上記転写ローラの図中左側方には、２本のローラに張架した紙搬送ベルト１３ａを図中反時計回りに無端移動せしめる紙搬送ユニット１３が配設されている。また、この紙搬送ユニット１３の更に左側方には、定着手段１４、排紙ローラ対１５が順次配設されている。上述のトナー像が静電転写せしめられた転写紙Ｐは、感光体１や転写ローラ４の回転に伴って転写ニップから紙搬送ユニット１３の紙搬送ベルト１３ａ上に送られた後、定着手段１４内に入る。この定着手段１４は、内部にハロゲンランプ等の熱源を有し且つ互いに当接しながら等速で回転する定着ローラ１４ａ対によって定着ニップを形成している。これら定着ローラ１４ａは、それぞれ図示しない表面温度センサによる検知結果に基づいて熱源への電源供給がＯＮ／ＯＦＦ制御されることで、その表面温度が所定値（例えば１６５〜１８５［℃］）に維持される。定着手段１４内に入った転写紙Ｐは、上記定着ニップに挟まれて加熱処理及び加圧処理が施されることで、その表面にトナー像が定着せしめられる。そして、定着手段１４内から排紙ローラ対１５を経て機外へと排出される。
【００３４】
上記転写ニップで転写紙Ｐ上に静電転写されずに感光体１表面に残留した転写残トナーは、感光体クリーナー１７によって感光体１から除去される。このようにしてクリーニングされた感光体１表面は、図示しない除電手段によって除電された後、上記帯電チャージャによって一様帯電せしめられる。また、上記転写ニップで感光体１上から紙搬送ベルト１３ａ上に転移してしまったトナーは、紙搬送ユニット１３のベルトクリーニング装置１３ｂによって紙搬送ベルト１３ａ上から除去される。なお、感光体クリーナー１７は、ステアリン酸亜鉛棒を引っ掻いて得たステアリン酸亜鉛粉末を感光体１表面に塗布するステアリン酸亜鉛塗布手段を有している。クリーニング後の感光体１表面にステアリン酸亜鉛粉末を塗布することで、感光体１の表面摩擦係数を低下させて転写性を向上させることができる。
【００３５】
図１１は、上記転写ニップとその周囲とを示す側面図である。感光体１に向けて押圧される転写ローラ４は、ステンレス、鉄等からなる直径２０〜３０［ｍｍ］の図示しない芯金ローラと、これに被覆されたＥＰＤＭ、シリコーン、ＮＢＲ、ウレタン等からなるソリッド状の第１弾性層４ａとを有している。また、この第１弾性層４ａの上に被覆された第２弾性層４ｂ（第１弾性層よりも柔らかい）や、上記芯金ローラの両端面に突設せしめられた軸４ｃなども有している。両端の軸４ｃは、それぞれ軸受１８によって回動可能に支持されており、これら軸受１８はバネ１９によって感光体１に向けて付勢されている。この付勢により、転写ローラ４が感光体１に向けて押圧される。
【００３６】
上記第２弾性層４ｂは、厚み、硬度（ＡｓｋｅｒＣ／１ｋｇ荷重時）、体積固有抵抗が、それぞれ０．１［ｍｍ］、２５［度］、１×１０^９〜１×１０^１１［Ωｃｍ］に調整されている。また、上記第１弾性層４ａは、厚み、硬度（ＡｓｋｅｒＣ／１ｋｇ荷重時）、体積固有抵抗が、それぞれ２．０［ｍｍ］、７０［度］、第２弾性層４ｂよりも一桁小さい値に調整されている。第２弾性層４ｂの硬度が１５［度］未満になると、永久変形を引き起こし易くなってしまう。また、第２弾性層４ｂの硬度が４０［度］を超えると、弾性変形による上記エアーギャップの低減効果が急激に得られ難くなってくる。また、第１弾性層４ａの硬度が６０［度］未満で且つその厚みが０．５［ｍｍ］未満になると、転写ニップで転写紙Ｐと感光体１との密着性を向上させる効果が急激に弱まり始める。
【００３７】
本プリンタに用いられるトナーとしては、従来から公知の製造法によって製造されたものを用いることができる。例えば、粉砕法によるものを用いることができる。即ち、結着樹脂、磁性体、離型剤、着色剤、必要に応じて帯電制御剤等をミキサー等を用いて混合し、熱ロール、エクストルーダー等の混練機を用い混練する。そして、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で粉砕した後、分級して得られたトナーである。また例えば、重合法によって製造されたトナーでもよい。なかでも、変性ポリエステル樹脂を基材として用いた重合法によって製造されたトナーが好適である。
【００３８】
図１２は、感光体１と、これに向けて十分な圧力で押圧される転写ローラ４とによって形成される転写ニップを示す拡大模式図である。同図に示すように、転写ローラ４が感光体１に向けて十分な圧力で押圧されてなる転写ニップでは、転写ローラ４の第１弾性層４ａや第２弾性層４ｂが柔軟に弾性変形する。この弾性変形により、転写紙Ｐが、感光体１表面に担持されているトナー像Ｉの表層に接触するだけでなく、互いに隣り合っているトナー像Ｉの間の凹部に食い込むように押圧されて、感光体１表面やトナー像Ｉとの密着性が高められる。このことにより、感光体１と転写紙Ｐとの間に形成されるエアーギャップが低減されて、転写ニップ内やニップ前後における転写チリが抑えられる。
【００３９】
次に、本プリンタの特徴的な構成について説明する。
本発明者らは、以下に説明する実験例の実験結果に基づいて、本プリンタの特徴的な構成を創作するに至った。即ち、まず、Ａ〜Ｆという６種類のトナーを製造した。
【００４０】
トナーＡについては、次のようにして製造した。
［トナーバインダーの合成］
冷却管、攪拌機および窒素導入管の付いた反応槽中に、ビスフェノールＡエチレンオキサイド２モル付加物７２４重量部、イソフタル酸２７６重量部及びジブチルチンオキサイド２重量部を入れた。そして、常圧且つ２３０［℃］の条件下で８時間重縮合反応させた後、更に１０〜１５［ｍｍＨｇ］まで減圧して５時間反応させた。次いで、１６０［℃］まで冷却した後、３２重量部の無水フタル酸を加えて２時間反応させた。そして、更に８０［℃］まで冷却した後、酢酸エチル中にてイソフォロンジイソシアネート１８８重量部と２時間反応させてイソシアネート含有プレポリマーを得た。次に、このイソシアネート含有プレポリマー２６７重量部と、イソホロンジアミン１４重量部とを５０［℃］で２時間反応させて、重量平均分子量６４０００のウレア変性ポリエステル（１）を得た。一方、既に述べた手法と同様にして、ビスフェノールＡエチレンオキサイド２モル付加物７２４重量部と、テレフタル酸２７６重量部とを常圧且つ２３０［℃］の条件下で８時間重縮合反応させた。そして、１０〜１５［ｍｍＨｇ］まで減圧して５時間反応させて、ピーク分子量５０００の変性されていないポリエステル（ａ）を得た。次いで、酢酸エチルとメチルエチルケトン（以下、ＭＥＫという）との１：１混合溶剤を用意した。そして、上述のウレア変性ポリエステル（１）２００重量部と、上述の変性されていないポリエステル（ａ）８００重量部とを、この混合溶剤に溶解、混合して、トナーバインダー（Ａ）の溶液を得た。次に、これを一部減圧乾燥して、トナーバインダー（Ａ）を単離した。そのガラス転移温度（以下、Ｔｇという）、酸化は、それぞれ６２［℃］、１０であった。
【００４１】
［トナーの合成］
ビーカー内に上述のトナーバインダー（Ａ）の溶液２４０重量部と、ペンタエリスリトールテトラベヘネート（融点８１℃、溶融粘度２５ｃｐｓ）２０重量部と、カーボンブラック１０重量部とを入れた。そして、６０［℃］の温度環境下にてＴＫ式ホモミキサーを用いて１２０００［ｒｐｍ］の速度で攪拌して、それらを均一に溶解、分散させた。これをトナー材料溶液とした。次いで、他のビーカー内にイオン交換水７０６重量部と、ハイドロキシアパタイト１０％懸濁液（日本化学工業社製のスーパタイト１０）２９４重量部と、ドデシルベンゼンスルホン酸ナトリウム０．２重量部とを入れて均一に溶解した。そして、これを６０［℃］まで昇温せしめた後、ＴＫ式ホモミキサーにて１２０００［ｒｐｍ］の速度で攪拌しながら、上述のトナー材料溶液を投入して１０分間攪拌した。次に、この混合液を攪拌棒及び温度計の付設されたコルベンに移し、９８［℃］まで昇温せしめて一部溶剤を除去した。そして、室温に戻してから上記ＴＫ式ホモミキサーにて１２０００［ｒｐｍ］の速度で攪拌して、トナーの形状を整えた後、更に溶剤を完全に除去した。その後、濾別、洗浄、乾燥した後、風力分級して、母体トナー粒子を得た。更に、この母体トナー粒子１００重量部に疎水性シリカ０．５重量部をヘンシェルミキサーにて混合して、トナーＡを得た。このトナーＡの形状係数ＳＦ−１は１４０、平均円形度は０．９２、分散度は１．３９、凝集度は２５［％］であった。
【００４２】
なお、形状係数ＳＦ−１は、粒子の丸さの度合いを示す指標であり、次のようにして求めることができる。即ち、例えば日立製作所製ＦＥ−ＳＥＭ（Ｓ−８００）などの顕微装置を用いて１０００倍の拡大観察エリアを得る。そして、この拡大観察エリアにて、トナー粒子を無作為に１００個サンプリングしながらその画像を順次撮影していく。そして、得られた撮影画像の電子データを、インターフェース経由で例えばニコレ社製ＬｕｚｅｘＩＩＩなどの画像解析装置に転送して、各粒子について径の絶対最大長ＭＸＬＮＧと投影面積ＡＲＥＡとを解析してそれらの平均値を算出する。この絶対最大長ＭＸＬＮＧとは、図１３に示すように、トナー粒子の２次元投影画像において径が最大となる箇所の長さのことである。真の楕円であれば、長径の長さとなる。解析された絶対最大長ＭＸＬＮＧと投影面積ＡＲＥＡとを次に示す式に代入して、トナー粒子１００個あたりの平均を算出すれば、形状係数ＳＦ−１を求めることができる。真球の形状係数ＳＦ−１は１００である。
【数４】
ＳＦ−１＝（ＭＸＬＮＧ）^２／ＡＲＥＡ×（π／４）×１００
【００４３】
また、トナーの平均円形度については、株式会社ＳＹＳＭＥＸ製フロー式粒子像分析装置ＦＰＩＡ−２１００を用いて、次のようにして測定した。即ち、１級塩化ナトリウムを用いて１％ＮａＣｌ水溶液を調整した後、これを０．４５［μｍ］のフィルターで濾過する。得られた濾液５０〜１００［ｍｌ］に分散剤として界面活性剤、好ましくはアルキルベンゼンスルフォン酸塩を０．１〜５ｍｌ加えた後、これに試料（トナー粉体）を１〜１０［ｍｇ］加える。更に、超音波分散機にてトナー分散処理を１分間施して、５０００〜１５０００［個／μｌ］のトナー濃度の被検材料を得た。この被検材料中のトナーをＣＣＤカメラで撮像して得た２次元画像のトナー粒子面積と同一の面積を有する円の直径を円相当径として求める。そして、この円相当径で０．６［μｍ］以上のトナー粒子をＣＣＤの撮影精度に鑑みた有効被検粒子としてその円形度を算出した。具体的には、ＣＣＤカメラによる２次元のトナー粒子像と同じ投影面積をもつ円の周囲長を投影像の周囲長で除算することによって円形度を算出した。そして、各トナー粒子の円形度の累積値を全トナー粒子数で除算することで平均円形度を求めた。
【００４４】
また、トナーの分散度については、トナーの重量平均径を個数平均径で除算することで求めた。これらの径については、Ｃｏｕｌｔｅｒ ＭＵＬＴＩＳＩＺＥＲ ２ｅを用い、これに１００［μｍ］の径のアパーチャーをセットして測定した。
【００４５】
また、トナーの凝集度については、パウダテスター（ホソカワミクロン株式会社製ＰＴ−Ｎ型）を用いて測定した。この測定における操作方法については基本的にテスターの取り扱い説明書に従ったが、次に列記する点については変更した。
・使用ふるい：７５μｍ、４５μｍ、２２μｍの３通りについて試験した
・振動時間：３０秒
【００４６】
トナーＢについては、次のようにして製造した。
［トナーバインダーの合成］
ビスフェノールＡエチレンオキサイド２モル付加物３３４重量部と、ビスフェノールＡプロピレンオキサイド２モル付加物３３４重量部と、イソフタル酸２７４重量部と、無水トリメリット酸２０重量部とを混ぜ合わせた。そして、トナーＡと同様にして、重縮合反応させた後、更にイソホロンジイソシアネート１５４重量部を反応させてプレポリマーを得た。そして、このプレポリマー２１３重量部と、イソホロンジアミン９．５重量部と、ジブチルアミン０．５重量部とをトナーＡと同様に反応させて、重量平均分子量７９０００のウレア変性ポリエステル（２）を得た。次いで、このウレア変性ポリエステル（２）２００重量部と、上述の変性されていないポリエステル（ａ）８００重量部とを、酢酸エチルとＭＥＫとの１：１混合溶剤２０００重量部に溶解、混合して、トナーバインダー（Ｂ）の溶液を得た。そして、これを一部減圧乾燥して、トナーバインダー（Ｂ）を単離した。そのピーク分子量は５０００、Ｔｇは６２℃、酸価は１０であった。
【００４７】
［トナーの合成］
溶解温度や分散温度を５０［℃］に変えた点の他は、トナーＡと同様にして、母体トナー粒子を得た。そして、この母体トナー粒子１００重量部に、サリチル酸誘導体の亜鉛塩からなる帯電制御剤１．０重量部を混合し、加温雰囲気中で攪拌て粒子表面に帯電制御剤を固着させた。次に、このトナー粒子１００重量部に疎水性シリカ１．０重量部と疎水性酸化チタン０．５重量部とをヘンシェルミキサーにて混合して、トナーＢを得た。このトナーＢの形状係数ＳＦ−１は１３０、平均円形度は０．９２、分散度は１．３７、凝集度は２４［％］であった。
【００４８】
トナーＣについては、次のようにして製造した。
［トナーバインダーの合成］
上述のウレア変性ポリエステル（１）３０重量部と、上述の変性されていないポリエステル（ａ）９７０重量部とを、酢酸エチルとＭＥＫとの１：１混合溶剤２０００重量部に溶解、混合した。これによって得たトナーバインダー（Ｂ）の溶液を一部減圧乾燥して、トナーバインダー（Ｃ）を単離した。そのピーク分子量は５０００、Ｔｇは６２℃、酸価は１０であった。
【００４９】
［トナーの合成］
トナーバインダー（Ｃ）を用い、且つ、着色剤としてカーボンブラック８重量部を用いた点の他は、トナーＢと同様にしてトナーＣを得た。このトナーＣの形状係数ＳＦ−１は１２５、平均円形度は０．９６、分散度は１．３５、凝集度は２２％であった。
【００５０】
トナーＤについては、次のようにして製造した。
［トナーバインダーの合成］
上述のウレア変性ポリエステル（１）５００重量部と、上述の変性されていないポリエステル（ａ）５００重量部とを、酢酸エチルとＭＥＫとの１：１混合溶剤２０００重量部に溶解、混合した。これによって得たトナーバインダー（Ｄ）の溶液を一部減圧乾燥して、トナーバインダー（Ｄ）を単離した。そのピーク分子量は５０００、Ｔｇは６２［℃］、酸価は１０であった。
【００５１】
［トナーの合成］
トナーバインダー（Ｄ）を用い、且つ、着色剤としてカーボンブラック８重量部を用いた点の他は、トナーＡと同様にしてトナーＤを得た。このトナーＤの形状係数ＳＦ−１は１２０、平均円形度は０．９７、分散度は１．２１、凝集度は２２［％］であった。
【００５２】
トナーＥについては、次のようにして製造した。
［トナーバインダーの合成］
上述のウレア変性ポリエステル（１）７５０重量部と、上述の変性されていないポリエステル（ａ）２５０重量部とを、酢酸エチルとＭＥＫとの１：１混合溶剤２０００重量部に溶解、混合した。これによって得たトナーバインダー（Ｅ）の溶液を一部減圧乾燥して、トナーバインダー（Ｅ）を単離した。そのピーク分子量は５０００、Ｔｇは６２［℃］、酸価は１０であった。
【００５３】
［トナーの合成］
トナーバインダー（Ｅ）を用いた点の他は、トナーＡと同様にしてトナーＥを得た。このトナーＥの形状係数ＳＦ−１は１１５、平均円形度は０．９７、分散度は１．２０、凝集度は１８［％］であった。
【００５４】
トナーＦについては、次のようにして製造した。
［トナーの合成］
上記トナーバインダー（Ｅ）の母体トナー粒子１００重量部に対し、疎水性シリカ１．５重量部をヘンシェルミキサーにて混合して、トナーＦを得た。このトナーＦの形状係数ＳＦ−１は１１５、平均円形度は０．９７、分散度は１．２０、凝集度は７［％］であった。
【００５５】
重量平均粒径５０［μｍ］の球形フェライトの表面に、シリコーン樹脂をコートした後、熱乾燥して磁性キャリアを得た。そして、上述の６種類のトナーを、それぞれこの磁性キャリアと混合して６種類の二成分現像剤を得た。なお、トナーと磁性キャリアとの混合比率については、３．０〜５．０［ｗ％］に調整した。
【００５６】
株式会社リコー社製の電子写真プリンタ（Ｉｍａｇｉｏ ＮＥＯ７５０）を改造して、図９に示した構成と同様のプリンタ試験機を製造した。そして、上述の６種類の二成分現像剤をそれぞれ用いて、このプリンタ試験機でグレースケール画像（図１参照）を現像した。そして、それが転写紙Ｐ上に静電転写される前にプリント動作を中断させて、感光体１上における中間調部平均粒状度推定値を先に述べた方法と同様にして求めた。
【００５７】
次に、上述の６種類の二成分現像剤をそれぞれ用いて同様にしてグレースケール画像を現像した後、転写紙Ｐ上に静電転写した。但し、その転写紙Ｐを上記定着手段１４に進入させる前に試験機のプリント動作を停止させて、未定着のグレースケール画像を担持した転写紙Ｐ（以下、未定着転写紙という）を得た。同様の実験を、４段階の転写ニップ圧条件について、それぞれ転写電流条件を４段階に変化させて行って、合計で９６枚の未定着転写紙を得た（トナー６種類×転写ニップ圧条件４段階×転写電流条件４段階）。なお、転写ニップ圧条件は、０．０４、０．２０、１．００、２．００［Ｎ／ｍｍ^２］の４段階である。また、転写電流条件は、１０、２０、２００、４００［ｎＡ／ｍｍ^２］の４段階である。
【００５８】
得られた９６枚の未定着転写紙について、それぞれグレースケール画像の中間調部平均粒状度を測定した。このとき、グレースケール画像が未定着であるため、そのままではスキャナーで読み取らせる際に擦って画質を乱してしまうおそれがある。そこで、まず、測定用の穴のあいた約０．１［ｍｍ］厚のフィルムを用意して、これを未定着転写紙の画像担持面に密着させた後、フィルム密着面をスキャナー（ハイデンベルグ社製Ｎｅｘｓｃａｎ４１００）の原稿載置面に密着させた。このように、フィルムをスペーサとして機能させてグレースケール画像の測定対象領域を原稿載置面に接触させないようにして、１２００［ｄｐｉ］の解像度で読み込んだ。そして、得られた電子データに基づいて、転写後且つ定着前のグレースケール画像の中間調部平均粒状度を求めた。
【００５９】
また、次のようにして、転写後且つ定着前のグレースケール画像の転写率も求めた。即ち、まず、感光体１から転写紙Ｐにグレースケール画像が静電転写された時点でプリント動作を停止させ、それまでそのグレースケール画像が担持されていた感光体１領域に残留しているトナーを粘着テープで収集した。そして、この粘着テープの重量を測定し、この測定値から、トナー収集前に予め測定しておいた粘着テープだけの重量を差し引いた値を転写残トナー量とした。次に、トナー像が転写された転写紙Ｐを画像のある箇所で切り抜いて紙片とした後、その重量を測定した。そして、その紙片のグレースケール画像に圧縮エアーを吹きかけてトナーをほぼ全量吹き飛ばした後、再び紙片の重量を測定し、前者の重量から後者の重量を差し引いた値を、転写トナー量とした。このようにして求めた転写残トナー量と転写トナー量とを加算した値を、全トナー量とした。そして、次式とに基づいて転写率を求めた。
【数５】
転写率＝転写トナー量／全トナー量×１００［％］
【００６０】
次いで、上述の９６枚の未定着転写紙をそれぞれ上記定着手段１４に通して、プリントアウト紙を得た。更に、定着条件を変化させながら同様のプリントアウト紙を得た。先のプリントアウト紙のときと合わせると、合計で３段階の定着条件についてテストして、２８８枚のプリントアウト紙を得たのである。なお、これら定着条件については、定着ローラ１４ａ対のうち、トナー像に密着する方（請求項に記載した加熱部材として機能する方）として、次に列記するものに順次変更することで、３段階に変化させた。
▲１▼芯金ローラ上に厚み１ｍｍ、硬度２５［度］（アスカーＣ／１ｋｇ荷重時）のシリコーンゴムからなる表面層を設けたもの（以下、シリコーン表面ローラという）。
▲２▼芯金ローラ上に厚み２００［μｍ］のシリコーンゴムからなる中間層を設け、その上に厚み２０［μｍ］の四フッ化エチレン樹脂（Ｐｏｌｙｔｅｔｒａｆｌｕｏｒｏｅｔｈｙｌｅｎｅ）からなる表面層を設けたもの。以下、これをテフ（商標名テフロンの略）表面弾性ローラという。また、このローラの芯金ローラ上における２層合成硬度は、７０［度］（アスカーＣ／１ｋｇ荷重時）であった。
▲３▼芯金ローラ上に四フッ化エチレン樹脂からなる表面層を設けたもの（以下、テフ表面剛性ローラという）。そして、得られたプリントアウト紙に基づいて、定着後のグレースケール画像における中間調部平均粒状度を測定した。
【００６１】
次に示す表１は、トナーＡの性状と、このトナーＡを用いて得たグレースケール画像の現像後（転写前）における中間調部平均粒状度推定値とを示すものである。また、次に示す表２、表３、表４、表５、表６は、それぞれトナーＢ、Ｃ、Ｄ、Ｅ、Ｆについて、その性状と、グレースケール画像の現像後（転写前）における中間調部平均粒状度推定値との関係を示すものである。但し、これらには、転写率、転写後且つ定着前における中間調部平均粒状度、定着後における中間調部平均粒状度も関連付けて示している。
【表２】

【表３】

【表４】

【表５】

【表６】

【表７】

【００６２】
表２〜表７の間で、形状係数ＳＦ−１、平均円形度及び分散度と、感光体１上の現像後且つ転写前のグレースケール画像における中間調部平均粒状度推定値とを比較してみると、次のようなことがわかる。即ち、トナーとして、形状係数ＳＦ−１の小さなものを用いるほど、トナー像をザラツキ感のないものとして現像することができる。また、平均円形度が高いものを用いるほど、トナー像をザラツキ感のないものとして現像することができる。また、分散度の小さいものを用いるほど、トナー像をザラツキ感のないものとして現像することができる。よって、現像後且つ転写前のトナー像におけるザラツキ感をできるだけ抑えるためには、できるだけ形状係数ＳＦ−１が小さく、平均円形度が高く且つ分散度が小さいトナーを用いることが望ましい。
【００６３】
表２に示したように、最も条件の悪いトナーＡでも、現像後且つ転写前のトナー像（グレースケール画像）として、中間調部平均粒状度推定値が０．２５を大きく下回る０．１８のものを得ることができる。しかし、表２には示していないが、トナーＡを用いた場合、最終的な定着後のグレースケール画像として、中間調部平均粒状度０．２５以下のものを得ることはできなかった。また、表２に示したように、トナーＢを用いた場合、現像後且つ転写前の画像として中間調部平均粒状度推定値０．１７以下のものを得ることができる。しかし、これでも最終的な定着後の画像として、中間調部平均粒状度０．２５以下のものを得ることはできなかった。最終的な定着後の画像として、中間調部平均粒状度０．２５以下のものを得るためには、表４〜表７に示したように、現像後且つ転写前の画像として、中間調部平均粒状度推定値０．１５以下のものを得なければならないことがわかる。
【００６４】
表２〜表７で、静電転写による粒状度劣化値を比較してみると、次のようなことがわかる。即ち、静電転写だけに着目すれば、トナーの形状係数ＳＦ−１は転写後且つ定着前のトナー像における中間調部平均粒状度にそれほど影響を与えないことがわかる。また、平均円形度や分散度と、静電転写による粒状度劣化値とを比較してみても、静電転写だけに着目すれば、平均円形度や分散度は中間調部平均粒状度にそれほど影響を与えないことがわかる。静電転写の前段階の現像工程で形状係数ＳＦ−１、平均円形度、分散度がそれぞれ大きく影響しているので、結果として転写後且つ転写前の画像の中間調部平均粒状度が大きく異なってくるだけである。よって、静電転写だけに着目すれば、トナーの形状係数ＳＦ−１、平均円形度及び分散度はそれほど重要ではない。
【００６５】
これに対し、表３〜表７で転写ニップ圧や転写電流と、静電転写による粒状度劣化値とを比較してみると、前者が後者に大きな影響を与えることがわかる。具体的には、転写ニップ圧、転写電流の何れも、低すぎたり高すぎたりすると、転写後のグレースケール画像の中間調部平均粒状度を大きく悪化させてしまう。
【００６６】
転写ニップ圧が低すぎると転写後のグレースケール画像の中間調部平均粒状度が大きく悪化する理由は、次のように考えられる。即ち、既に述べたように、静電転写時においては、転写紙Ｐの画像部の周囲に少量のトナー粒子が付着して画像を乱す現象（以下、転写チリという）が少なからず起こる。従来、この転写チリは転写紙Ｐが転写ニップに挟まれていない状態において、転写ニップの前後で感光体１上のトナー像から少量のトナーが飛散して転写ニップに挟まれていない転写紙Ｐに付着することによるものと考えられていた。ところが、本発明者らは鋭意研究により、転写ニップの前後にて感光体１上のトナー像からトナーが飛散していないにもかかわらず、転写工程を経た後の転写紙Ｐ上に転写チリを引き起こしている現象を認めた。このことは、転写ニップ内においても転写チリの発生があることを示唆している。その理由は、転写ニップ内で微小ギャップが形成されることによるものと考えられる。具体的には、転写ニップ内において、感光体１表面のトナー担持領域が転写紙Ｐと密着していても、トナー担持領域の間のトナー非担持領域が転写紙Ｐに密着していないことがある。そして、このトナー非担持領域と転写紙Ｐとの間に微小ギャップが形成され、ここで転写チリが起こっていると考えられる。そこで、本プリンタでは、上記転写ローラ４として弾性層（第１弾性層４ａ、第２弾性層４ｂ）を設けたものを用いている。そして、転写ニップにて、上記トナー担持領域とトナー非担持領域とによって形成される微妙な凹凸に追従させて弾性層を柔軟に変形させることで、上述の微小ギャップの形成を抑えるようにしている。しかしながら、せっかく上記弾性層を設けていても、転写ニップ圧を低く設定し過ぎると、それを柔軟に変形させることができず、上述の微小ギャップにて相当量の転写チリが発生してしまう。このことが転写ニップ圧を低く設定し過ぎると、転写後のグレースケール画像の中間調部平均粒状度を大きく悪化させる原因であると考えられるのである。
【００６７】
また、転写ニップ圧が高すぎても転写後のグレースケール画像の中間調部平均粒状度が大きく悪化する理由は、次の通りである。即ち、転写ニップにおいては、トナー像の表面で感光体１に接触しているトナー粒子のうち、その下のトナー粒子とともに転写紙Ｐ側に転移しないで感光体１上に残ってしまうものが少なからず発生する。このトナー粒子の発生量は、転写ニップ圧が高まるほど多くなる傾向にあり、多くなり過ぎると転写後のトナー像中に白抜け部分を発生させるいわゆる版画という現象を引き起こしてしまう。転写ニップ圧が高すぎると、この版画がザラツキ感を視認させる程度まで悪化させてしまうのである。
【００６８】
また、転写電流が低すぎると転写後のグレースケール画像の中間調部平均粒状度が大きく悪化する理由は、次の通りである。即ち、転写率は転写電流が大きくなるほど高くなる。そして、転写電流が低すぎると、ザラツキ感を与えない程度の量のトナー転写量が得られなくなり、中間調部平均粒状度を大きく悪化させてしまうのである。
【００６９】
また、転写電流が高すぎても転写後のグレースケール画像の中間調部平均粒状度が大きく悪化する理由は、次の通りである。即ち、転写電流と、上述の転写チリの量とにも相関関係があり、前者が高くなるほど後者が多くなってくる。そして、転写電流が高すぎると、ザラツキ感を大きく与える程度の転写チリ量を発生させてしまうのである。
【００７０】
よって、転写後且つ定着前のトナー像にて、ザラツキ感を与えることのない中間調部平均粒状度０．２５以下の画質を得るためには、次のようにしなければならない。即ち、トナーとして適切な性状のものを用いて、現像後の中間調部平均粒状度推定値をできるだけ良好な現像を行うとともに、適切な転写ニップ圧及び転写電流で静電転写を行う必要がある。
【００７１】
また、表３〜表７に示したように、転写後且つ定着前のトナー像は、転写紙Ｐに定着されるときにも、中間調部平均粒状度を劣化させることがわかる。定着に伴ってトナー像がつぶれたり、光沢を帯びたりすることによるものと考えられる。トナー像に密着させる方の定着ローラとしては、上記▲１▼のものがその劣化を最も抑え、上記▲３▼のものが最も劣化を大きくすることがわかる。これらのことを検討すると、最終的に（定着後に）中間調部平均粒状度０．２５以下のトナー像を得るためには、次に掲げる条件１〜３の何れかを具備させなければならないことがわかる。
［条件１］
・トナーとして、形状係数ＳＦ−１が１２５以下、平均円形度０．９６以上、及び分散度１．３５以下という条件を具備するものを用いる。
・転写電流を２０〜２００［ｎＡ／ｍｍ^２］に設定する。
・転写ニップ圧を０．２０〜１．００［Ｎ／ｍｍ^２］に設定する。
・定着ローラ対１４ａのうち、トナー像に密着させる方として、上記▲１▼のものを用いる。
［条件２］
・トナーとして、形状係数ＳＦ−１が１２０以下、平均円形度０９７以上、及び分散度１．２１以下という条件を具備するものを用いる。
・転写電流を２０〜２００［ｎＡ／ｍｍ^２］に設定する。
・転写ニップ圧を０．２０〜１．００［Ｎ／ｍｍ^２］に設定する。
・定着ローラ対１４ａのうち、トナー像に密着させる方として、上記▲１▼のものを用いる。
［条件３］
・トナーとして、形状係数ＳＦ−１が１１５以下、平均円形度０９７以上、及び分散度１．２０以下という条件を具備するものを用いる。
・転写電流を２０〜２００［ｎＡ／ｍｍ^２］に設定する。
・転写ニップ圧を０．２０〜１．００［Ｎ／ｍｍ^２］に設定する。
・定着ローラ対１４ａのうち、トナー像に密着させる方として、上記▲１▼又は▲２▼のものを用いる。
【００７２】
そこで、本プリンタにおいては、トナーとして、条件１〜条件３の何れかに記載されたものを用いるようにユーザーに対して指定している。また、転写電流の値を２０〜２００［ｎＡ／ｍｍ^２］に設定するとともに、転写ニップ圧を０．２０〜１．００［Ｎ／ｍｍ^２］に設定している。更に、条件１又は２に記載されたトナーを用いるように指定した場合には、定着ローラ対１４ａのうちトナー像に密着させる方として上記▲１▼のものを設けている。一方、条件３に記載されたトナーを用いるように指定した場合には、かかるローラとして、上記▲１▼又は▲２▼のものを設けている。よって、指定したトナーが用いられる限り、濃度階調のある画像として、定着後の状態で低画質感を与えることのない高画質のものを確実に形成することができる。なお、トナーの指定については、例えば、条件１〜３の何れかに記載されたトナーをプリンタ（画像形成装置）とともに梱包して出荷することによって行えばよい。また例えば、かかるトナーの製品番号や商品名などを、プリンタ本体やこの取扱説明書などに明記することによって行ってもよい。また例えば、ユーザーに対して書面や電子データ等をもって上記製品番号や商品名などを通知することによって行ってもよい。また例えば、かかるトナーを内部のトナー収容手段にセットした状態でプリンタ本体を出荷することによって行ってもよい。
【００７３】
参考までに、重量平均粒径４．２、６．８、９．０［μｍ］のトナーによる転写後且つ定着前で、それぞれ中間調部平均粒状度が０．２０、０．４９、０．９０となったグレースケール画像の拡大模式図を、それぞれ図１４、図１５、図１６に示す。また、定着時における粒状度劣化値が０．０４、０．１０、０．１５となったグレースケール画像の画像部の拡大模式図を、それぞれ図１７、図１８、図１９に示す。
【００７４】
以上、実施形態に係るプリンタにおいては、現像後且つ転写前のトナー像の中間調部平均粒状度推定値が０．１５以下であり、且つ、定着後のトナー像の中間調部平均粒状度が０．２５以下であるので、定着後の濃度階調のある画像として、低画質感を与えることのない高画質のものを確実に形成することができる。
更に、上記条件１〜３の何れかを具備させるようにしている。よって、それぞれの条件で指定したトナーが用いられる限り、低画質感を与えることのない高画質のものを確実に形成することができる。
【００７５】
【発明の効果】
請求項１乃至６の発明によれば、定着後の濃度階調のある画像であって、低画質感を与えることのない画像を確実に形成することができるという優れた効果がある。
【図面の簡単な説明】
【図１】本発明者らの実験に用いられたグレースケール画像のディスプレイ上における映像を示す模式図。
【図２】同映像における面積率階調中心付近箇所の拡大図。
【図３】同面積率階調中心付近箇所におけるスケール像の拡大図。
【図４】グレースケール画像の各階調における平均明度ＬとＲＭＳ粒状度σＤとの関係を示すグラフ。
【図５】テストプリントされたグレースケール画像に対するザラツキ感の主観的評価と、中間調部平均粒状度と、全階調の粒状度の平均値との関係を示すグラフ。
【図６】２×２のドットからなるパターンがマトリクス状に７０個並べられたパターン画像を示す模式図。
【図７】同パターン画像をパターン毎に等間隔で分割する操作を説明する模式図。
【図８】画像の画像部面積の標準偏差σと、中間調部平均粒状度との関係を示すグラフ。
【図９】実施形態に係るプリンタの概略構成図。
【図１０】同プリンタの感光体と現像装置とを示す拡大構成図。
【図１１】同プリンタの転写ニップとその周囲とを示す側面図。
【図１２】同プリンタの感光体と、これに向けて十分な圧力で押圧される転写ローラとによって形成される転写ニップを示す拡大模式図。
【図１３】形状係数ＳＦ−１の算出方法を説明するための模式図。
【図１４】重量平均粒径４．２［μｍ］のトナーによる転写後且つ定着前のグレースケール画像を示す拡大模式図。
【図１５】重量平均粒径６．８［μｍ］のトナーによる転写後且つ定着前のグレースケール画像を示す拡大模式図。
【図１６】重量平均粒径９．０［μｍ］のトナーによる転写後且つ定着前のグレースケール画像を示す拡大模式図。
【図１７】定着時における粒状度劣化値が０．０４グレースケール画像の画像部の拡大模式図。
【図１８】定着時における粒状度劣化値が０．１０グレースケール画像の画像部の拡大模式図。
【図１９】定着時における粒状度劣化値が０．１５グレースケール画像の画像部の拡大模式図。
【符号の説明】
１ 感光体（潜像担持体）
４ 転写ローラ（転写手段の一部）
１４ 定着手段
１４ａ 定着ローラ対
２０ 現像装置（現像手段）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming method for forming an image by an electrophotographic method and an image forming apparatus using the same.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, as an image forming method for forming an image by an electrophotographic method, those described in Patent Document 1 and Patent Document 2 are known. For example, an image is formed by the following process. That is, first, an exposure process or the like is performed on a latent image carrier such as a photoconductor to carry an electrostatic latent image, and then the electrostatic latent image is developed by electrostatic adhesion of toner. Next, after the toner image obtained by the development is electrostatically transferred to a recording medium such as transfer paper, the toner image is heated and fixed to the recording medium by close contact with a heating member such as a fixing roller.
[0003]
Such an electrophotographic image forming method has a merit that an image can be easily formed based on digitized image information, but has a demerit that the image quality is inevitably deteriorated as compared with the offset printing method. In particular, in an image having a density gradation such as a photograph or a picture, the roughness is more noticeable than that obtained by the offset printing method, and it is easy to give a viewer an impression that the image quality is low. For this reason, in the electrophotographic method, how to suppress the low image quality is an important issue.
[0004]
An RMS granularity standardized in “ANSI PH-2.40-1985” is known as an index representing the roughness of an image, and is calculated based on the following equation.
(Equation 1)
RMS granularity σD = [(1 / N) × Σ (Di-D) ² ] ^1/2
* However, N: number of data, Di: density distribution, D: average density (D = 1 / NΣDi)
[0005]
Further, as an index indicating the roughness, a granularity GS defined by Xerox's Dooley and Shaw is also known. This is a numerical value obtained by integrating a cascade value of “Winner Spectrum (hereinafter, referred to as WS (f)”) and a visual spatial frequency characteristic (VTF: Visual Transfer Function). WS (f) is a set average of the square of the Fourier spectrum obtained by Fourier-transforming the density fluctuation from the average density obtained by scanning the image with a microdensitometer. The granularity GS is obtained by the following equation (the details are described in "Dooley, Rshaw Noise Perception in Electrophotography, J. Appl. Photogr. Eng. 5, 4 (1979), pp 190-196)".
(Equation 2)
Granularity GS = exp (−1.8D) ∫ (WS (f)) ^1/2 VTF (f) df
* However, D: average density, f: spatial frequency [c / mm], VTF (f): visual spatial frequency characteristic
[Patent Document 1]
JP 2002-202638 A [Patent Document 2]
JP 2002-287545 A
[Problems to be solved by the invention]
However, even if the images are printed out by the same image forming apparatus, some of these images have relatively good RMS granularity σD and granularity GS, and some do not. For this reason, it has been difficult to evaluate the performance of the image forming apparatus based on the RMS granularity σD and the granularity GS of the printed out image. In addition, sufficient consideration has not been given to what kind of image does not give a feeling of low image quality. Further, at present, there is no electrophotographic image forming apparatus that is commercially available that can reliably form an image without giving a low image quality.
[0008]
The present invention has been made in view of the above background, and an object of the present invention is to reliably form an image having a density gradation without giving a feeling of low image quality. An object of the present invention is to provide an electrophotographic image forming method. Another object of the present invention is to provide an electrophotographic image forming apparatus capable of reliably forming such an image.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 includes a step of supporting a latent image on a latent image carrier, a step of developing the latent image on the latent image carrier using toner, and a step of developing the latent image. A step of electrostatically transferring the toner image on the carrier to the recording medium, and a step of fixing the toner image on the recording medium by bringing a heating member into close contact with the toner image electrostatically transferred on the recording medium. Performing an image forming method of forming a toner image on the recording medium, wherein the estimated halftone average granularity of the toner image after development and before electrostatic transfer is 0.15 or less; Further, the average granularity of the halftone portion of the toner image after fixing is 0.25 or less.
Further, the invention of claim 2 provides a latent image carrier for carrying a latent image, developing means for developing a latent image on the latent image carrier using toner, and a toner image on the latent image carrier. An image forming apparatus comprising: a transfer unit that electrostatically transfers a toner image onto a recording medium; and a fixing unit that fixes a toner image on the recording material by bringing a heating member into close contact with the toner image electrostatically transferred onto the recording material. The estimated halftone portion granularity of the toner image after the development and before the electrostatic transfer is 0.15 or less, and the halftone portion average granularity of the toner image after fixing is 0.15. 25 or less.
The invention according to claim 3 is the image forming apparatus according to claim 2, wherein the toner has a shape factor SF-1 of 125 or less, an average circularity of 0.96 or more, and a dispersion degree of 1.35 or less. And the transfer unit transfers a transfer current of ²⁰ to 200 [nA / mm ² ] between the latent image carrier and a pressing member that presses the recording body toward the latent image carrier. While pressing the recording medium toward the latent image carrier at a pressure of 0.20 to 1.00 [N / mm ² ], the toner image on the latent image carrier is The electrostatic transfer is performed, and the fixing unit uses, as the pressing member, a member whose surface is covered with silicon rubber.
According to a fourth aspect of the present invention, there is provided the image forming apparatus according to the second aspect, further comprising a toner storage unit configured to store a toner used for developing a latent image on the latent image carrier. The toner accommodating unit accommodates the condition that the coefficient SF-1 is 125 or less, the average circularity is 0.96 or more, and the dispersity is 1.35 or less. The recording medium is moved toward the latent image carrier by a distance of 0.20 to 200 [nA / mm ² ] while a transfer current of ²⁰ to 200 [nA / mm ² ] is passed between the recording medium and the pressing member for pressing the recording medium. The toner image on the latent image carrier is electrostatically transferred to the recording body while being pressed at a pressure of 1.00 [N / mm ² ], and the fixing unit is used as the pressing member. Using silicon rubber coated on the surface It is characterized by the following.
The invention according to claim 5 is the image forming apparatus according to claim 2, wherein the toner has a condition that a shape factor SF-1 is 120 or less, an average circularity is 097 or more, and a dispersion degree is 1.21 or less. The transfer unit applies a transfer current of ²⁰ to 200 [nA / mm ² ] between the latent image carrier and a pressing member that presses the recording body toward the latent image carrier. While pressing the recording medium toward the latent image carrier at a pressure of 0.20 to 1.00 [N / mm ² ], the toner image on the latent image carrier is electrostatically applied to the recording medium. The transfer is performed, and the fixing unit uses, as the pressure member, one whose surface is coated with silicon rubber.
The invention according to claim 6 is the image forming apparatus according to claim 2, wherein the toner has a condition that a shape factor SF-1 is 115 or less, an average circularity is 097 or more, and a dispersion degree is 1.20 or less. The transfer unit applies a transfer current of ²⁰ to 200 [nA / mm ² ] between the latent image carrier and a pressing member that presses the recording body toward the latent image carrier. While pressing the recording medium toward the latent image carrier at a pressure of 0.20 to 1.00 [N / mm ² ], the toner image on the latent image carrier is electrostatically applied to the recording medium. The fixing means may be one in which silicon rubber is coated on the surface, or a surface layer made of ethylene tetrafluoride resin (Polytetrafluoroethylene) is coated on the elastic layer. Was It is characterized by using a thing.
[0010]
The present inventors have conducted intensive studies as described below and completed these inventions. That is, first, grayscale electronic data having 15 area ratio gradations obtained by dithering 106 screen lines of 600 [dpi] shown in FIG. 1 were prepared. The fifteen area ratio gradations are composed of area ratios of 3, 6, 9, 12.5, 16, 20, 25, 30, 41, 50, 59, 70, 80, 91, and 100 [%]. I have. FIG. 2 is an enlarged view of the area near the center of the area ratio gradation (area ratio = 41%) in the gray scale image on the display of the personal computer based on the electronic data. FIG. 3 shows an enlarged scale image near the center of the area ratio gradation.
[0011]
Next, the present inventors print out a grayscale image based on the above-mentioned electronic data using the first tester of an electrophotographic printer, and print out the average lightness L and the RMS granularity σD for each area ratio. And measured. Similarly, using a No. 2 test machine of an electrophotographic printer, a gray scale image was printed out in the same manner, and the average lightness L and the RMS granularity σD were measured for each area ratio (gradation on the display). I tried. Although the resolution of the second tester is 600 dpi, which is the same as that of the first tester, it has been found from a preliminary investigation that the roughness of the printout image is larger than that of the first tester. I have. The average lightness L is an average value of the read values L *.
[0012]
FIG. 4 is a graph showing the relationship between the average lightness L and the RMS granularity σD at each gradation of the gray scale image printed out by the first and second testers. As shown in the figure, it can be seen that there is no noticeable difference in the RMS granularity σD between the two grayscale images where the average lightness L is less than 20. Further, it can be seen that there is no noticeable difference in the RMS granularity σD between the portions where the average lightness L exceeds 80. These are due to the following reasons. That is, in a digital printout image in which a density gradation is expressed by a difference in the density of a repetitive pattern in an image, one of the factors affecting the graininess of the image is that a small amount of toner particles adhere irregularly around the image portion. That would be. This irregular attachment of toner particles is likely to occur when the density of the repetitive pattern is medium. When the density of the repetitive pattern exceeds a predetermined upper limit value, it becomes visible to the human eye as a solid portion, and the image portion (one pattern) and the non-image portion (between patterns) in the solid portion are identified. Becomes difficult. For this reason, irregular adhesion of the toner particles to the periphery of the image area is less likely to be visually recognized as roughness. On the other hand, when the density of the repetitive pattern is below a predetermined lower limit, the distance between the patterns becomes considerably large, and irregular adhesion of toner particles appears not to be contaminated between the patterns but to be captured in the patterns. As a result, it is difficult to visually recognize it as a rough feeling. Therefore, in the digital printout image, regardless of whether toner particles are irregularly attached around the image portion inside the digital printout image, the gradation portion where the average lightness L is less than 20 or the average lightness L is 80 That is, it is difficult to give a rough feeling to a portion where the tone exceeds. In other words, regardless of the performance of the electrophotographic image forming apparatus, good image quality without graininess can be obtained at a gradation position where the average lightness L is less than 20 or a gradation position where the average lightness L is more than 80. Can be
[0013]
On the other hand, in the two grayscale images, there is a large difference in the RMS granularity σD between gray scale portions where the average lightness L is 20 to 80 (hereinafter, referred to as a halftone portion). It can be seen that the first tester clearly outputs a good pattern with less roughness (a pattern with a small RMS granularity σD). As described above, the rough feeling mainly occurs in the halftone portion where the average lightness L is 20 to 80. Therefore, even if an image is printed out by the same image forming apparatus, an image having a relatively small area ratio of the halftone portion has good image quality, but an image having a relatively large area ratio has a low grainy feeling. It will be image quality. Similar results were obtained when the granularity GS was obtained instead of the RMS granularity σD. The reason why the RMS granularity σD and the granularity GS may be relatively good and not good even if the images are printed out by the same image forming apparatus is as described above. It was found that the difference was due to the difference in the area ratio.
[0014]
From these facts, in order to accurately know the performance of the electrophotographic image forming apparatus, it is not necessary to evaluate the roughness of the entire printed out image, but to evaluate the halftone part (average lightness L20 to 80). It can be said that it is necessary to evaluate the feeling of roughness alone.
[0015]
Next, the present inventors decided to evaluate the roughness of the grayscale image using an index different from the RMS granularity σD and the granularity GS. Specifically, first, the output grayscale image was read at a resolution of 1200 [dpi] by a scanner (Nexscan 4100 manufactured by Heidenberg). Then, the average lightness L and the granularity at each area ratio were examined. The granularity was calculated based on the following instead of the RMS granularity σD and the granularity GS described above. The average lightness L is an average value of the read values L *.
[Equation 3]
Granularity = exp (aL + b) ∫ (WS _L (f)) ^1/2 VTF (f) df + c
* However, L: average lightness, f: spatial frequency [c / mm], WS _L (f): power spectrum of lightness fluctuation, VTF (f): visual spatial frequency characteristic, a: coefficient (= 0.1044) ), B: coefficient (= 0.8944), c: coefficient (= −0.262)
[0016]
The average lightness L is used in place of the density D, the NWS is calculated in two dimensions, and then one-dimensionalized to evaluate the roughness. According to this equation, it was possible to obtain a roughness index having better linearity in a color space and adaptability to a color image than the above-described RMS granularity σD or granularity GS using the density D. The details of the granularity are described in “Japan Hardcopy '96”, Collection of Papers p189, “Noise Evaluation Method for Halftone Color Images”.
[0017]
Table 1 shows an example of the relationship between the area ratio of the image portion, the average lightness L, and the granularity according to the above equation (3).
[Table 1]

[0018]
Also in Table 1, it can be seen that the granularity of the portion having the average brightness of 40 to 80 is larger than the granularity of the other portions. In Table 1, the average value of the granularity is 0.32. On the other hand, when the average of only the six data (shown in bold) whose average lightness L is in the range of 40 to 80 is calculated, it becomes 0.43. A difference of 0.1 or more appears.
[0019]
Next, the average lightness L and the granularity according to the above equation (3) were measured in the same manner for the grayscale images printed out by various testing machines of the image forming apparatus. Then, the relationship between the result of averaging the granularity for all of the 15 area ratio gradations and the result of averaging the granularity of only the portion where the average brightness becomes 40 to 80 (hereinafter referred to as a halftone portion) is examined. saw. Further, for each grayscale image, a subjective evaluation of roughness was performed by a plurality of testers. The result is shown in FIG. In this figure, the larger the numerical value of the rank of roughness (1 to 5), the better the image is without roughness. As shown in the figure, in the evaluation method in which the granularity is averaged for all 15 area ratio gradations, the correlation between the rank of the roughness feeling and the average value is poor (correlation coefficient = 0.7527), and the index of the roughness feeling is used. It is understood that it is not suitable as. On the other hand, in the evaluation method in which the granularity is averaged only for the halftone portion, the correlation between the rank of the roughness and the average value is extremely good (correlation coefficient = 0.9124), and is excellent as an index of the roughness. You can see that it is. In this specification, this average value is defined as the halftone portion average granularity. The present inventors have conducted intensive studies and found that when the average granularity of the halftone portion was 0.25 or less, the graininess was not given. Therefore, if the image has an average gray level of 0.25 or less after being fixed on a recording medium such as a transfer sheet, the image does not give a low quality impression to humans.
[0020]
On the other hand, in general, in an electrophotographic image forming apparatus, a toner image immediately after development is formed by causing a small amount of toner particles to adhere irregularly around an image portion of a recording medium when the toner image is transferred to a recording medium such as transfer paper. Degrades image quality. Further, when the toner image transferred onto the recording medium is fixed thereto by close contact with the heating member, phenomena such as crushing of toner particles, glossiness, and spread of an attached area are caused to deteriorate image quality. Therefore, in order to obtain a fixed toner image that does not give a feeling of low image quality to humans, it is basically necessary to obtain a toner image having a halftone portion average granularity of 0.25 or less at the time of development. There is.
[0021]
In order to evaluate whether or not the toner image has just been developed, it is necessary to determine the average granularity of the halftone portion. To this end, the toner image must be read on a latent image carrier such as a photoreceptor by a reading means such as a scanner and the image information must be digitized. However, it is very difficult to read the toner image on the latent image carrier. This is because a desired reading accuracy cannot be obtained due to the curvature of the surface of the latent image carrier, or an unfixed toner image is rubbed and disturbed. Therefore, the present inventors have tried to estimate the average granularity of the halftone portion of the toner image immediately after development as follows. That is, first, a pattern image in which 70 patterns of 2 × 2 (= 4) dots shown in FIG. 6 are arranged in a matrix is printed out (transferred and fixed) on a transfer paper by a tester of an electrophotographic printer. )did. Then, after the obtained printout paper was read by the above-mentioned scanner, the above-mentioned halftone portion average granularity was measured based on the electronic data. Next, the matrix on the electronic data is divided at regular intervals in a grid pattern as shown in FIG. 6, and each divided area data (70 pieces) is binarized as shown in FIG. The area was analyzed to calculate the standard deviation σ of the image area. These calculations were performed for printout papers of various types of test machines, respectively, and the relationship between the standard deviation σ and the halftone portion average granularity was examined. Next, after developing the same pattern image in each testing machine, the machine was stopped and allowed to stand for several hours before transferring from the photoconductor to the transfer paper, and then the photoconductor was removed from the testing machine main body. In order not to disturb the unfixed image of the transfer paper, place a 0.1 mm thick film with a hole corresponding to the reading position on the contact glass of the scanner so that the unfixed image does not contact the contact glass. As described above, a transfer paper was placed on the film and scanned by a scanner. Then, after examining the standard deviation σ of the image portion area and the halftone portion average granularity, together with the data after fixing checked earlier, the standard deviation σ in all data, the halftone portion average granularity and Was plotted on a two-dimensional plane to obtain an approximate line of both.
[0022]
The reason why the σ and the average granularity are measured after the pattern image immediately after development is left on the photoconductor for several hours is as follows. In other words, when a photoconductor is used as the latent image carrier, if the photoconductor carrying the toner image immediately after development is moved from the inside of the machine to a bright place outside, the background portion of the photoconductor (non-exposed portion) The toner may be scattered due to the rapid change in the potential in the above case. Therefore, the photoconductor is taken out to a bright outside after being left for several hours to sufficiently attenuate the electric charge of the background portion.
[0023]
The above-mentioned approximate straight line is shown in FIG. As shown in the figure, the granularity estimated from the standard deviation σ of the image area based on the fixed image and the granularity estimated from the standard deviation σ of the image area based on the unfixed image are: It can be seen that the correlation is good. Therefore, the developed image on the photoconductor is microscopically photographed, the standard deviation σ of the image area is calculated, and the calculated result is applied to the graph shown in FIG. Can be estimated. In the present specification, this estimated value is defined as an estimated value of the halftone portion average granularity in an image after development and before transfer.
[0024]
Therefore, the estimated value of the average granularity of the halftone portion in the toner image after development and before the transfer is set to 0.15 or less, and the estimated value of the average granularity of the halftone portion in the toner image after fixing is set to 0.25 or less. According to the inventions of the items 1 to 6, it is possible to reliably form a high-quality image without giving a low image quality as an image having a density gradation after fixing.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment in which the present invention is applied to a color electrophotographic printer (hereinafter, referred to as a printer) as an image forming apparatus will be described.
First, a basic configuration of the printer will be described. FIG. 9 is a schematic configuration diagram of the printer according to the present embodiment. In the figure, a photosensitive member 1 serving as a latent image carrier for carrying a latent image is a drum having a diameter of 100 [mm] having an organic photosensitive layer made of amorphous or the like on its surface, and has a linear velocity of 330 [mm / sec]. Is rotated clockwise in the figure. Then, after the surface is uniformly charged by the charging charger 2, the laser optical device 16 carries an electrostatic latent image by a scanning exposure process based on image information. This image information is sent from a personal computer (not shown) or the like. The electrostatic latent image formed on the photoconductor 1 is developed into a toner image by a developing device 20 as a developing unit, and is then electrostatically transferred onto a transfer paper P as a recording body at a transfer nip described later. .
[0026]
FIG. 10 is an enlarged configuration diagram illustrating the photoconductor 1 and the developing device 20. In FIG. 1, a developing device 20 disposed on the side of the photoconductor 1 includes a toner replenisher 21 and a developing device 25 which are detachable from each other. The toner replenishing device 21 functions as a toner storing unit that stores toner therein, and includes an agitator 22, a gear-shaped toner replenishing roller 23, a replenishing regulating plate 24, and the like. Then, the toner is sent to the toner supply roller 23 while loosening the toner contained therein by the rotation of the agitator 22. The fed toner is supplied to the developing device 25 after the thickness on the roller is regulated by the supply regulating plate 24 while being rotated by the toner supply roller 23 rotated by a drive system (not shown).
[0027]
The developing device 25 includes a developing roller 26, a stirring paddle 27, a stirring roller 28, a regulating blade 29, a conveying screw 30, a toner density sensor (hereinafter, referred to as a T sensor) 31, and the like. Further, it also has a separator 32 disposed on the side of the developing roll 26. The developing device 25 contains a two-component developer containing a toner and a magnetic carrier made of a spherical ferrite having a diameter of 50 [μm]. The toner replenished from the toner replenisher 21 into the developing device 25 falls on a stirring roller 28 that is driven to rotate by a drive system (not shown). The stirring roller 28 sends the falling toner toward the stirring paddle 27 while mixing and stirring the toner with the two-component developer. At this time, the newly supplied toner is frictionally charged by rubbing with the magnetic carrier, the stirring roller 28 and the like. A stirring paddle 27 rotated and driven by a drive system (not shown) sends the two-component developer in the container toward the developing roll 26 while stirring. The developing roll 26 is driven by a drive system (not shown) to rotate the surface of the developing roller 26 at a linear speed of 660 [mm / sec] in the same direction as the surface of the drum at the facing position. It has a pipe 26a. Further, it has a magnet roller 26b which is fixed inside the pipe so as not to rotate along with it and in which a plurality of magnetic poles separated in the circumferential direction are formed. Among these plural magnetic poles, the peak magnetic force of the main developing magnetic pole located at a position facing a later-described developing area is adjusted to 120 [mT].
[0028]
The developing roller 26 serving as a developing member is disposed so that a part of the peripheral surface is exposed from an opening provided in the casing and faces the photoconductor 1. Then, the two-component developer sent from the stirring paddle 27 is carried on the surface of the non-magnetic pipe 26a under the influence of the magnetic force generated by the magnet roller 26b. While the carried two-component developer rotates together with the non-magnetic pipe 26a, the thickness of the layer on the pipe is regulated by the regulating blade 29 disposed so as to maintain a predetermined gap with the developing roll 26. Then, the sheet is conveyed to a developing area located at a position facing the photoconductor 1. A developing bias is applied to the non-magnetic pipe 26a by a power source (not shown). By this application, in the developing area, a developing potential for electrostatically moving toner from the pipe side to the drum side acts between the non-magnetic pipe 26a and the electrostatic latent image on the photosensitive member 1. Further, a non-development potential for electrostatically moving toner from the drum side to the pipe side acts between the non-magnetic pipe 26a and the non-image portion (non-latent image portion) of the photoconductor 1. Therefore, the two-component developer transported to the developing area causes the toner to adhere to only the electrostatic latent image on the photoconductor 1, and develops the electrostatic latent image into a toner image. The two-component developer that has passed through the developing area with the non-magnetic pipe 26a along with the non-magnetic pipe 26a is collected in the developing device 101 with the non-magnetic pipe 26a.
[0029]
As described above, the thickness of the two-component developer carried on the nonmagnetic pipe 26 a of the developing roll 26 is regulated by the regulating blade 29 on the pipe. Due to this regulation, the two-component developer, which is prevented from rotating together with the non-magnetic pipe 26a, stays upstream of the regulation blade 29 in the pipe rotation direction. Then, by being pressed by the subsequent two-component developer, the developer overflows onto the separator 32 disposed on the side of the developing roll 26. The overflowed two-component developer moves along the slope of the upper surface of the separator 32 and is guided toward the transport screw 30. The transport screw 30 agitates and transports the guided two-component developer in the axial direction (the depth direction in the figure). Thereby, so-called lateral stirring of the two-component developer is performed. In contrast to the horizontal stirring, the stirring roll 26 and the stirring paddle 27 perform so-called vertical stirring in which the two-component developer is stirred while being conveyed in the rotational circumferential direction. The transport screw 30 drops the two-component developer onto the stirring roller 28 while laterally stirring. By this drop, the vertical circulation of the two-component developer in the developing device is realized.
[0030]
A T sensor 31 is provided below the stirring roller 28, and outputs a signal corresponding to the magnetic permeability of the two-component developer stirred and conveyed by the stirring roller 28 to a control unit (not shown). Since the toner concentration of the two-component developer correlates with the magnetic permeability, the T sensor 31 detects the toner concentration of the two-component developer as a result. The control unit operates the toner replenisher 21 as appropriate so that the output signal from the T sensor 31 approaches a predetermined target value, thereby reducing the toner concentration of the two-component developer with the development. To recover. However, since the magnetic permeability of the two-component developer fluctuates due to environmental changes such as humidity and changes in the bulk of the two-component developer, the control unit appropriately corrects the target value. Specifically, the target value is corrected according to the image density of the reference toner image formed on the photoconductor 1 at a predetermined timing. The image density is grasped by, for example, an output from a reflection type photo sensor for detecting the light reflectance of the reference toner image.
[0031]
A transfer unit having a transfer roller 4 and the like is provided below the photosensitive member 1 in the drawing. The transfer unit includes, in addition to the illustrated transfer roller 4, a driving unit that rotationally drives the transfer roller 4, a power supply (not illustrated) that applies a transfer bias to the transfer roller 4, and the like. The transfer roller 4 is rotationally driven so that the surface of the transfer roller 4 is moved in the same direction as the surface of the photoconductor 1 at the abutting portion while the transfer roller 4 is in contact with the photoconductor 1 at a predetermined pressure to form a transfer nip. In this transfer nip, a transfer electric field is formed by the influence of the transfer bias. The toner image developed on the photoconductor 1 enters the transfer nip as the photoconductor 1 rotates.
[0032]
Below the transfer means, a plurality of paper feed cassettes 10 for accommodating a plurality of transfer papers P, which are recording bodies, are arranged so as to vertically overlap each other. The paper feed cassette 10 rotates the paper feed roller 10a pressed against the uppermost transfer paper P at a predetermined timing to feed the transfer paper P to the paper feed path. In the paper feed path, the transferred transfer paper P passes through a plurality of pairs of transport rollers 11 and is stopped between the rollers of the pair of registration rollers 12. The registration roller pair 12 sends the sandwiched transfer paper P toward the transfer nip at a timing at which the transfer paper P can be superimposed on the toner image formed on the photoconductor 1 as described above. As a result, the toner image on the photosensitive member 1 and the transfer paper P sent out by the registration roller pair 12 are brought into close contact with each other in synchronization with the transfer nip. Then, the image is electrostatically transferred onto the transfer sheet P, which is a pressed body, under the influence of the above-described transfer electric field and nip pressure (transfer pressure).
[0033]
On the left side of the transfer roller in the figure, a paper transport unit 13 for endlessly moving a paper transport belt 13a stretched over two rollers in a counterclockwise direction in the figure is provided. Further, on the left side of the paper transport unit 13, a fixing unit 14 and a pair of paper discharge rollers 15 are sequentially disposed. The transfer paper P on which the toner image has been electrostatically transferred is sent from the transfer nip to the paper transport belt 13a of the paper transport unit 13 with the rotation of the photoconductor 1 and the transfer roller 4, and then the fixing means 14 Get in. The fixing means 14 has a heat source such as a halogen lamp therein and forms a fixing nip by a pair of fixing rollers 14a which rotate at a constant speed while being in contact with each other. The surface temperature of these fixing rollers 14a is maintained at a predetermined value (for example, 165 to 185 [° C.]) by ON / OFF control of power supply to a heat source based on a detection result of a surface temperature sensor (not shown). Is done. The transfer paper P entering the fixing means 14 is subjected to a heat treatment and a pressure treatment while being sandwiched by the fixing nip, so that the toner image is fixed on the surface thereof. Then, the sheet is discharged from the inside of the fixing unit 14 to the outside of the apparatus via the sheet discharge roller pair 15.
[0034]
The untransferred toner remaining on the surface of the photoconductor 1 without being electrostatically transferred onto the transfer paper P at the transfer nip is removed from the photoconductor 1 by the photoconductor cleaner 17. The surface of the photoconductor 1 thus cleaned is neutralized by a neutralizing unit (not shown), and then uniformly charged by the charging charger. Further, the toner transferred from the photosensitive member 1 to the paper transport belt 13a at the transfer nip is removed from the paper transport belt 13a by the belt cleaning device 13b of the paper transport unit 13. The photoreceptor cleaner 17 has a zinc stearate coating unit that applies a zinc stearate powder obtained by scratching a zinc stearate rod to the surface of the photoreceptor 1. By applying the zinc stearate powder to the surface of the photoconductor 1 after the cleaning, the surface friction coefficient of the photoconductor 1 can be reduced, and the transfer property can be improved.
[0035]
FIG. 11 is a side view showing the transfer nip and its surroundings. The transfer roller 4 pressed toward the photoreceptor 1 is made of a core metal roller (not shown) made of stainless steel, iron, or the like and having a diameter of 20 to 30 [mm], and EPDM, silicone, NBR, urethane, or the like coated thereon. And a solid first elastic layer 4a. Also, a second elastic layer 4b (softer than the first elastic layer) coated on the first elastic layer 4a, and shafts 4c protruding from both end surfaces of the cored roller are provided. I have. The shafts 4 c at both ends are rotatably supported by bearings 18, and these bearings 18 are urged toward the photoconductor 1 by springs 19. With this bias, the transfer roller 4 is pressed toward the photoconductor 1.
[0036]
The second elastic layer 4b has a thickness, a hardness (under AskerC / 1 kg load), and a volume resistivity of 0.1 [mm], 25 [degrees], 1 × 10 ⁹ to 1 × 10 ¹¹ [Ωcm], respectively. Has been adjusted. The first elastic layer 4a has a thickness, a hardness (under AskerC / 1 kg load), and a volume specific resistance of 2.0 [mm] and 70 [degrees], respectively, one digit smaller than the second elastic layer 4b. Has been adjusted. When the hardness of the second elastic layer 4b is less than 15 [degrees], permanent deformation is likely to occur. On the other hand, when the hardness of the second elastic layer 4b exceeds 40 [degrees], the effect of reducing the air gap due to elastic deformation becomes difficult to be rapidly obtained. When the hardness of the first elastic layer 4a is less than 60 [degrees] and the thickness thereof is less than 0.5 [mm], the effect of improving the adhesion between the transfer paper P and the photoconductor 1 at the transfer nip is sharply increased. Begins to weaken.
[0037]
As the toner used in the present printer, a toner manufactured by a conventionally known manufacturing method can be used. For example, a pulverization method can be used. That is, a binder resin, a magnetic substance, a release agent, a colorant, and if necessary, a charge control agent and the like are mixed using a mixer or the like, and kneaded using a kneader such as a hot roll or an extruder. Then, the toner is cooled and solidified, pulverized by pulverization using a jet mill, turbojet, kryptron, or the like, and then classified to obtain a toner. Further, for example, a toner manufactured by a polymerization method may be used. Among them, a toner produced by a polymerization method using a modified polyester resin as a base material is preferable.
[0038]
FIG. 12 is an enlarged schematic diagram illustrating a transfer nip formed by the photoconductor 1 and the transfer roller 4 pressed with a sufficient pressure toward the photoconductor 1. As shown in the figure, in the transfer nip where the transfer roller 4 is pressed against the photoconductor 1 with a sufficient pressure, the first elastic layer 4a and the second elastic layer 4b of the transfer roller 4 are elastically deformed flexibly. . Due to this elastic deformation, the transfer paper P is pressed not only to come into contact with the surface layer of the toner image I carried on the surface of the photoreceptor 1 but also to bite into a concave portion between the toner images I adjacent to each other. Thus, the adhesion to the surface of the photoconductor 1 and the toner image I is improved. As a result, the air gap formed between the photoconductor 1 and the transfer paper P is reduced, and transfer dust in the transfer nip and before and after the nip is suppressed.
[0039]
Next, the characteristic configuration of the printer will be described.
The present inventors have created a characteristic configuration of the present printer based on experimental results of experimental examples described below. That is, first, six types of toners A to F were manufactured.
[0040]
The toner A was manufactured as follows.
[Synthesis of toner binder]
In a reaction vessel equipped with a cooling pipe, a stirrer, and a nitrogen introducing pipe, 724 parts by weight of bisphenol A ethylene oxide 2 mol adduct, 276 parts by weight of isophthalic acid, and 2 parts by weight of dibutyltin oxide were put. Then, after performing a polycondensation reaction for 8 hours under the condition of normal pressure and 230 [° C.], the pressure was further reduced to 10 to 15 [mmHg], and the reaction was performed for 5 hours. Next, after cooling to 160 ° C., 32 parts by weight of phthalic anhydride was added and reacted for 2 hours. After further cooling to 80 ° C., the mixture was reacted with 188 parts by weight of isophorone diisocyanate in ethyl acetate for 2 hours to obtain an isocyanate-containing prepolymer. Next, 267 parts by weight of the isocyanate-containing prepolymer and 14 parts by weight of isophoronediamine were reacted at 50 ° C. for 2 hours to obtain a urea-modified polyester (1) having a weight average molecular weight of 64000. On the other hand, in the same manner as described above, 724 parts by weight of bisphenol A ethylene oxide 2 mol adduct and 276 parts by weight of terephthalic acid were subjected to a polycondensation reaction at normal pressure and 230 [° C.] for 8 hours. Then, the pressure was reduced to 10 to 15 [mmHg] and the reaction was carried out for 5 hours to obtain an unmodified polyester (a) having a peak molecular weight of 5000. Next, a 1: 1 mixed solvent of ethyl acetate and methyl ethyl ketone (hereinafter, referred to as MEK) was prepared. Then, 200 parts by weight of the urea-modified polyester (1) and 800 parts by weight of the unmodified polyester (a) are dissolved and mixed in the mixed solvent to obtain a solution of the toner binder (A). Was. Next, this was partially dried under reduced pressure to isolate the toner binder (A). Its glass transition temperature (hereinafter referred to as Tg) and oxidation were 62 [° C.] and 10, respectively.
[0041]
[Synthesis of Toner]
In a beaker, 240 parts by weight of the above-mentioned solution of the toner binder (A), 20 parts by weight of pentaerythritol tetrabehenate (melting point: 81 ° C., melt viscosity: 25 cps), and 10 parts by weight of carbon black were placed. Then, the mixture was stirred at a speed of 12000 [rpm] using a TK homomixer under a temperature environment of 60 [° C.] to uniformly dissolve and disperse them. This was used as a toner material solution. Next, 706 parts by weight of ion-exchanged water, 294 parts by weight of a 10% suspension of hydroxyapatite (Supatite 10 manufactured by Nippon Chemical Industry Co., Ltd.), and 0.2 part by weight of sodium dodecylbenzenesulfonate were placed in another beaker. And dissolved uniformly. After the temperature was raised to 60 [° C.], the above-described toner material solution was charged and stirred for 10 minutes while stirring at a speed of 12000 [rpm] using a TK homomixer. Next, this mixed solution was transferred to a kolben provided with a stirring rod and a thermometer, and the temperature was raised to 98 ° C. to partially remove the solvent. Then, after returning to room temperature, the mixture was stirred at a speed of 12000 [rpm] with the TK homomixer to adjust the shape of the toner, and then the solvent was completely removed. Then, after filtering, washing, and drying, the particles were classified by air to obtain base toner particles. Further, 0.5 parts by weight of hydrophobic silica was mixed with 100 parts by weight of the base toner particles using a Henschel mixer to obtain a toner A. The toner A had a shape factor SF-1 of 140, an average circularity of 0.92, a dispersity of 1.39, and a cohesion of 25 [%].
[0042]
The shape factor SF-1 is an index indicating the degree of roundness of particles, and can be obtained as follows. That is, for example, a magnification observation area of 1000 times is obtained by using a microscope such as FE-SEM (S-800) manufactured by Hitachi, Ltd. Then, in this enlarged observation area, the images are sequentially photographed while randomly sampling 100 toner particles. Then, the obtained electronic data of the captured image is transferred to an image analysis device such as LuzexIII manufactured by Nicole via an interface, and the absolute maximum length MXLNG of the diameter and the projected area AREA are analyzed for each particle to analyze those. Calculate the average value. The absolute maximum length MXLNG is, as shown in FIG. 13, the length of the portion where the diameter becomes maximum in the two-dimensional projected image of the toner particles. If it is a true ellipse, it will be the length of the major axis. By substituting the analyzed absolute maximum length MXLNG and the projected area AREA into the following equation and calculating the average per 100 toner particles, the shape factor SF-1 can be obtained. The shape factor SF-1 of the true sphere is 100.
(Equation 4)
SF-1 = (MXLNG) ² / AREA × (π / 4) × 100
[0043]
The average circularity of the toner was measured as follows using a flow type particle image analyzer FPIA-2100 manufactured by SYSMEX CORPORATION. That is, a 1% NaCl aqueous solution is prepared using primary sodium chloride, and then filtered through a 0.45 [μm] filter. A surfactant (preferably 0.1 to 5 ml of an alkylbenzene sulfonate) is added as a dispersant to 50 to 100 [ml] of the obtained filtrate, and then 1 to 10 [mg] of a sample (toner powder) is added thereto. . Further, a toner dispersion treatment was performed for 1 minute by an ultrasonic disperser to obtain a test material having a toner concentration of 5000 to 15000 [particles / μl]. The diameter of a circle having the same area as the toner particle area of a two-dimensional image obtained by imaging the toner in the test material with a CCD camera is determined as a circle equivalent diameter. The circularity of the toner particles having an equivalent circle diameter of 0.6 [μm] or more was calculated as effective test particles in consideration of the imaging accuracy of the CCD. Specifically, the circularity was calculated by dividing the perimeter of a circle having the same projected area as the two-dimensional toner particle image by the CCD camera by the perimeter of the projected image. Then, the average circularity was determined by dividing the cumulative value of the circularity of each toner particle by the total number of toner particles.
[0044]
Further, the degree of dispersion of the toner was determined by dividing the weight average diameter of the toner by the number average diameter. These diameters were measured using a Coulter MULTIZER 2e with an aperture having a diameter of 100 [μm] set therein.
[0045]
The degree of aggregation of the toner was measured using a powder tester (PT-N type manufactured by Hosokawa Micron Corporation). The operation method in this measurement basically followed the tester's instruction manual, but the following points were changed.
Sieve used: 75 μm, 45 μm, and 22 μm were tested. Vibration time: 30 seconds.
The toner B was manufactured as follows.
[Synthesis of toner binder]
334 parts by weight of bisphenol A ethylene oxide 2 mol adduct, 334 parts by weight of bisphenol A propylene oxide adduct, 274 parts by weight of isophthalic acid, and 20 parts by weight of trimellitic anhydride were mixed. Then, a polycondensation reaction was carried out in the same manner as in the toner A, and 154 parts by weight of isophorone diisocyanate was further reacted to obtain a prepolymer. Then, 213 parts by weight of the prepolymer, 9.5 parts by weight of isophoronediamine, and 0.5 part by weight of dibutylamine are reacted in the same manner as in the case of toner A to obtain a urea-modified polyester (2) having a weight average molecular weight of 79,000. Was. Next, 200 parts by weight of the urea-modified polyester (2) and 800 parts by weight of the unmodified polyester (a) are dissolved and mixed in 2000 parts by weight of a 1: 1 mixed solvent of ethyl acetate and MEK. Thus, a solution of the toner binder (B) was obtained. Then, this was partially dried under reduced pressure to isolate the toner binder (B). Its peak molecular weight was 5000, Tg was 62 ° C., and acid value was 10.
[0047]
[Synthesis of Toner]
Base toner particles were obtained in the same manner as in toner A, except that the melting temperature and the dispersion temperature were changed to 50 [° C]. Then, 1.0 part by weight of a charge control agent composed of a zinc salt of a salicylic acid derivative was mixed with 100 parts by weight of the base toner particles, and the mixture was stirred in a warm atmosphere to fix the charge control agent on the particle surface. Next, 1.0 part by weight of hydrophobic silica and 0.5 part by weight of hydrophobic titanium oxide were mixed with 100 parts by weight of the toner particles using a Henschel mixer to obtain a toner B. The toner B had a shape factor SF-1 of 130, an average circularity of 0.92, a dispersity of 1.37, and an agglomeration of 24 [%].
[0048]
The toner C was manufactured as follows.
[Synthesis of toner binder]
30 parts by weight of the urea-modified polyester (1) and 970 parts by weight of the unmodified polyester (a) were dissolved and mixed in 2000 parts by weight of a 1: 1 mixed solvent of ethyl acetate and MEK. The resulting solution of the toner binder (B) was partially dried under reduced pressure to isolate the toner binder (C). Its peak molecular weight was 5000, Tg was 62 ° C., and acid value was 10.
[0049]
[Synthesis of Toner]
A toner C was obtained in the same manner as the toner B, except that the toner binder (C) was used and 8 parts by weight of carbon black was used as a coloring agent. The shape factor SF-1 of this toner C was 125, the average circularity was 0.96, the degree of dispersion was 1.35, and the degree of aggregation was 22%.
[0050]
The toner D was manufactured as follows.
[Synthesis of toner binder]
500 parts by weight of the urea-modified polyester (1) and 500 parts by weight of the unmodified polyester (a) were dissolved and mixed in 2000 parts by weight of a 1: 1 mixed solvent of ethyl acetate and MEK. The resulting solution of the toner binder (D) was partially dried under reduced pressure to isolate the toner binder (D). Its peak molecular weight was 5000, Tg was 62 [° C.], and acid value was 10.
[0051]
[Synthesis of Toner]
A toner D was obtained in the same manner as the toner A, except that the toner binder (D) was used and 8 parts by weight of carbon black was used as a coloring agent. The shape factor SF-1 of this toner D was 120, the average circularity was 0.97, the degree of dispersion was 1.21, and the degree of aggregation was 22%.
[0052]
The toner E was manufactured as follows.
[Synthesis of toner binder]
750 parts by weight of the urea-modified polyester (1) and 250 parts by weight of the unmodified polyester (a) were dissolved and mixed in 2000 parts by weight of a 1: 1 mixed solvent of ethyl acetate and MEK. The resulting solution of the toner binder (E) was partially dried under reduced pressure to isolate the toner binder (E). Its peak molecular weight was 5000, Tg was 62 [° C.], and acid value was 10.
[0053]
[Synthesis of Toner]
A toner E was obtained in the same manner as the toner A, except that the toner binder (E) was used. The shape factor SF-1 of this toner E was 115, the average circularity was 0.97, the degree of dispersion was 1.20, and the degree of aggregation was 18%.
[0054]
The toner F was manufactured as follows.
[Synthesis of Toner]
Toner F was obtained by mixing 1.5 parts by weight of hydrophobic silica with 100 parts by weight of the base toner particles of the toner binder (E) using a Henschel mixer. The shape factor SF-1 of this toner F was 115, the average circularity was 0.97, the dispersion was 1.20, and the agglomeration was 7%.
[0055]
The surface of a spherical ferrite having a weight average particle diameter of 50 [μm] was coated with a silicone resin, and then dried by heating to obtain a magnetic carrier. Then, the above-mentioned six kinds of toners were mixed with this magnetic carrier to obtain six kinds of two-component developers. The mixing ratio between the toner and the magnetic carrier was adjusted to 3.0 to 5.0 [w%].
[0056]
An electrophotographic printer (Imagio NEO750) manufactured by Ricoh Co., Ltd. was modified to produce a printer tester having the same configuration as that shown in FIG. A grayscale image (see FIG. 1) was developed with this printer tester using each of the above-described six types of two-component developers. Then, the printing operation was interrupted before it was electrostatically transferred onto the transfer paper P, and the estimated halftone portion average granularity on the photoreceptor 1 was determined in the same manner as described above.
[0057]
Next, a grayscale image was similarly developed using each of the above-described six types of two-component developers, and then electrostatically transferred onto the transfer paper P. However, the printing operation of the tester was stopped before the transfer paper P entered the fixing means 14 to obtain a transfer paper P carrying an unfixed gray scale image (hereinafter, referred to as an unfixed transfer paper). . A similar experiment was performed by changing the transfer current conditions in four steps for each of four transfer nip pressure conditions to obtain a total of 96 unfixed transfer papers (6 types of toner × transfer nip pressure condition 4). Stage × transfer current condition 4 stages). The transfer nip pressure conditions are four stages of 0.04, 0.20, 1.00, and 2.00 [N / mm ² ]. The transfer current conditions are four stages of 10, 20, 200, and 400 [nA / mm ² ].
[0058]
For each of the 96 unfixed transfer papers thus obtained, the average granularity of the halftone portion of the grayscale image was measured. At this time, since the gray scale image has not been fixed, there is a possibility that the image quality will be disturbed by rubbing when the image is read by the scanner as it is. Therefore, first, a film having a thickness of about 0.1 [mm] having a hole for measurement is prepared, and the film is brought into close contact with the image bearing surface of the unfixed transfer paper, and then the film contact surface is scanned with a scanner (Heidenberg Co., Ltd.). Nexscan 4100). In this way, the film was read at a resolution of 1200 [dpi] while the film functioned as a spacer so that the measurement target area of the grayscale image did not come into contact with the document placing surface. Then, based on the obtained electronic data, an average granularity of a halftone portion of the gray scale image after transfer and before fixing was obtained.
[0059]
Further, the transfer rate of the gray scale image after the transfer and before the fixing was also determined as follows. That is, first, when the grayscale image is electrostatically transferred from the photoconductor 1 to the transfer paper P, the printing operation is stopped, and the toner remaining in the photoconductor 1 area where the grayscale image has been carried so far. Was collected with adhesive tape. Then, the weight of the pressure-sensitive adhesive tape was measured, and the value obtained by subtracting the weight of only the pressure-sensitive adhesive tape, which was measured before collecting the toner, from the measured value was defined as the transfer residual toner amount. Next, the transfer paper P on which the toner image was transferred was cut out at a portion of the image to form a paper piece, and the weight was measured. Then, compressed air was blown onto the gray scale image of the paper piece to blow off almost all of the toner, and then the weight of the paper piece was measured again. The value obtained by subtracting the latter weight from the former weight was defined as the transfer toner amount. The value obtained by adding the transfer residual toner amount and the transfer toner amount obtained in this way was defined as the total toner amount. Then, the transfer rate was determined based on the following equation.
(Equation 5)
Transfer rate = transferred toner amount / total toner amount × 100 [%]
[0060]
Next, the 96 unfixed transfer papers were passed through the fixing means 14 to obtain printout papers. Further, the same printout paper was obtained while changing the fixing conditions. When combined with the previous printout paper, a total of three levels of fixing conditions were tested to obtain 288 printout papers. The fixing conditions are changed in three steps by sequentially changing the fixing condition of the pair of fixing rollers 14a to the one listed in the following as the one that adheres to the toner image (the one that functions as the heating member described in the claims). Was changed to
(1) A core metal roller provided with a surface layer made of silicone rubber having a thickness of 1 mm and a hardness of 25 [degrees] (when Asker C / 1 kg load) (hereinafter referred to as a silicone surface roller).
{Circle over (2)} An intermediate layer made of silicone rubber having a thickness of 200 [μm] provided on a cored roller, and a surface layer made of a 20 [μm] -thick ethylene tetrafluoride resin (Polytetrafluoroethylene) provided thereon. Hereinafter, this is referred to as a Tef (abbreviation of Teflon) surface elastic roller. The two-layer composite hardness of this roller on the cored roller was 70 [degrees] (Asker C / 1 kg load).
{Circle around (3)} A core metal roller provided with a surface layer made of tetrafluoroethylene resin (hereinafter referred to as a Teff surface rigid roller). Then, based on the obtained printout paper, the average granularity of the halftone portion in the gray scale image after fixing was measured.
[0061]
Table 1 below shows the properties of the toner A and the estimated halftone portion average granularity after development (before transfer) of the grayscale image obtained using the toner A. Tables 2, 3, 4, 5, and 6 show the properties of toners B, C, D, E, and F, respectively, and the intermediate properties after development (before transfer) of a grayscale image. It shows the relationship with the estimated average granularity of the tone portion. However, the transfer rate, the average granularity of the halftone portion after the transfer and before the fixing, and the average granularity of the halftone portion after the fixing are also shown in association with these.
[Table 2]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

[Table 7]

[0062]
In Tables 2 to 7, the shape factor SF-1, the average circularity and the degree of dispersion were compared with the estimated halftone average granularity in the gray scale image after development and before transfer on the photoreceptor 1. The following shows the following. That is, as the toner having a smaller shape factor SF-1 is used, the toner image can be developed with less roughness. Further, as the toner having a higher average circularity is used, the toner image can be developed with less roughness. Further, as the toner having a smaller degree of dispersion is used, the toner image can be developed with less roughness. Therefore, in order to minimize the feeling of roughness in the toner image after development and before transfer, it is desirable to use a toner having a shape factor SF-1 as small as possible, a high average circularity, and a small degree of dispersion.
[0063]
As shown in Table 2, even in the toner A having the worst condition, the toner image (gray scale image) after the development and before the transfer has a halftone portion average granularity estimated value of 0.18, which is much lower than 0.25. You can get things. However, although not shown in Table 2, when toner A was used, it was not possible to obtain a gray scale image after final fixing having an average granularity of 0.25 or less in a halftone portion. Further, as shown in Table 2, when the toner B is used, an image having an estimated halftone portion average granularity of 0.17 or less can be obtained as an image after development and before transfer. However, even in this case, it was not possible to obtain a final fixed image having an average granularity of 0.25 or less in the halftone portion. As shown in Tables 4 to 7, as an image after development and before transfer, as an image after final fixing, an image having an average granularity of 0.25 or less in the halftone portion was obtained. It can be seen that an average granularity estimated value of 0.15 or less must be obtained.
[0064]
Comparing the granularity degradation values due to electrostatic transfer in Tables 2 to 7, the following can be seen. That is, if attention is paid only to the electrostatic transfer, it is understood that the toner shape factor SF-1 does not significantly affect the average granularity of the halftone portion in the toner image after the transfer and before the fixing. Also, when comparing the average circularity or dispersion and the granularity deterioration value due to electrostatic transfer, if attention is paid only to electrostatic transfer, the average circularity or dispersion is not so large as the halftone part average granularity. It turns out that it has no effect. Since the shape factor SF-1, the average circularity, and the degree of dispersion greatly influence each other in the development process prior to the electrostatic transfer, the average granularity of the halftone portion of the image after the transfer and before the transfer greatly differs as a result. It just comes. Therefore, focusing only on the electrostatic transfer, the shape factor SF-1, the average circularity, and the degree of dispersion of the toner are not so important.
[0065]
On the other hand, when the transfer nip pressure and transfer current are compared with the granularity deterioration value due to the electrostatic transfer in Tables 3 to 7, it can be seen that the former has a great influence on the latter. Specifically, if both the transfer nip pressure and the transfer current are too low or too high, the average granularity of the halftone portion of the gray scale image after transfer will be significantly deteriorated.
[0066]
The reason why the average granularity of the halftone portion of the gray scale image after the transfer is significantly deteriorated when the transfer nip pressure is too low is considered as follows. That is, as described above, at the time of electrostatic transfer, a phenomenon that a small amount of toner particles adhere to the periphery of the image portion of the transfer paper P and disturb the image (hereinafter, referred to as transfer dust) occurs to a considerable extent. Conventionally, when the transfer paper P is not sandwiched by the transfer nip, a small amount of toner scatters from the toner image on the photoconductor 1 before and after the transfer nip, and the transfer paper P is not sandwiched by the transfer nip. It was thought to be due to adhesion to. However, the inventors of the present invention have conducted intensive studies and have found that even though the toner has not scattered from the toner image on the photoconductor 1 before and after the transfer nip, the transfer dust is transferred onto the transfer paper P after the transfer process. The phenomena that caused it were recognized. This suggests that there is transfer dust even in the transfer nip. It is considered that the reason is that a minute gap is formed in the transfer nip. Specifically, in the transfer nip, even if the toner carrying area on the surface of the photoconductor 1 is in close contact with the transfer paper P, the toner non-carrying area between the toner carrying areas is not in close contact with the transfer paper P. is there. Then, a minute gap is formed between the toner non-carrying region and the transfer paper P, and it is considered that transfer dust occurs here. Therefore, in the present printer, a transfer roller provided with an elastic layer (a first elastic layer 4a and a second elastic layer 4b) is used. In the transfer nip, the elastic layer is flexibly deformed by following the fine irregularities formed by the toner-carrying region and the toner-non-carrying region, thereby suppressing the formation of the minute gap. . However, even if the elastic layer is provided, if the transfer nip pressure is set too low, the transfer nip pressure cannot be flexibly deformed, and a considerable amount of transfer dust occurs in the above-mentioned minute gap. This is considered to be the cause of significantly deteriorating the average granularity of the halftone portion of the gray scale image after transfer if the transfer nip pressure is set too low.
[0067]
Further, the reason why the average granularity of the halftone portion of the gray scale image after transfer is significantly deteriorated even when the transfer nip pressure is too high is as follows. That is, in the transfer nip, among the toner particles in contact with the photoconductor 1 on the surface of the toner image, there are few toner particles that remain on the photoconductor 1 without being transferred to the transfer paper P side together with the toner particles thereunder. Occur. The amount of generated toner particles tends to increase as the transfer nip pressure increases. If the amount of toner particles increases too much, a phenomenon called a print, which causes white spots in a transferred toner image, is caused. If the transfer nip pressure is too high, the print will deteriorate to the point where the graininess can be visually recognized.
[0068]
The reason why the average granularity of the gray scale image after transfer is significantly deteriorated when the transfer current is too low is as follows. That is, the transfer rate increases as the transfer current increases. If the transfer current is too low, it is impossible to obtain a toner transfer amount that does not give a rough feeling, and the average granularity of the halftone portion is greatly deteriorated.
[0069]
Further, the reason why the average granularity of the halftone portion of the gray scale image after transfer is significantly deteriorated even when the transfer current is too high is as follows. That is, there is also a correlation between the transfer current and the amount of transfer dust, and the higher the former, the greater the latter. If the transfer current is too high, an amount of transfer dust that gives a large roughness is generated.
[0070]
Therefore, in order to obtain an image quality having a halftone portion average granularity of 0.25 or less without giving a grainy feeling in a toner image after transfer and before fixing, the following must be performed. That is, it is necessary to use a toner having an appropriate property as the toner and to perform the development as good as possible with the estimated value of the average gradation of the halftone portion after the development, and to perform the electrostatic transfer with an appropriate transfer nip pressure and transfer current. .
[0071]
Also, as shown in Tables 3 to 7, it can be seen that the toner image after the transfer and before the fixing deteriorates the halftone portion average granularity even when the toner image is fixed on the transfer paper P. It is considered that the toner image is crushed or glossy with fixing. As for the fixing roller that is brought into close contact with the toner image, it can be seen that the above (1) minimizes the deterioration, and the above (3) causes the greatest deterioration. Considering these facts, in order to finally obtain (after fixing) a toner image with an average granularity of 0.25 or less in the halftone portion, one of the following conditions 1 to 3 must be satisfied. I understand.
[Condition 1]
A toner having a condition that the shape factor SF-1 is 125 or less, the average circularity is 0.96 or more, and the dispersion degree is 1.35 or less is used.
And transfer current is set to 20~200 [nA / mm ^2].
・ Set the transfer nip pressure to 0.20 to 1.00 [N / mm ² ].
The fixing roller pair 14a uses the above-mentioned (1) as the one that is brought into close contact with the toner image.
[Condition 2]
A toner having a condition that the shape factor SF-1 is 120 or less, the average circularity is 097 or more, and the dispersion degree is 1.21 or less is used.
And transfer current is set to 20~200 [nA / mm ^2].
・ Set the transfer nip pressure to 0.20 to 1.00 [N / mm ² ].
The fixing roller pair 14a uses the above-mentioned (1) as the one that is brought into close contact with the toner image.
[Condition 3]
The toner having a condition that the shape factor SF-1 is 115 or less, the average circularity is 097 or more, and the dispersity is 1.20 or less is used.
And transfer current is set to 20~200 [nA / mm ^2].
・ Set the transfer nip pressure to 0.20 to 1.00 [N / mm ² ].
Of the fixing roller pair 14a, the one of the above (1) or (2) is used as the one that is brought into close contact with the toner image.
[0072]
Therefore, in the present printer, the user is instructed to use any one of the conditions 1 to 3 as the toner. In addition, the value of the transfer current is set to 20 to 200 [nA / mm ² ], and the transfer nip pressure is set to 0.20 to 1.00 [N / mm ² ]. Further, when it is specified that the toner described in the condition 1 or 2 is used, the fixing roller pair 14a is provided with the above-mentioned (1) as the one to be brought into close contact with the toner image. On the other hand, when it is specified to use the toner described in the condition 3, the roller of the above (1) or (2) is provided as such a roller. Therefore, as long as the designated toner is used, it is possible to reliably form a high-quality image that does not give a low-quality image in a state after fixing as an image having a density gradation. The designation of the toner may be performed, for example, by packing and shipping the toner described in any one of Conditions 1 to 3 together with a printer (image forming apparatus). Alternatively, for example, the product number or the product name of the toner may be specified in the printer main body or the instruction manual. Further, for example, the notification may be performed by notifying the user of the product number or the product name using a document or electronic data. Further, for example, the printing may be performed by shipping the printer main body in a state where the toner is set in the internal toner storing unit.
[0073]
For reference, after transfer with a toner having a weight average particle size of 4.2, 6.8, and 9.0 [μm] and before fixing, the average granularity of the halftone portion is 0.20, 0.49, and 0.4, respectively. FIGS. 14, 15 and 16 show enlarged schematic diagrams of the grayscale image that has become 90, respectively. Further, FIGS. 17, 18 and 19 show enlarged schematic views of the image portion of the grayscale image in which the granularity deterioration values at the time of fixing are 0.04, 0.10 and 0.15, respectively.
[0074]
As described above, in the printer according to the embodiment, the estimated halftone portion average granularity of the toner image after the development and before the transfer is 0.15 or less, and the average halftone portion granularity of the toner image after the fixing is less than 0.15. Since it is 0.25 or less, it is possible to reliably form a high-quality image that does not give a low image quality as an image having a density gradation after fixing.
Further, any one of the above conditions 1 to 3 is satisfied. Therefore, as long as the toner specified under each condition is used, it is possible to reliably form a high-quality image without giving a low image quality.
[0075]
【The invention's effect】
According to the first to sixth aspects of the present invention, there is an excellent effect that an image having a density gradation after fixing and which does not give a low image quality can be reliably formed.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an image on a display of a grayscale image used in an experiment performed by the present inventors.
FIG. 2 is an enlarged view of a portion near the center of the area ratio gradation in the image.
FIG. 3 is an enlarged view of a scale image at a position near the center of the area ratio gradation.
FIG. 4 is a graph showing a relationship between average lightness L and RMS granularity σD at each gradation of a grayscale image.
FIG. 5 is a graph showing a relationship between a subjective evaluation of roughness in a test-printed grayscale image, an average granularity of a halftone portion, and an average value of granularities of all gradations.
FIG. 6 is a schematic diagram showing a pattern image in which a pattern of 2 × 2 dots is arranged in a matrix of 70 pieces.
FIG. 7 is a schematic diagram illustrating an operation of dividing the pattern image at equal intervals for each pattern.
FIG. 8 is a graph showing a relationship between a standard deviation σ of an image portion area of an image and an average granularity of a halftone portion.
FIG. 9 is a schematic configuration diagram of a printer according to the embodiment.
FIG. 10 is an enlarged configuration diagram showing a photoconductor and a developing device of the printer.
FIG. 11 is a side view showing a transfer nip of the printer and its surroundings.
FIG. 12 is an enlarged schematic diagram illustrating a transfer nip formed by a photoconductor of the printer and a transfer roller pressed with a sufficient pressure toward the photoconductor.
FIG. 13 is a schematic diagram for explaining a method of calculating a shape factor SF-1.
FIG. 14 is an enlarged schematic diagram showing a grayscale image after transfer with a toner having a weight average particle size of 4.2 [μm] and before fixing.
FIG. 15 is an enlarged schematic diagram showing a grayscale image after transfer with a toner having a weight average particle size of 6.8 [μm] and before fixing.
FIG. 16 is an enlarged schematic diagram showing a grayscale image after transfer with a toner having a weight average particle size of 9.0 [μm] and before fixing.
FIG. 17 is an enlarged schematic diagram of an image portion of a grayscale image having a granularity degradation value of 0.04 at the time of fixing.
FIG. 18 is an enlarged schematic diagram of an image portion of a grayscale image having a granularity degradation value of 0.10 during fixing.
FIG. 19 is an enlarged schematic diagram of an image portion of a grayscale image having a granularity degradation value of 0.15 during fixing.
[Explanation of symbols]
1 Photoconductor (latent image carrier)
4 Transfer roller (part of transfer means)
14 Fixing Means 14a Fixing Roller Pair 20 Developing Device (Developing Means)

Claims (6)

A step of carrying the latent image on the latent image carrier, a step of developing the latent image on the latent image carrier using toner, and a step of electrostatically transferring the toner image on the latent image carrier to a recording medium And a step of fixing a toner image on the recording medium by bringing a heating member into close contact with the toner image electrostatically transferred on the recording medium to form a toner image on the recording medium. A forming method,
The estimated halftone average granularity of the toner image after development and before the electrostatic transfer is 0.15 or less, and the average halftone granularity of the toner image after fixing is 0.25 or less. An image forming method, comprising: Latent image carrier for carrying a latent image, developing means for developing the latent image on the latent image carrier using toner, and transfer means for electrostatically transferring the toner image on the latent image carrier to a recording medium And an image forming apparatus comprising: a fixing unit that fixes a toner image on the recording medium by bringing a heating member into close contact with the toner image electrostatically transferred onto the recording medium.
The estimated halftone average granularity of the toner image after development and before the electrostatic transfer is 0.15 or less, and the average halftone granularity of the toner image after fixing is 0.25 or less. An image forming apparatus, comprising: The image forming apparatus according to claim 2,
As the toner, a toner satisfying a condition that the shape factor SF-1 is 125 or less, the average circularity is 0.96 or more, and the dispersion degree is 1.35 or less is specified.
The transfer unit applies a transfer current of ²⁰ to 200 [nA / mm ² ] between the latent image carrier and a pressing member that presses the recording body toward the latent image carrier, and transfers the recording body to the latent image carrier. The toner image on the latent image carrier is electrostatically transferred to the recording body while being pressed against the image carrier at a pressure of 0.20 to 1.00 [N / mm ² ];
An image forming apparatus is characterized in that the fixing means uses, as the pressure member, one whose surface is covered with silicone rubber. The image forming apparatus according to claim 2,
A toner storage unit that stores toner used for developing the latent image on the latent image carrier,
As the toner, a toner having a condition that the shape factor SF-1 is 125 or less, the average circularity is 0.96 or more, and the dispersity is 1.35 or less is accommodated in the toner accommodating means.
The transfer unit applies a transfer current of ²⁰ to 200 [nA / mm ² ] between the latent image carrier and a pressing member that presses the recording body toward the latent image carrier, and transfers the recording body to the latent image carrier. The toner image on the latent image carrier is electrostatically transferred to the recording body while being pressed against the image carrier at a pressure of 0.20 to 1.00 [N / mm ² ];
An image forming apparatus is characterized in that the fixing means uses, as the pressure member, one whose surface is covered with silicone rubber. The image forming apparatus according to claim 2,
As the toner, a toner satisfying a condition that the shape factor SF-1 is 120 or less, the average circularity is 097 or more, and the dispersity is 1.21 or less is specified.
The transfer unit applies a transfer current of ²⁰ to 200 [nA / mm ² ] between the latent image carrier and a pressing member that presses the recording body toward the latent image carrier, and transfers the recording body to the latent image carrier. The toner image on the latent image carrier is electrostatically transferred to the recording body while being pressed against the image carrier at a pressure of 0.20 to 1.00 [N / mm ² ];
An image forming apparatus is characterized in that the fixing means uses, as the pressure member, one whose surface is covered with silicone rubber. The image forming apparatus according to claim 2,
As the toner, toner having a condition that the shape factor SF-1 is 115 or less, the average circularity is 097 or more, and the dispersion degree is 1.20 or less is specified.
The transfer unit applies a transfer current of ²⁰ to 200 [nA / mm ² ] between the latent image carrier and a pressing member that presses the recording body toward the latent image carrier, and transfers the recording body to the latent image carrier. The toner image on the latent image carrier is electrostatically transferred to the recording body while being pressed against the image carrier at a pressure of 0.20 to 1.00 [N / mm ² ];
Further, the fixing means uses, as the pressure member, one having a surface coated with silicone rubber or one having an elastic layer coated with a surface layer made of polytetrafluoroethylene (Polytetrafluoroethylene). An image forming apparatus, comprising:

Images