JP2004070208A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device in which a wide color reproducing range is realized over the region from halftone gradation to the region having high brightness that is especially important when a natural image or the like is outputted and a high image quality image is formed. <P>SOLUTION: A color image forming is conducted by using six different kinds of toner, i.e., cyan, light cyan, magenta, light magenta, yellow and black. Density and hue are mutually made different for shade toner cyan, magenta, light toner light cyan and light magenta. The image forming in a high brightness region is conducted by using light toner only. In the halftone gradation region, the image forming is conducted using both shade and light toner. <P>COPYRIGHT: (C)2004,JPO

Description

【００６６】
トナーの形状係数ＳＦ−１はトナー粒子の丸さの度合を示し、球形から徐々に不定形となる。ＳＦ−２はトナー粒子の凹凸度合を示し、トナー表面の凹凸が顕著となる。
【００６７】
分子量は、ＧＰＣにより測定される。具体的なＧＰＣの測定方法としては、予めトナーをソックスレー抽出器を用いテトラヒドロフラン（ＴＨＦ）で２０時間抽出を行ったサンプルを用い、カラム構成は昭和電工製Ａ−８０１，８０２，８０３，８０４，８０５，８０６，８０７を連結し標準ポリスチレン樹脂の検量線を用い分子量分布を測定し得る。
【００６８】
重量平均分子量（Ｍｗ）と数平均分子量（Ｍｎ）の比率（Ｍｗ／Ｍｎ）は、２〜１００を示す樹脂が好ましい。
【００６９】
トナーのガラス転移点（Ｔｇ）は定着性、保存性の点から５０℃〜７５℃（さらに好ましくは、５２℃〜７０℃）が好ましい。
【００７０】
トナーのガラス転移点の測定にはたとえば、パーキンエルマー社製のＤＳＣ−７のような高精度の内熱式入力補償型の示差走査熱量計で測定を行う。測定方法は、ＡＳＴＭ　Ｄ３４１８−８２に準じて行う。本実施形態においては、試料を１回昇温させ前履歴をとった後、急冷し、再度温度速度１０℃／ｍｉｎ、温度０〜２００℃の範囲で昇温させたときに測定されるＤＳＣ曲線を用いる。
【００７１】
本実施形態に使用される結着樹脂は、ポリスチレン；ポリ−ｐ−クロルスチレン，ポリビニルトルエンの如きスチレン置換体の単重合体；スチレン−ｐ−クロルスチレン共重合体，スチレン−ビニルトルエン共重合体，スチレン−ビニルナフタリン共重合体，スチレン−アクリル酸エステル共重合体，スチレン−メタクリル酸エステル共重合体，スチレン−α−クロルメタクリル酸メチル共重合体，スチレン−アクリロニトリル共重合体，スチレン−ビニルメチルエーテル共重合体，スチレン−ビニルエチルエーテル共重合体，スチレン−ビニルメチルケトン共重合体，スチレン−ブタジエン共重合体，スチレン−イソプレン共重合体，スチレン−アクリロニトリル−インデン共重合体の如きスチレン系共重合体；ポリ塩化ビニル，フェノール樹脂，天然変性フェノール樹脂，天然樹脂変性マレイン酸樹脂，アクリル樹脂，メタクリル樹脂，ポリ酢酸ビニール，シリコーン樹脂，ポリエステル樹脂，ポリウレタン，ポリアミド樹脂，フラン樹脂，エポキシ樹脂，キシレン樹脂，ポリビニルブチラール，テルペン樹脂，クマロンインデン樹脂，石油系樹脂等が使用できる。架橋されたスチレン系樹脂も好ましい結着樹脂である。
【００７２】
スチレン系共重合体のスチレンモノマーに対するコモノマーとしては、アクリル酸，アクリル酸メチル，アクリル酸エチル，アクリル酸ブチル，アクリル酸ドデシル，アクリル酸オクチル，アクリル酸−２−エチルヘキシル，アクリル酸フェニル，メタクリル酸，メタクリル酸メチル，メタクリル酸エチル，メタクリル酸ブチル，メタクリル酸オクチル，アクリロニトリル，メタクリロニトリル，アクリルアミドのような二重結合を有するモノカルボン酸もしくはその置換体；マレイン酸，マレイン酸ブチル，マレイン酸メチル，マレイン酸ジメチルのような二重結合を有するジカルボン酸及びその置換体；塩化ビニル，酢酸ビニル，安息香酸ビニルのようなビニルエステル類；エチレン，プロピレン，ブチレンのようなエチレン系オレフィン類；ビニルメチルケトン，ビニルヘキシルケトンのようなビニルケトン類；ビニルメチルエーテル，ビニルエチルエーテル，ビニルイソブチルエーテルのようなビニルエーテル類；等のビニル単量体が挙げられる。これらは、単独もしくは組み合わせて用いられる。
【００７３】
架橋剤としては、２個以上の重合可能な二重結合を有する化合物が用いられる。例えば、ジビニルベンゼン，ジビニルナフタレンのような芳香族ジビニル化合物；エチレングリコールジアクリレート，エチレングリコールジメタクリレート，１，３−ブタンジオールジメタクリレートのような二重結合を２個有するカルボン酸エステル；ジビニルアニリン，ジビニルエーテル，ジビニルスルフィド，ジビニルスルホンの如きジビニル化合物；３個以上のビニル基を有する化合物；が挙げられる。これらは単独もしくは混合して使用される。
【００７４】
定着時の定着部材からの離型性の向上、定着性の向上の点から次のようなワックス類をトナー粒子中に含有させることも好ましい。パラフィンワックス及びその誘導体，マイクロクリスタリンワックス及びその誘導体，フィッシャートロプシュワックス及びその誘導体，ポリオレフィンワックス及びその誘導体，カルナバワックス及びその誘導体である。誘導体には酸化物や、ビニル系モノマーとのブロック共重合体，グラフト変性物が挙げられる。
【００７５】
その他、長鎖アルコール，長鎖脂肪酸，酸アミド，エステルワックス，ケトン，硬化ヒマシ油及びその誘導体，植物系ワックス，動物系ワックス，鉱物系ワックス，ペトロラクタム等も場合により使用しても良い。
【００７６】
トナーを作製するには、結着樹脂，ワックス，着色剤としての顔料，染料，又は磁性体，必要に応じて荷電制御剤の如き添加剤をヘンシェルミキサー，ボールミルの如き混合器により十分混合してから加熱ロール，ニーダー，エクストルーダーの如き熱混練機を用いて溶融混練して樹脂を溶融せしめた中に顔料，染料又は磁性体を分散又は溶解せしめ、冷却固化後、粉砕，分級を行なってトナーを得ることが出来る。分級工程においては生産効率上、多分割分級機を用いることが好ましい。
【００７７】
さらに、特公昭５６−１３９４５号公報等に記載のディスク又は多流体ノズルを用い溶融混合物を空気中に霧化し球状トナーを得る方法や、特公昭３６−１０２３１号公報，特開昭５９−５３８５６号公報，特開昭５９−６１８４２号公報に述べられている懸濁重合方法を用いて直接トナーを生成する方法や、単量体には可溶で得られる重合体が不溶な水系有機溶剤を用い直接トナーを生成する分散重合方法又は水溶性極性重合開始剤存在下で直接重合しトナーを生成するソープフリー重合方法に代表される乳化重合方法を用いてトナー粒子を製造しても良い。
【００７８】
さらに、Ｃｙａｎ，Ｍａｇｅｎｔａ，Ｙｅｌｌｏｗ，Ｂｌａｃｋの各トナーの着色剤について説明する。
【００７９】
黒色着色剤としてカーボンブラック，磁性体，以下に示すイエロー／マゼンタ／シアン着色剤を用い黒色に調色されたものが挙げられる。
【００８０】
イエロー着色剤としては、縮合アゾ化合物，イソインドリノン化合物，アンスラキノン化合物，アゾ金属錯体，メチン化合物，アリルアミド化合物に代表される化合物が挙げられる。具体的には、Ｃ．Ｉ．ピグメントイエロー１２，１３，１４，１５，１７，６２，７４，８３，９３，９４，９５，９７，１０９，１１０，１１１，１２０，１２７，１２８，１２９，１４７，１６８，１７４，１７６，１８０，１８１，１９１が好適に用いられる。
【００８１】
マゼンタ着色剤としては、縮合アゾ化合物，ジケトピロロピロール化合物，アンスラキノン，キナクリドン化合物，塩基染料レーキ化合物，ナフトール化合物，ベンズイミダゾロン化合物，チオインジゴ化合物，ペリレン化合物が挙げられる。具体的には、Ｃ．Ｉ．ピグメントレッド２，３，５，６，７，２３，４８；２，４８；３，４８；４，５７；１，８１；１，１４４，１４６，１６６，１６９，１７７，１８４，１８５，２０２，２０６，２２０，２２１，２５４が特に好ましい。
【００８２】
シアン着色剤としては、上述したように、銅フタロシアニン化合物及びその誘導体，アンスラキノン化合物，塩基染料レーキ化合物等が挙げられる。具体的には、Ｃ．Ｉ．ピグメントブルー１，７，１５，１５：１，１５：２，１５：３，１５：４，６０，６２，６６が特に好適に利用できる。
【００８３】
磁性体としては、鉄，コバルト，ニッケル，銅，マグネシウム，マンガン，アルミニウム，珪素などの元素を含む金属酸化物がある。中でも四三酸化鉄，γ−酸化鉄の如き酸化鉄を主成分とするものが好ましい。トナーの帯電性コントロールの点から硅素元素またはアルミニウム元素の如き金属元素を含有していてもよい。これら磁性粒子は、窒素吸着法によるＢＥＴ比表面積が好ましく２〜３０ｍ^２／ｇ、特に３〜２８ｍ^２／ｇが好ましく、モース硬度が５〜７の磁性体が好ましい。
【００８４】
上述した濃トナーと淡トナーは、それぞれ着色剤を異ならせることによって、濃度レベルおよび色相角を適宜異ならせたものである。または、同一の着色剤を用い、かつ、その含有量を異ならせることによっても、濃度レベルおよび色相角を適宜異ならせることもできる。この場合、淡トナーの着色剤の含有量を、濃トナーの着色剤の含有量の１／５以下にすることで、好ましい濃度レベルに設定することができる。
【００８５】
次に、上記した画像形成装置の画像形成動作について説明する。
【００８６】
ここでは、Ｒｅｄ（Ｒ），Ｇｒｅｅｎ（Ｇ），Ｂｌｕｅ（Ｂ）の３色からなる入力画像を、Ｃｙａｎ（ＤＣ），ＬｉｇｈｔＣｙａｎ（ＬＣ），Ｍａｇｅｎｔａ（ＤＭ），ＬｉｇｈｔＭａｇｅｎｔａ（ＬＭ），Ｙｅｌｌｏｗ（Ｙ），Ｂｌａｃｋ（Ｋ）の６種類のトナーを用いて画像形成を行う場合について述べる。すなわち、シアンをＬＣとＤＣの２種類のトナーを用いて出力し、マゼンタをＬＭとＤＭの２種類のトナーを用いて出力する。
【００８７】
画像形成装置は、原稿読取装置（スキャナ部）によって原稿上のカラー画像を読み取り、ＣＣＤでＲＧＢに色分解された入力画像信号を得る。あるいは、画像形成装置がプリンタ機能を有する場合には、コンピュータからＲＧＢのプリントデータ（入力画像信号）を得る場合もある。なお、ここではＲＧＢの入力画像を用いているが、これは単に原稿読取装置やコンピュータのプリンタドライバの仕様に基づくものにすぎない。入力画像としては、ＲＧＢ画像の他にも、ＣＭＹＫ画像，ＣＭＹＫ＋ＬＣ＋ＬＭ画像，Ｌ＊ａ＊ｂ＊画像や、特色用のチャンネルを含む画像なども入力可能である。
【００８８】
画像形成を行う際には、入力されたＲＧＢの色信号を、画像形成用の（出力デバイスで出力可能な）ＣＭＹＫ＋ＬＣ＋ＬＭの色信号に変換しなければならない。
【００８９】
図１２に、色変換方式の一手法を示す。
【００９０】
図１２では、入力画像のＲＧＢ信号をＣＭＹＫの４色に色分解した後、特定色（ＣとＭ）について濃淡２つの版データに分版し、最終的に、Ｙ，Ｋ，ＬＣ，ＤＣ，ＬＭ，ＤＭの６色の色信号を得る。そして、６色の色信号に対して、所定のγ補正を施した後、ハーフトーン処理を行って、ＰＷＭ回路に入力する。
【００９１】
かかる色変換方式の場合、ＲＧＢの色信号をＣやＭの一次色に変換した後、ＬＣ＋ＤＣ，ＬＭ＋ＤＭのように濃淡それぞれの色信号に分解しているので、濃淡２種類のトナーの色相が大きく異なると、単色のグラデーションやハイライト部分などにおいて色相が不均一となり、見た目に違和感を生ずるおそれがある。しかしながら、本実施形態では２種類のトナーの色相の変位量を３０度以下、より好適には２０度以下にしているので、そのような出力画像の品位低下を抑制しつつ、良好な諧調性・粒状性と広い色再現を実現することが可能となる。
【００９２】
なお、濃淡２つの版データへの変換方法については、トナーの濃度レベル等により様々な組み合わせが考えられる。図８に基本となる直線的な諧調変換方法を示す。
【００９３】
図示したとおり、ハイライトで先に淡トナーが立ち上がり、中間調付近から濃トナーが入り始め、しばらく濃淡の組み合わせで諧調を再現しながら、高濃度部では淡トナーの使用が制限されていく。このときの濃淡の諧調の組み合わせは、粒状性や諧調性、色域などの画像品質と、トナー消費量の関係より決定される。またここでは簡単のため、直線的な諧調を図示したが、実際にはトーンジャンプを防止する観点から、濃淡各トナーの濃度の入り始めは緩やかなカーブを描くことが好適である。
【００９４】
図１３は、色変換方式の別の手法を示したものである。
【００９５】
ここでは、ダイレクトマッピングにより、入力画像のＲＧＢ信号から直接Ｙ，Ｋ，ＬＣ，ＤＣ，ＬＭ，ＤＭの６色の信号に色分解している。
【００９６】
ダイレクトマッピングとは、ルックアップテーブル（ＬＵＴ）を参照して、入力信号（入力画像の色情報）から出力デバイスの出力信号（画像形成用の色情報）へダイレクトに変換する色変換方式をいう。たとえば、色空間内のＬ＊ａ＊ｂ＊やＲＧＢなどの３つの入力信号を与えることにより、その色を再現するために必要な出力色空間内の信号値を、ＣＭＹＫの４色あるいはＣＭＹＫ＋ＬＣ＋ＬＭの６色などのかたちで出力する。
【００９７】
この色変換方式はマトリクス演算を必要とせず、非線形な変換が可能となることから、ＵＣＲの設定など色変換の自由度が大幅に向上し、トナーの載り量をコントロールしながら、所望の色再現を可能にすることができる。
【００９８】
また、ダイレクトマッピングによれば、入力画像のＲＧＢ信号から直接濃トナーと淡トナーそれぞれの色信号を発生させるので、図１２の方法で懸念されるような濃淡トナーの色相の違いによる出力品位の低下を招くこともない。
【００９９】
以上述べたように、本実施形態の画像形成装置によれば、濃度および色相が互いに異なる濃トナーと淡トナーを用いて、高明度領域では淡トナーのみにより画像形成を行い、中間調領域では淡トナーと濃トナーを併せて画像形成を行うので、良好な諧調性・粒状性を実現できるとともに、特に自然画像等を出力する際に重要となる、中間調から明度の高い領域にかけて、広い色再現範囲を実現し、高品質な画像形成を行うことが可能となる。
【０１００】
【実施例】
（実施例１）
ＣＭＫＹの各色のうち、シアンとマゼンタの２色について、濃度レベルの異なる２種類のトナーを用い、Ｃｙａｎ，ＬｉｇｈｔＣｙａｎ，Ｍａｇｅｎｔａ，ＬｉｇｈｔＭａｇｅｎｔａ，Ｙｅｌｌｏｗ，Ｂｌａｃｋの６種類のトナーでカラー画像形成を行う画像形成装置を構成した。
【０１０１】
ここで、濃トナーには顔料系の着色剤、淡トナーには染料系の着色剤というように、互いに異なる着色剤を用いることとし、濃トナーと淡トナーの濃度レベルおよび色相角を互いに異ならせた。具体的には、下記の原材料を用いて各トナーを作製した。
【０１０２】
＜Ｃｙａｎ＞
ポリエステル樹脂（１００重量部）　／　フタロシアニン顔料　（３重量部）
＜ＬｉｇｈｔＣｙａｎ＞
ポリエステル樹脂（１００重量部）　／　アントラキノン染料　（０．６重量部）
＜Ｍａｇｅｎｔａ＞
ポリエステル樹脂（１００重量部）　／　キナクリドン顔料　（３重量部）
＜ＬｉｇｈｔＭａｇｅｎｔａ＞
ポリエステル樹脂（１００重量部）　／　アントラキノン染料　（０．６重量部）
【０１０３】
上記原料をヘンシェルミキサーにより予備混合を行い、二軸押し出し式混練機により溶融混連し、冷却後ハンマーミルを用いて１〜２ｍｍ程度に粗粉砕した。次いで、エアージェット方式による微粉砕機で微粉砕した。得られた微粉砕物を分級し、シリカを外添して、重量平均粒子５．６μｍのＣｙａｎ，ＬｉｇｈｔＣｙａｎ，Ｍａｇｅｎｔａ，ＬｉｇｈｔＭａｇｅｎｔａの各粒子を得た。
【０１０４】
このとき、濃トナーと淡トナーの色相角の変位量は、ＣＩＥＬＡＢ空間で淡トナーの最低明度をＬｐ，画像形成を行うシートの明度をＬｍとしたとき、
（Ｌｍ−Ｌｐ）×０．２＋Ｌｐ
で求まる明度において、３度であった。
【０１０５】
以上の様な構成のトナーを用いて画像形成を行ったところ、表１に示すとおり、比較例に比べて色再現域を約３０％増加させることが可能となり、写真画質に迫る広い色再現範囲を実現することができた。
【０１０６】
なお、比較例１は、Ｃｙａｎ，Ｍａｇｅｎｔａ，Ｙｅｌｌｏｗ，Ｂｌａｃｋの４色のトナーを用いた、キヤノン株式会社製ＣＬＣ１１００複写機の結果である。また、比較例２はシアンとマゼンタに濃淡トナーを使用し、そのときの濃淡トナーの色相角の変位量がない場合の結果である。また、比較例３はシアンとマゼンタに濃淡トナーを使用し、そのときの濃淡トナーの色相角の変位量が、上記の
（Ｌｍ−Ｌｐ）×０．２＋Ｌｐ
で求まる明度において、１度〜２度の場合の結果である。
【０１０７】
色再現域の広さの比較は、比較例１の色再現域の体積を１００としたときの相対値で評価した。
【０１０８】
【表１】

【０１０９】
（実施例２）
ＣＭＫＹの各色のうち、シアンとマゼンタの２色について、濃度レベルの異なる２種類のトナーを用い、Ｃｙａｎ，ＬｉｇｈｔＣｙａｎ，Ｍａｇｅｎｔａ，ＬｉｇｈｔＭａｇｅｎｔａ，Ｙｅｌｌｏｗ，Ｂｌａｃｋの６種類のトナーでカラー画像形成を行う画像形成装置を構成した。
【０１１０】
ここで、濃トナーと淡トナーにはともに顔料系の同一の着色剤を用いることとし、着色剤の含有量を変化させることで、濃トナーと淡トナーの濃度レベルおよび色相角を互いに異ならせた。具体的には、下記の原材料を用いて各トナーを作製した。
【０１１１】
＜Ｃｙａｎ＞
ポリエステル樹脂（１００重量部）　／　フタロシアニン顔料　（４重量部）
＜ＬｉｇｈｔＣｙａｎ＞
ポリエステル樹脂（１００重量部）　／　フタロシアニン顔料　（０．７重量部）
＜Ｍａｇｅｎｔａ＞
ポリエステル樹脂（１００重量部）　／　キナクリドン顔料　（５重量部）
＜ＬｉｇｈｔＭａｇｅｎｔａ＞
ポリエステル樹脂（１００重量部）　／　キナクリドン顔料　（１重量部）
【０１１２】
上記原料をヘンシェルミキサーにより予備混合を行い、二軸押し出し式混練機により溶融混連し、冷却後ハンマーミルを用いて１〜２ｍｍ程度に粗粉砕した。次いで、エアージェット方式による微粉砕機で微粉砕した。得られた微粉砕物を分級し、シリカを外添して、重量平均粒子５．６μｍのＣｙａｎ，ＬｉｇｈｔＣｙａｎ，Ｍａｇｅｎｔａ，ＬｉｇｈｔＭａｇｅｎｔａの各粒子を得た。
【０１１３】
得られたトナーの特性を図１４のａ＊−ｂ＊平面図に示す。図からＣｙａｎとＬｉｇｈｔＣｙａｎ、ＭａｇｅｎｔａとＬｉｇｈｔＭａｇｅｎｔａの各々の色相角が、ＣＩＥＬＡＢ空間上で５度以上であることが理解できる。このとき、濃トナーと淡トナーの色相角の変位量は、ＣＩＥＬＡＢ空間で淡トナーの最低明度をＬｐ，画像形成を行うシートの明度をＬｍとしたとき、
（Ｌｍ−Ｌｐ）×０．２＋Ｌｐ
で求まる明度において、３度以上であった。
【０１１４】
また各々の諧調性を図１５に示す。図示した淡トナーを用いて高明度領域を、濃トナーを用いて低明度領域を作像することが可能となる。
【０１１５】
以上の様な構成のトナーを用いて画像形成を行ったところ、表２に示すとおり、比較例に比べて色再現域を約２０％増加させることが可能となり、写真画質に迫る広い色再現範囲を実現することができた。なお、比較例１，比較例２，比較例３については、上記実施例１のものと同様である。
【０１１６】
【表２】

【０１１７】
【発明の効果】
以上説明したように、本発明は、濃度および色相が互いに異なる濃トナーと淡トナーを用いて、高明度領域では淡トナーのみにより画像形成を行い、中間調領域では淡トナーと濃トナーを併せて画像形成を行うので、良好な諧調性・粒状性を実現できるとともに、特に自然画像等を出力する際に重要となる、中間調から明度の高い領域にかけて、広い色再現範囲を実現し、写真画質にも迫る高品質な画像形成を行うことが可能となる。
【図面の簡単な説明】
【図１】電子写真方式の画像形成装置の基本構成を説明するための説明図である。
【図２】本発明の実施形態に係る画像形成装置の一構成例を示す概略構成図である。
【図３】本発明の実施形態に係る画像形成装置の他の構成例を示す概略構成図である。
【図４】濃トナーと淡トナーの色相が等しい場合の色再現範囲を示す模式図である。
【図５】濃トナーと淡トナーの色相が互いに異なる場合の色再現範囲を示す模式図である。
【図６】濃トナーと淡トナーの色相が互いに異なり、且つ、中間調領域での濃トナーと淡トナーの重なる領域が広い場合の色再現範囲を示す模式図である。
【図７】所定の明度での一次色の色相角を示す模式図である。
【図８】濃トナーと淡トナーの諧調カーブの一例を示す図である。
【図９】図８の諧調カーブにより得られた画像の濃度カーブを示す図である。
【図１０】図８とは異なる諧調カーブにより得られた画像の濃度カーブを示す図である。
【図１１】高明度領域における濃トナーと淡トナーの明度の相違を示す図である。
【図１２】色変換方式の一手法を説明する図である。
【図１３】色変換方式の他の手法（ダイレクトマッピング）を説明する図である。
【図１４】実施例２に係るトナーの特性を示すａ＊−ｂ＊平面図である。
【図１５】実施例２に係るトナーの諧調性を示す図である。
【符号の説明】
１０　シート
１１　感光ドラム
１２　帯電器
１４　転写帯電器
１５　定着器
１６　クリーニング部材
１７　画像露光器
１８　中間転写体
１９　現像器
ＳＴ　画像形成ステーション[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electrophotographic image forming apparatus used for, for example, a copying machine, a printer, a facsimile, a digital proof, and the like.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, as a high-speed and high-quality image forming apparatus, a copying machine and a laser beam printer employing an electrophotographic system have been known. Recent advances in digital technology have increased the demand for higher image quality of electrophotographic image forming apparatuses from the POD market to the office and home consumer markets. In addition to images that have undergone screen processing such as printing, silver Demands for image characteristics having high gradation, a wide color reproduction range, and good granularity as in salt photography have been further increased. Particularly, with the rapid spread of CG images and digital cameras, realization of a wide color reproduction range approaching silver halide photography is desired.
[0003]
This type of electrophotographic image forming apparatus irradiates an image carrier with light by a laser beam or the like, and at that time, an image is recorded according to an amount of the irradiated light. From a binary image such as a character, Any image can be formed up to an image including a halftone such as a photograph. In reproducing the intermediate density at this time, various patterns can be formed on the image carrier by using a pulse width modulation method (PWM method) and an image processing method such as a dither method or a density pattern method. it can.
[0004]
Charged toner particles are made to adhere to the obtained pattern on the image carrier, and further transferred and fixed on a sheet such as paper to obtain a final output image. As the toner used at this time, four colors of cyan (Cyan), magenta (Magenta), yellow (Yellow), and black (Black) are generally used. Toner has been subjected to various improvements to improve various image characteristics such as granularity, gradation, density, saturation, and gloss.
[0005]
For the purpose of further improving image quality, JP-A-5-35038, JP-A-8-171252, JP-A-2000-231279, JP-A-2000-305339, JP-A-2000-347476, As disclosed in Japanese Patent Application Laid-Open No. 2001-290319, there is disclosed a technique for using a plurality of toners having different density levels to realize further improvement in graininess and gradation reproducibility from highlight to halftone region. ing.
[0006]
In addition, using a developer in which a plurality of toners with different density levels are mixed in advance, and using the difference in the amount of charge and the particle size, from the highlight to the high density region, it is made with the toner type suitable for each gradation. Imaging systems are also disclosed in the art.
[0007]
However, this method
-It is not possible to "select toners having different density levels accurately according to gradation", and it is not possible to sufficiently separate functions due to differences in toner types.
-After outputting a plurality of images and consuming a large amount of toner, the weight ratio of the mixed plurality of toners is different from the initial one and greatly different from the initial image quality.
When two or more types of light and shade toners having different density ranges are used, it is not necessary to use light toner in the high density area, and it is preferable not to use it in the sense of reducing consumption. It cannot be controlled not to use toner.
Have such problems.
[0008]
[Problems to be solved by the invention]
According to the above-mentioned conventional technology, it is possible to improve “granularity from highlight to halftone region” and “gradation reproducibility” by forming an image using two or more types of toner having different density ranges. it can.
[0009]
However, a wide color reproduction range, which is particularly problematic in forming a photographic image, has not yet been realized. Therefore, a so-called color gamut must be compressed at the time of image formation, and in the case of a copying machine, the color tone of the read document cannot be reproduced as it is, so that satisfactory output quality cannot be obtained.
[0010]
The present invention has been made in view of the above-described circumstances, and aims to realize a wide color reproduction range from a halftone to a high brightness region, which is particularly important when outputting a natural image or the like. Another object of the present invention is to provide an image forming apparatus capable of forming a high quality image.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, in an electrophotographic image forming apparatus that forms a color image using a plurality of toners, at least one color has a dark toner and a light hue different in density and hue from each other. An image is formed using only the light toner in a high brightness area using toner, and an image is formed by combining the dark toner and the light toner in a halftone area.
[0012]
It is preferable that the lightness at the point where the saturation of the dark toner and the light toner is equal is different from each other.
[0013]
More preferably, the brightness of the dark toner and the brightness of the light toner are different from each other at least in a region having a brightness of 60 or more in the CIELAB space.
[0014]
The displacement of the hue angle between the dark toner and the light toner is preferably 3 degrees or more, more preferably 5 degrees or more in the CIELAB space.
[0015]
Alternatively, in the CIELAB space, when the minimum lightness of the light toner is Lp and the lightness of a sheet on which an image is formed is Lm,
(Lm-Lp) × 0.2 + Lp
It is also preferable that the displacement amount of the hue angle between the dark toner and the light toner at the brightness obtained by the above is 3 degrees or more.
[0016]
It is preferable that an area where image formation is performed by combining the dark toner and the light toner is equal to or more than ５ of all gradations of the color.
[0017]
In an image forming apparatus that forms a color image with at least three colors of cyan, magenta and yellow, it is preferable to use the dark toner and the light toner for two colors of cyan and magenta.
[0018]
The dark toner and the light toner may be toners having different colorants, or may be toners having the same colorant and different contents of the colorant.
[0019]
At this time, it is preferable that the content of the colorant in the light toner is 1/5 or less of the content of the colorant in the dark toner.
[0020]
It is preferable that color signals of the dark toner and the light toner are generated by direct mapping from color signals of an input image.
[0021]
Alternatively, it is also preferable that the color signal of the input image is converted into a color signal for image formation, and the color signal for image formation is separated into color signals of the dark toner and the light toner.
[0022]
The displacement of the hue angle between the dark toner and the light toner is preferably 30 degrees or less, more preferably 20 degrees or less in the CIELAB space.
[0023]
In the region where the image density to be formed is 0.3 or less, it is preferable to form an image only with the light toner.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Preferred embodiments of the present invention will be illustratively described in detail below with reference to the drawings. The electrophotographic image forming apparatus described below is suitable for use in laser beam printers, copiers, laser facsimile machines, digital proofs, and the like.
[0025]
First, a basic configuration of an electrophotographic image forming apparatus will be described with reference to FIG.
[0026]
The image forming apparatus includes a photosensitive drum 11 as an image carrier, and a charger 12, an image exposing unit 17, a developing unit 19, a transfer charger 14, a fixing unit 15, and a cleaning member 16 disposed therearound. An electrophotographic image forming apparatus is configured.
[0027]
The photosensitive drum 11 may be of a function separation type having a two-layer structure such as a charge generation layer and a charge transport layer, or a single layer type, with a conductive support base as the lowermost layer.
[0028]
The charger 12 is a charging unit that uniformly charges the photosensitive drum 11. As the charging means, a corona charging method using a corona charger consisting of a wire and an electric field control grid, a roller charging method of applying a direct current or a superimposed bias of direct current and alternating current to a charging roller in contact with an image carrier, and the like Is mentioned.
[0029]
The image exposing unit 17 is an exposing unit that forms an electrostatic latent image by exposing the charged surface of the photosensitive drum 11 to an image. As the exposure means, a scanner type using a semiconductor laser, an LED that performs image exposure through a selfoc lens that is a light condensing device, a device using an EL element, a plasma light emitting element, or the like, Various optical systems can be suitably used.
[0030]
The developing device 19 is a developing unit that forms a toner image (visible image) by attaching toner to the electrostatic latent image on the photosensitive drum 11. As a developing method, a magnetic one-component non-contact developing method in which a magnetic toner is conveyed by a magnetic force and fly-developed on an image carrier in a non-contact manner at a developing nip, or a developing process is performed by contacting the image carrier with a developing nip. A non-magnetic one-component non-contact development method in which non-magnetic toner is regulated and charged by a blade, carried on a developing sleeve, transported, and the toner is developed in a non-contact manner in a developing nip. Alternatively, a non-magnetic one-component contact developing method in which a developing process is performed by contacting an image carrier with a developing nip, and a non-magnetic toner is mixed with a carrier that is a magnetic powder, and is similarly transported to a developing nip by a developing sleeve to perform a developing process. Various developing methods such as a two-component developing method can be used.
[0031]
The transfer charger 14 is a transfer unit that transfers the toner image on the photosensitive drum 11 to the sheet 10 such as paper. As a transfer method, a transfer method using an electric force or a mechanical force can be used. As a method of performing transfer using electric force, a corona transfer method in which a DC bias of the opposite polarity to the charged polarity of the toner is applied by a corona wire to perform the transfer, a roller is brought into contact, and a bias of the opposite polarity to the toner is applied. And a roller transfer method for applying the pressure.
[0032]
2 and 3 are schematic configuration diagrams illustrating a configuration example of the image forming apparatus according to the present embodiment. As shown in the figure, in the present embodiment, a color image is formed using a plurality of toners by an image forming apparatus having a plurality of developing units 19.
[0033]
The image forming apparatus of FIG. 2 has a configuration in which a plurality of (six in FIG. 2) image forming stations ST each including a photosensitive drum 11, a charger 12, a developing unit 19, and a transfer charger 14 are arranged in the sheet conveying direction. It is. The developing units of each image forming station ST are loaded with toners of different colors or densities. In this apparatus, a color image is formed by sequentially superimposing toner images formed in each image forming station ST on a sheet.
[0034]
The image forming apparatus shown in FIG. 3 is an apparatus having a configuration in which a plurality of developing devices 19 (six in the figure) are arranged around a single photosensitive drum 11. Each developing device 19 is loaded with a toner of a different color or density. In this apparatus, the developing unit 19 is sequentially switched to superimpose toner images of respective colors formed on the intermediate transfer body 18, and the superimposed toner images are collectively transferred to a sheet to form a color image.
[0035]
As the image forming apparatus, in addition to those shown here, any configuration may be suitably used as long as it has two or more developing units and performs image formation using two or more types of toner. be able to.
[0036]
In the present embodiment, in the image forming apparatus having the above configuration, dark toner and light toner having different density levels are used for at least one color. By mainly using the light toner in the high-brightness region (highlight region), it is possible to improve the granularity of the high-brightness region and realize high gradation reproducibility.
[0037]
Various combinations can be considered for the combination of toners used in each developing device. Representative combinations are shown below. In the following combinations, LightCyan, LightMagenta, Yellow, and LightBlack toners are light toners whose density levels are lower than dark toners of Cyan, Magenta, DarkYellow, and Black, respectively.
[0038]
1. Cyan, LightCyan, Magenta, Yellow, Black (5 colors in total)
2. Cyan, LightCyan, Magenta, LightMagenta, Yellow, Black (6 colors in total)
3. Cyan, LightCyan, Magenta, LightMagenta, Yellow, DarkYellow, Black (7 colors in total)
4. Cyan, LightCyan, Magenta, LightMagenta, Yellow, DarkYellow, Black, LightBlack (8 colors in total)
5. LightBlue, Cyan, LightRed, Magenta, LightGreen, DarkYellow, Black (7 colors in total)
6. LightCyan, Blue, LightMagenta, Red, Yellow, Green, Black (7 colors in total)
7. Black, LightBlack (2 colors in total)
etc.
[0039]
Note that any other combination can be adopted as the combination of the toner. For example, three or more types of toners having different density levels are used, the range of color expression is widened using special color toners such as orange, gold, silver, and white, and glossiness is obtained using a colorless toner containing no colorant. It is of course possible to improve the method.
[0040]
Further, in the present embodiment, the hue of the dark toner and the hue of the light toner are set so that the hue angles are different from each other. By making the hues of a plurality of types of toner having different density levels different from each other in this manner, it is possible to expand the color reproduction range in the saturation direction in a lightness region near a point where dark toner and light toner are switched.
[0041]
That is, by using dark toner and light toner having different densities and hues, and separating the functions of the toner used for the high brightness area and the low brightness area, the granularity of the high brightness area (highlight area) is improved. At the same time, a wide color reproduction range can be realized by realizing high gradation reproducibility and giving a displacement amount to the hue angle between the dark toner and the light toner.
[0042]
The color reproduction range at this time will be described with reference to FIGS. FIGS. 4 to 6 are diagrams schematically illustrating the color reproduction range of the image forming apparatus using the dark toner and the light toner in the CIELAB space. FIG. 4 shows a case where the hues of the dark toner and the light toner are equal. FIG. 5 shows a case where the hues of the dark toner and the light toner are different from each other. FIG. 6 shows a case where the hues of the dark toner and the light toner are different from each other, and the area where the dark toner and the light toner overlap in the halftone area is wide.
[0043]
By making the hues of the dark toner and the light toner different from each other as shown in FIG. 5 and FIG. 6, the color reproduction range is wider than in the case where the hues of the dark toner and the light toner are equal (FIG. 4). Understand. In particular, as shown in FIG. 6, a region where the color gamuts of the dark toner and the light toner overlap in the halftone region is widened as compared with the case of FIG. 5 in which the hues of the dark toner and the light toner are different but the overlap in the halftone region is small. Thus, it can be understood that the color reproduction range is significantly widened.
[0044]
The displacement of the hue angle between the dark toner and the light toner is preferably 30 degrees or less, more preferably 20 degrees or less on the a * -b * plane. If the hue is too different, the discontinuity of the color tone becomes noticeable between the portion using only the light toner, the portion using both the light toner and the dark toner, and the portion using only the dark toner, which may cause a problem in the quality of the output image. Because there is.
[0045]
Further, the displacement of the hue angle between the dark toner and the light toner is preferably 3 degrees or more, more preferably 5 degrees or more on the a * -b * plane. If the displacement of the hue angle is too small, the effect of expanding the color reproduction range cannot be obtained.
[0046]
Furthermore, when a portion where the gamma of each of the dark and light toners overlaps is provided and the dark and light toners are used together in the intermediate lightness region, the minimum lightness of the light toner is Lp, and the whiteness of the sheet on which an image is formed When the brightness is Lm,
(Lm-Lp) × 0.2 + Lp
It is also preferable that the displacement amount of the hue angle at the brightness obtained by the above is set to 3 degrees or more.
[0047]
The line indicating the hue angle of the primary color at the above lightness is as shown in FIG. In the figure, solid lines indicate primary color lines of Cyan, Magenta, and Yellow, respectively, and dotted lines indicate primary color lines of LightCyan, LightMagenta. By setting the hue angle of the dark and light toners as described above, the hue angle in the portion where both the dark and light toners are used can be made appropriate, and the end point of each line of the dark and light toners in the figure It is possible to widen the color reproduction range in the saturation direction in an intermediate lightness region (a portion connected by a circle) obtained by connecting the two.
[0048]
The ratio of using the dark toner and the light toner is that only the light toner is used in the high brightness area in the CIELAB space, the dark toner and the light toner are used together in the halftone area, and the dark toner and the light toner are used in the low brightness area. It is preferable to form an image by using the toner together or using only the dark toner.
[0049]
FIG. 8 shows an example of the gradation curve of the dark toner and the light toner. The horizontal axis represents the tone value of the image before the separation into the dark toner and the light toner, and the vertical axis represents the tone value when the image is separated into the dark toner and the light toner. Separation refers to dividing image data of a certain color (also referred to as a plate or a channel) into two image data for dark toner and light toner.
[0050]
In the example of FIG. 8, in a high brightness area (highlight area) having a small gradation value, an image is formed using only light toner. Then, up to the gradation value 128, the gradation of the light toner is increased, and when the gradation value exceeds 128, the gradation of the light toner is reduced. On the other hand, with respect to the dark toner, the tone of the dark toner is increased from the point where the tone value exceeds 128. That is, in the halftone area, image formation is performed by combining the light toner and the dark toner.
[0051]
The density curve of the image thus obtained is shown in the graph of FIG. The horizontal axis is the gradation value of the image as in FIG. 8, and the vertical axis is the density of the image. It can be seen that good gradation reproducibility is obtained by using only the light toner in the high brightness region and using the dark toner and the light toner together in the halftone region.
[0052]
Various gradation curves other than those shown in FIG. 8 can be used for the gradation curves of the dark toner and the light toner. At this time, it is preferable that the area where the image formation is performed by combining the dark toner and the light toner is at least 1/5 of the full gradation of the color in order to realize good gradation and a wide color reproduction range.
[0053]
However, as shown in FIG. 10, it is not preferable to use the dark toner and the light toner together from the highlight region because the graininess of the highlight region is reduced (the toner graininess appears). Therefore, in a gradation where the image density to be formed is 0.3 or less, it is preferable to use only the light toner and set the usage rate of the dark toner to 0%.
[0054]
Note that the above-described technology mainly realizes a wide color reproduction range from halftone to high brightness, which is important when outputting an actual natural image. The technique is different from the technique in which the color reproduction range is extended to an area with low brightness by increasing the amount of the applied image.
[0055]
As described above, in the present embodiment, the color reproduction range is expanded by making the hues of the dark toner and the light toner different from each other. In addition, the lightness of the dark toner and the light toner are made different from each other. It has been clarified by the present inventors that the color reproduction range can be further expanded.
[0056]
Specifically, a point (√ (a *) where the saturation of the dark toner and that of the light toner in the CIELAB space are equal. ² + B * ² ) Are set so that the values of the lightness (L *) at the points ()) are different from each other. More preferably, it is preferable that the dark toner and the light toner have different lightness in a high lightness region (a lightness region of 60 or more). FIG. 11 shows an example. The horizontal axis is the saturation (√ (a * ² + B * ² )), And the vertical axis represents lightness (L *). As shown in the figure, the lightness of the light toner is set to be relatively higher in the high lightness region than the lightness of the dark toner having the same saturation.
[0057]
By making the lightness of the dark toner and the lightness of the light toner different from each other, it is possible to further expand the color reproduction range in the saturation direction in the high lightness region. The vivid coloration in the high-brightness area not only makes it possible to significantly improve the texture of transparent photographic images such as the sky and the sea, but is also frequently used for company and product logos. Vivid coloration can also be achieved.
[0058]
In the present specification, the lightness value / density value is a value measured using a spectral densitometer MODEL: 528 manufactured by X-Rite. The values of L * a * b * are measured using a spectrodensitometer MODEL: 528 manufactured by X-Rite under the measurement conditions of an observation light source D50 and an observation field of 2 °.
[0059]
Next, a method for producing the dark toner and the light toner will be described in detail.
[0060]
Examples of the cyan colorant that can be used in the light cyan toner and the dark cyan toner include a copper phthalocyanine compound and its derivative, an anthraquinone compound, and a basic dye lake compound. Specifically, C.I. I. Pigment Blue 1,7,15,15: 1,15: 2,15: 3,15: 4,60,62,66 can be particularly preferably used. These colorants may be mixed with a yellow colorant, a magenta colorant, and the like, which will be described later, to obtain a cyan toner having preferable preferable a *, b *, and L * values. These colorants can be used alone or as a mixture or in the form of a solid solution.
[0061]
The resin component contained in the toner preferably has a peak in a molecular weight range of 600 to 50,000 in a molecular weight distribution of gel permeation chromatography (GPC) of a tetrahydrofuran (THF) -soluble component.
[0062]
In the molecular weight distribution of gel permeation chromatography (GPC), the binder resin used in the toner has a low molecular weight peak in the range of 3,000 to 15,000, which indicates that the shape of the toner produced by the pulverization method is affected by heat and mechanical shock. It is preferable for controlling by force.
[0063]
When the peak of the low molecular weight exceeds 15,000, it is difficult to control the shape factors SF-1 and SF-2 within a preferable range, and the transfer efficiency is not sufficiently improved. If the molecular weight is less than 3000, fusing is likely to occur during the surface treatment of the toner particles.
[0064]
The shape factors SF-1 and SF-2 were randomly sampled by using a FE-SEM (S-800) manufactured by Hitachi, Ltd., and 100 toner images enlarged at a magnification of 500 times. The parameter is defined by a value obtained by performing the analysis by introducing it into an image analysis device (Luzex3) manufactured by Nicolet and performing calculation according to the following equation.
[0065]
(Equation 1)

[0066]
The shape factor SF-1 of the toner indicates the degree of roundness of the toner particles, and gradually changes from spherical to irregular. SF-2 indicates the degree of unevenness of the toner particles, and the unevenness of the toner surface becomes remarkable.
[0067]
Molecular weight is measured by GPC. As a specific GPC measurement method, a sample obtained by previously extracting the toner with tetrahydrofuran (THF) using a Soxhlet extractor for 20 hours is used, and the column configuration is A-801, 802, 803, 804, 805 manufactured by Showa Denko. , 806 and 807, and the molecular weight distribution can be measured using a standard polystyrene resin calibration curve.
[0068]
A resin having a ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 2 to 100 is preferable.
[0069]
The glass transition point (Tg) of the toner is preferably from 50 ° C. to 75 ° C. (more preferably, from 52 ° C. to 70 ° C.) from the viewpoint of fixability and storage stability.
[0070]
The glass transition point of the toner is measured by, for example, a differential scanning calorimeter of a high precision internal heating type input compensation type such as DSC-7 manufactured by PerkinElmer. The measurement method is performed according to ASTM D3418-82. In this embodiment, the DSC curve measured when the sample is heated once to obtain a pre-history, rapidly cooled, and then heated again at a temperature rate of 10 ° C./min and a temperature range of 0 to 200 ° C. Used.
[0071]
The binder resin used in this embodiment is polystyrene; a polystyrene-substituted homopolymer such as poly-p-chlorostyrene and polyvinyltoluene; a styrene-p-chlorostyrene copolymer, and a styrene-vinyltoluene copolymer. Styrene-vinyl naphthalene copolymer, styrene-acrylate copolymer, styrene-methacrylate copolymer, styrene-α-methyl methacrylate copolymer, styrene-acrylonitrile copolymer, styrene-vinyl methyl Styrene-based copolymers such as ether copolymers, styrene-vinyl ethyl ether copolymers, styrene-vinyl methyl ketone copolymers, styrene-butadiene copolymers, styrene-isoprene copolymers, styrene-acrylonitrile-indene copolymers Polymer; polyvinyl chloride, phenol Resin, natural modified phenolic resin, natural resin modified maleic resin, acrylic resin, methacrylic resin, polyvinyl acetate, silicone resin, polyester resin, polyurethane, polyamide resin, furan resin, epoxy resin, xylene resin, polyvinyl butyral, terpene resin, Coumarone indene resin, petroleum resin and the like can be used. Crosslinked styrenic resins are also preferred binder resins.
[0072]
Comonomers for the styrene monomer of the styrene copolymer include acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, phenyl acrylate, methacrylic acid, Monocarboxylic acid having a double bond such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, octyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide or a substituted product thereof; maleic acid, butyl maleate, methyl maleate, Dicarboxylic acids having a double bond such as dimethyl maleate and substituted products thereof; vinyl esters such as vinyl chloride, vinyl acetate and vinyl benzoate; ethylene-based olefins such as ethylene, propylene and butylene S; vinyl methyl ketone, vinyl ketones such as vinyl hexyl ketone, vinyl methyl ether, vinyl ethyl ether, vinyl ethers such as vinyl isobutyl ether; vinyl monomers can be mentioned. These are used alone or in combination.
[0073]
As the crosslinking agent, a compound having two or more polymerizable double bonds is used. For example, aromatic divinyl compounds such as divinylbenzene and divinylnaphthalene; carboxylic esters having two double bonds such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, and 1,3-butanediol dimethacrylate; divinylaniline; Divinyl compounds such as divinyl ether, divinyl sulfide and divinyl sulfone; and compounds having three or more vinyl groups. These may be used alone or as a mixture.
[0074]
From the viewpoint of improving the releasability from the fixing member at the time of fixing and improving the fixing property, it is also preferable to include the following waxes in the toner particles. Paraffin wax and its derivatives, microcrystalline wax and its derivatives, Fischer-Tropsch wax and its derivatives, polyolefin wax and its derivatives, carnauba wax and its derivatives. Derivatives include oxides, block copolymers with vinyl monomers, and graft-modified products.
[0075]
In addition, long-chain alcohols, long-chain fatty acids, acid amides, ester waxes, ketones, hydrogenated castor oil and derivatives thereof, plant-based waxes, animal-based waxes, mineral-based waxes, petrolactam, and the like may be optionally used.
[0076]
To prepare a toner, a binder resin, wax, a pigment as a colorant, a dye, or a magnetic substance, and if necessary, additives such as a charge control agent are sufficiently mixed by a mixer such as a Henschel mixer or a ball mill. The pigment, dye or magnetic material is dispersed or dissolved in a resin by melting and kneading using a hot kneader such as a heating roll, kneader, extruder, etc., and after cooling and solidifying, pulverizing and classifying the toner Can be obtained. In the classifying step, it is preferable to use a multi-segment classifier in terms of production efficiency.
[0077]
Further, a method described in JP-B-56-13945, etc., in which a molten mixture is atomized into air using a disk or a multi-fluid nozzle to obtain a spherical toner, and JP-B-36-10231, JP-A-59-53856. JP-A-59-61842, JP-A-59-61842, a method of directly forming a toner using a suspension polymerization method, and a method using a water-soluble organic solvent in which a monomer is soluble and a polymer obtained is insoluble. The toner particles may be produced by an emulsion polymerization method typified by a dispersion polymerization method for directly producing a toner or a soap-free polymerization method for producing a toner by directly polymerizing in the presence of a water-soluble polar polymerization initiator.
[0078]
Further, the colorant of each of the cyan, magenta, yellow, and black toners will be described.
[0079]
Examples of the black colorant include carbon black, a magnetic material, and a black color tone using a yellow / magenta / cyan colorant shown below.
[0080]
Examples of the yellow colorant include compounds represented by condensed azo compounds, isoindolinone compounds, anthraquinone compounds, azo metal complexes, methine compounds, and allylamide compounds. Specifically, C.I. I. Pigment Yellow 12, 13, 14, 15, 17, 62, 74, 83, 93, 94, 95, 97, 109, 110, 111, 120, 127, 128, 129, 147, 168, 174, 176, 180, 181 and 191 are preferably used.
[0081]
Examples of the magenta colorant include condensed azo compounds, diketopyrrolopyrrole compounds, anthraquinones, quinacridone compounds, basic dye lake compounds, naphthol compounds, benzimidazolone compounds, thioindigo compounds, and perylene compounds. Specifically, C.I. I. Pigment Red 2,3,5,6,7,23,48; 2,48; 3,48; 4,57; 1,81; 1,144,146,166,169,177,184,185,202, 206, 220, 221, 254 are particularly preferred.
[0082]
As described above, the cyan colorant includes a copper phthalocyanine compound and its derivative, an anthraquinone compound, a basic dye lake compound and the like. Specifically, C.I. I. Pigment Blue 1,7,15,15: 1,15: 2,15: 3,15: 4,60,62,66 can be particularly preferably used.
[0083]
Examples of the magnetic material include metal oxides containing elements such as iron, cobalt, nickel, copper, magnesium, manganese, aluminum, and silicon. Among them, those containing iron oxide as a main component, such as ferric oxide and γ-iron oxide, are preferred. It may contain a metal element such as a silicon element or an aluminum element from the viewpoint of controlling the chargeability of the toner. These magnetic particles preferably have a BET specific surface area of 2 to 30 m by a nitrogen adsorption method. ² / G, especially 3 to 28 m ² / G is preferred, and a magnetic material having a Mohs hardness of 5 to 7 is preferred.
[0084]
The dark toner and the light toner described above have different density levels and hue angles as appropriate by using different colorants. Alternatively, the density level and the hue angle can be appropriately changed by using the same colorant and changing the content thereof. In this case, by setting the content of the colorant of the light toner to 1/5 or less of the content of the colorant of the dark toner, a preferable density level can be set.
[0085]
Next, an image forming operation of the above-described image forming apparatus will be described.
[0086]
Here, an input image composed of three colors of Red (R), Green (G), and Blue (B) is converted into cyan (DC), light cyan (LC), magenta (DM), light magenta (LM), and yellow (Y). , Black (K) is used to form an image. That is, cyan is output using two types of toners, LC and DC, and magenta is output using two types of toners, LM and DM.
[0087]
The image forming apparatus reads a color image on a document by a document reading device (scanner unit) and obtains an input image signal which is separated into RGB by a CCD. Alternatively, when the image forming apparatus has a printer function, RGB print data (input image signal) may be obtained from a computer. Although the RGB input image is used here, it is merely based on the specifications of the document reading device and the printer driver of the computer. As an input image, in addition to the RGB image, a CMYK image, a CMYK + LC + LM image, an L * a * b * image, an image including a channel for a special color, and the like can be input.
[0088]
When an image is formed, the input RGB color signals must be converted into CMYK + LC + LM color signals for image formation (which can be output by an output device).
[0089]
FIG. 12 shows one method of the color conversion method.
[0090]
In FIG. 12, after the RGB signals of the input image are separated into four colors of CMYK, specific colors (C and M) are separated into two density data, and finally Y, K, LC, DC, Six color signals of LM and DM are obtained. Then, after performing a predetermined γ correction on the six color signals, halftone processing is performed, and the halftone processing is input to the PWM circuit.
[0091]
In the case of such a color conversion method, the RGB color signals are converted into primary colors of C and M and then separated into color signals of dark and light, such as LC + DC and LM + DM. If they are different, the hue becomes non-uniform in a gradation or a highlight portion of a single color, which may cause a sense of discomfort to the eye. However, in the present embodiment, the hue displacement of the two types of toner is set to 30 degrees or less, more preferably 20 degrees or less. It is possible to realize granularity and wide color reproduction.
[0092]
Note that various combinations can be considered for the method of converting the data into two versions of light and shade depending on the toner density level and the like. FIG. 8 shows a basic linear gradation conversion method.
[0093]
As shown in the drawing, the light toner first rises at the highlight, the dark toner starts to enter near the halftone, and while the gradation is reproduced for a while by the combination of the light and shade, the use of the light toner is restricted in the high density part. The combination of shades of light and shade at this time is determined based on the relationship between image quality such as graininess, tone, color gamut, and toner consumption. Although a linear gradation is shown here for simplicity, it is preferable to draw a gradual curve at the beginning of the density of each density toner from the viewpoint of actually preventing a tone jump.
[0094]
FIG. 13 shows another method of the color conversion method.
[0095]
Here, color separation is directly performed on the RGB signals of the input image into signals of six colors of Y, K, LC, DC, LM, and DM by direct mapping.
[0096]
The direct mapping refers to a color conversion method for directly converting an input signal (color information of an input image) to an output signal of an output device (color information for image formation) with reference to a lookup table (LUT). For example, by giving three input signals such as L * a * b * and RGB in the color space, the signal values in the output color space required to reproduce the color can be converted into four colors of CMYK or CMYK + LC + LM. Output in the form of six colors.
[0097]
This color conversion method does not require a matrix operation and allows non-linear conversion, greatly improving the degree of freedom in color conversion such as UCR settings, and controlling the amount of toner applied to achieve the desired color reproduction. Can be made possible.
[0098]
In addition, according to the direct mapping, the color signals of the dark toner and the light toner are directly generated from the RGB signals of the input image. Also does not invite.
[0099]
As described above, according to the image forming apparatus of the present embodiment, an image is formed using only the light toner in the high brightness area using the dark toner and the light toner having different densities and hues, and the light is formed in the halftone area. Since the image is formed using both toner and dark toner, good gradation and graininess can be achieved, and wide color reproduction from mid-tones to high brightness areas, which is especially important when outputting natural images, etc. The range can be realized, and high-quality image formation can be performed.
[0100]
【Example】
(Example 1)
Among two colors of CMKY, an image forming apparatus for forming a color image with six kinds of toners of Cyan, LightCyan, Magenta, LightMagenta, Yellow and Black using two kinds of toners having different density levels for two colors of cyan and magenta. Was configured.
[0101]
Here, different colorants are used, such as a pigment-based colorant for the dark toner and a dye-based colorant for the light toner, and the dark toner and the light toner have different density levels and hue angles. Was. Specifically, each toner was prepared using the following raw materials.
[0102]
<Cyan>
Polyester resin (100 parts by weight) / phthalocyanine pigment (3 parts by weight)
<LightCyan>
Polyester resin (100 parts by weight) / Anthraquinone dye (0.6 parts by weight)
<Magenta>
Polyester resin (100 parts by weight) / quinacridone pigment (3 parts by weight)
<LightMagenta>
Polyester resin (100 parts by weight) / Anthraquinone dye (0.6 parts by weight)
[0103]
The raw materials were premixed by a Henschel mixer, melt-blended by a twin-screw extruder, cooled, and coarsely ground to about 1 to 2 mm using a hammer mill. Next, it was pulverized by a pulverizer using an air jet method. The obtained finely pulverized product was classified and silica was externally added to obtain particles of Cyan, LightCyan, Magenta, and LightMagenta having a weight average particle size of 5.6 μm.
[0104]
At this time, the displacement amount of the hue angle between the dark toner and the light toner is as follows, where the minimum lightness of the light toner in the CIELAB space is Lp, and the lightness of the sheet on which an image is formed is Lm.
(Lm-Lp) × 0.2 + Lp
In the brightness obtained by the above, it was 3 degrees.
[0105]
When an image was formed using the toner having the above configuration, as shown in Table 1, the color gamut could be increased by about 30% as compared with the comparative example, and a wide color gamut close to the image quality of a photograph was obtained. Was realized.
[0106]
Comparative Example 1 is a result of a CLC1100 copying machine manufactured by Canon Inc. using four color toners of Cyan, Magenta, Yellow, and Black. Comparative Example 2 is a result in the case where dark and light toners are used for cyan and magenta, and there is no displacement amount of the hue angle of the dark and light toners at that time. In Comparative Example 3, the density toner was used for cyan and magenta, and the displacement amount of the hue angle of the density toner at that time was the above-mentioned amount.
(Lm-Lp) × 0.2 + Lp
This is the result in the case of 1 to 2 degrees in the brightness obtained by:
[0107]
The comparison of the width of the color gamut was evaluated by a relative value when the volume of the color gamut of Comparative Example 1 was set to 100.
[0108]
[Table 1]

[0109]
(Example 2)
Among two colors of CMKY, an image forming apparatus for forming a color image with six kinds of toners of Cyan, LightCyan, Magenta, LightMagenta, Yellow and Black using two kinds of toners having different density levels for two colors of cyan and magenta. Was configured.
[0110]
Here, the same pigment-based colorant was used for both the dark toner and the light toner, and the density level and the hue angle of the dark toner and the light toner were made different from each other by changing the content of the colorant. . Specifically, each toner was prepared using the following raw materials.
[0111]
<Cyan>
Polyester resin (100 parts by weight) / phthalocyanine pigment (4 parts by weight)
<LightCyan>
Polyester resin (100 parts by weight) / phthalocyanine pigment (0.7 parts by weight)
<Magenta>
Polyester resin (100 parts by weight) / quinacridone pigment (5 parts by weight)
<LightMagenta>
Polyester resin (100 parts by weight) / quinacridone pigment (1 part by weight)
[0112]
The raw materials were premixed by a Henschel mixer, melt-blended by a twin-screw extruder, cooled, and coarsely ground to about 1 to 2 mm using a hammer mill. Next, it was pulverized by a pulverizer using an air jet method. The obtained finely pulverized product was classified and silica was externally added to obtain particles of Cyan, LightCyan, Magenta, and LightMagenta having a weight average particle size of 5.6 μm.
[0113]
The characteristics of the obtained toner are shown in the a * -b * plan view of FIG. From the figure, it can be understood that each hue angle of Cyan and LightCyan, and Magenta and LightMagenta is 5 degrees or more in CIELAB space. At this time, the displacement amount of the hue angle between the dark toner and the light toner is as follows, where the minimum lightness of the light toner in the CIELAB space is Lp, and the lightness of the sheet on which an image is formed is Lm.
(Lm-Lp) × 0.2 + Lp
Was 3 degrees or more.
[0114]
FIG. 15 shows each gradation. It is possible to form a high brightness area using the illustrated light toner and a low brightness area using the dark toner.
[0115]
When an image was formed using the toner having the above configuration, as shown in Table 2, the color gamut could be increased by about 20% as compared with the comparative example, and a wide color gamut close to the image quality of a photograph was obtained. Was realized. Note that Comparative Example 1, Comparative Example 2, and Comparative Example 3 are the same as those in Example 1 described above.
[0116]
[Table 2]

[0117]
【The invention's effect】
As described above, the present invention uses dark toner and light toner having different densities and hues to form an image using only light toner in a high brightness region, and combines light toner and dark toner in a halftone region. Since image formation is performed, good gradation and graininess can be achieved, and a wide color reproduction range is realized from midtones to high brightness areas, which is especially important when outputting natural images, etc. It is possible to perform high quality image formation approaching.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram illustrating a basic configuration of an electrophotographic image forming apparatus.
FIG. 2 is a schematic configuration diagram illustrating a configuration example of an image forming apparatus according to an embodiment of the present invention.
FIG. 3 is a schematic configuration diagram showing another configuration example of the image forming apparatus according to the embodiment of the present invention.
FIG. 4 is a schematic diagram showing a color reproduction range when the hues of a dark toner and a light toner are equal.
FIG. 5 is a schematic diagram illustrating a color reproduction range when the hues of the dark toner and the light toner are different from each other.
FIG. 6 is a schematic diagram illustrating a color reproduction range when the hues of the dark toner and the light toner are different from each other and the overlapping area of the dark toner and the light toner in the halftone area is wide.
FIG. 7 is a schematic diagram showing a hue angle of a primary color at a predetermined brightness.
FIG. 8 is a diagram illustrating an example of a gradation curve of a dark toner and a light toner.
9 is a diagram showing a density curve of an image obtained by the gradation curve of FIG. 8;
FIG. 10 is a diagram showing a density curve of an image obtained by a gradation curve different from that in FIG. 8;
FIG. 11 is a diagram illustrating a difference in lightness between a dark toner and a light toner in a high lightness region.
FIG. 12 is a diagram illustrating one method of a color conversion method.
FIG. 13 is a diagram illustrating another method (direct mapping) of the color conversion method.
FIG. 14 is an a * -b * plan view showing characteristics of a toner according to Example 2.
FIG. 15 is a diagram illustrating the gradation of a toner according to the second embodiment.
[Explanation of symbols]
10 sheets
11 Photosensitive drum
12 Charger
14 Transfer charger
15 Fixing device
16 Cleaning member
17 Image exposure unit
18 Intermediate transfer member
19 Developing device
ST Image forming station

Claims (16)

In an electrophotographic image forming apparatus that forms a color image using a plurality of toners,
For at least one color,
Using dark toner and light toner whose density and hue are different from each other,
In the high brightness region, an image is formed only with the light toner,
An image forming apparatus, wherein an image is formed by combining the dark toner and the light toner in a halftone area. 2. The image forming apparatus according to claim 1, wherein the lightness at the point where the saturation of the dark toner and the light toner is equal is different. The image forming apparatus according to claim 2, wherein the lightness of the dark toner and the lightness of the light toner are different from each other at least in a region having a lightness of 60 or more in the CIELAB space. 4. The image forming apparatus according to claim 1, wherein a displacement amount of a hue angle between the dark toner and the light toner is 3 degrees or more in a CIELAB space. 4. The image forming apparatus according to claim 1, wherein a displacement amount of a hue angle between the dark toner and the light toner is 5 degrees or more in a CIELAB space. In CIELAB space,
The minimum lightness of the light toner is Lp,
When the brightness of a sheet on which an image is formed is Lm,
(Lm-Lp) × 0.2 + Lp
4. The image forming apparatus according to claim 1, wherein the amount of displacement of the hue angle between the dark toner and the light toner at the lightness determined by (3) is 3 degrees or more. 7. The image forming apparatus according to claim 1, wherein an area where image formation is performed by combining the dark toner and the light toner is １ ／ or more of all tones of the color. 8. apparatus. In an image forming apparatus which forms a color image with at least three colors of cyan, magenta and yellow,
The image forming apparatus according to any one of claims 1 to 7, wherein the dark toner and the light toner are used for two colors of cyan and magenta. The image forming apparatus according to any one of claims 1 to 8, wherein the dark toner and the light toner are toners having different colorants. 9. The image forming apparatus according to claim 1, wherein the dark toner and the light toner have the same colorant and have different contents of the colorant. 9. . 11. The image forming apparatus according to claim 9, wherein the content of the colorant in the light toner is 1/5 or less of the content of the colorant in the dark toner. The image forming apparatus according to claim 1, wherein color signals of the dark toner and the light toner are generated by direct mapping from color signals of an input image. 12. The color signal of an input image is converted into a color signal for image formation, and the color signal for image formation is separated into color signals of the dark toner and the light toner, respectively. An image forming apparatus according to any one of the preceding claims. 14. The image forming apparatus according to claim 1, wherein a displacement amount of a hue angle between the dark toner and the light toner is equal to or less than 30 degrees in a CIELAB space. 14. The image forming apparatus according to claim 1, wherein a displacement amount of a hue angle between the dark toner and the light toner is equal to or less than 20 degrees in a CIELAB space. The image forming apparatus according to any one of claims 1 to 15, wherein an image is formed using only light toner in an area where the formed image density is 0.3 or less.

Images