JP2004287047A - Developing device, image forming apparatus, process cartridge, and developer carrier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress adhesion of a carrier in a developer to the surface of a latent image carrier even when the carrier in the developer has a small diameter. <P>SOLUTION: On the downstream side of a main magnetic pole P1b which forms a magnetic field for causing a developer to form a carrier brush in a developing region, in the surface rotating direction of a developing sleeve 81, an auxiliary magnetic pole P1c whose polarity is opposite to that of the main magnetic pole is disposed. On a developing sleeve surface from the main magnetic pole P1b to the auxiliary magnetic pole P1c, the sum of the absolute value of normal magnetic flux density and the absolute value of tangential magnetic flux density is allowed to increase monotonously, whereby carrier adhesion can be effectively suppressed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【数３】

【００６２】
そして、現像スリーブ８１の軸方向をｘ方向とし、現像スリーブ表面移動方向におけるその表面の接線方向をｙ方向とし、現像スリーブ表面の法線方向をｚ方向とする。なお、ｙ方向は現像スリーブが表面移動する向きを正とし、ｚ方向は現像スリーブの中心軸から離れる向きを正とする。このとき、キャリアを現像スリーブ８１側に拘束する拘束力となるのは、キャリアに働く磁力Ｆのｚ方向成分Ｆｚである。そして、磁束密度Ｂのｘ方向成分Ｂｘは０に近似できる。よって、キャリアを現像スリーブ８１側に拘束する拘束力Ｆｚは、上記数２及び上記数３に示した演算式より、下記の数４に示すとおりとなる。
【数４】
Ｆｚ ≒ ｋ・｛（Ｂｙ・（∂Ｂｚ／∂ｙ）＋Ｂｚ・（∂Ｂｚ／∂ｚ）｝
【００６３】
上記数４に示す演算式から、上記キャリア離脱領域においてキャリアに働く拘束力Ｆｚを求める。このキャリア離脱領域は、主磁極Ｐ１ｂと下流側補助磁極Ｐ１ｃとのちょうど真ん中の領域であって、磁気ブラシの先端部分が通過する部分である。そのため、このキャリア離脱領域では、磁力線の向きはほぼｙ方向に対して平行であり、磁束密度Ｂのｚ方向成分は０に近似できる。そうすると、キャリア離脱領域においてキャリアに働く拘束力Ｆｚは、上記数４に示す演算式より、下記の数５に示すとおりとなる。
【数５】
Ｆｚ ≒ ｋ・Ｂｙ・（∂Ｂｚ／∂ｙ）
【００６４】
ところで、現像領域の中心においては、磁気ブラシの本来の長さよりも現像ギャップＰｇの方が狭いため、磁気ブラシは短く規制された状態にある。そのため、磁気ブラシは、現像領域の中心を通過した後、現像領域の中心から離れるにつれて徐々に長くなる。したがって、磁気ブラシの先端部分は、現像領域の中心から離れるにつれて現像スリーブ表面から遠ざかることになる。したがって、磁気ブラシの先端部分を構成するキャリアに働く磁力は、現像領域の中心から離れるにつれて弱まることになる。なお、感光体ドラム２０の表面も、現像領域の中心から離れるにつれて現像スリーブ表面から遠ざかるため、現像領域の中心から離れるにつれてキャリアに働く静電力も弱まることになる。しかし、この静電力の弱まる度合いは、磁気ブラシに働く磁力が弱まる度合いに比べて小さい。よって、現像領域の中心から離れるにつれて現像剤からキャリアが離脱しやすくなる。したがって、現像剤からのキャリア離脱を抑制するには、現像領域の中心から離れるほど拘束力Ｆｚを強くする必要がある。
【００６５】
上記数５に示す演算式から、キャリア離脱領域における拘束力Ｆｚを高めるためには、キャリア離脱領域における磁束密度Ｂのｙ方向成分Ｂｙを高める必要がある。また、キャリア離脱領域において、キャリア離脱領域における磁束密度Ｂの絶対値は、下記の数６に示す演算式から求めることができ、その値は、そのｙ方向成分Ｂｙの絶対値に近似できる。そして、上述したように現像剤からのキャリア離脱を抑制するには現像領域の中心から離れるほど拘束力Ｆｚを強くする必要があることを考慮すると、キャリア離脱領域において磁束密度Ｂの絶対値が現像領域の中心から離れるほど大きくなれば、現像剤からのキャリア離脱を抑制することができることになる。なお、このキャリア離脱領域は、主磁極Ｐ１ｂと下流側補助磁極Ｐ１ｃの中間点にあるため、主磁極Ｐ１ｂと下流側補助磁極Ｐ１ｃとの間において最も磁束密度Ｂが小さくなる箇所である。したがって、このキャリア離脱領域で磁束密度Ｂの絶対値が現像領域の中心から離れるほど大きくなるということは、主磁極Ｐ１ｂから下流側補助磁極Ｐ１ｃに向かって、磁束密度Ｂの絶対値が常に増加することを意味する。そして、磁束密度Ｂの絶対値は、そのｘ方向成分Ｂｘが０に近似できることから、そのｙ方向成分Ｂｘとｚ方向成分Ｂｚとを合成したものの絶対値に近似することができる。また、磁束密度Ｂの絶対値が常に増加するということは、ｙ方向成分Ｂｘの絶対値とｚ方向成分Ｂｚの絶対値との和が単調増加することに等しい。更に、このように単調増加するということは、現像スリーブ表面上における磁束密度のｙ方向成分Ｂｘの絶対値とｚ方向成分Ｂｚの絶対値との和が単調増加することと同義である。したがって、主磁極Ｐ１ｂから下流側補助磁極Ｐ１ｃに向かって、現像スリーブ表面における法線方向磁束密度の絶対値と接線方向磁束密度の絶対値との和が単調増加するような磁束密度分布を得ることで、現像剤からのキャリア離脱を抑制することができ、感光体ドラム２０へのキャリア付着を抑制することができるものと考えられる。
【数６】
Ｂ ＝ （Ｂｘ^２＋Ｂｙ^２＋Ｂｚ^２）^１／２
≒ ｜Ｂｙ｜
【００６６】
なお、本発明者らの研究の結果、主磁極Ｐ１ｂにより現像スリーブ表面上に生じる最大の法線方向磁束密度をＢｍ_１とし、下流側の補助磁極Ｐ１ｃにより現像スリーブ表面上に生じる最大の法線方向磁束密度をＢｍ_２としたとき、下記の数７に示す関係式を満たせば、キャリア付着をより効果的に抑制することができることも判明した。
【数７】
１００［ｍＴ］ ≦ Ｂｍ_１ ≦ Ｂｍ_２ ≦ １６０［ｍＴ］
【００６７】
〔比較例１〕
次に、キャリアの体積平均粒径が小さくなるほどキャリア付着が起きやすくなることを確認するために行った比較実験（以下、「比較例１」という。）について説明する。本比較例１では、上述した実験例１及び実験例２で用いたキャリアよりも体積平均粒径が小さいキャリアを用いて、キャリア付着の評価を行った。具体的には、上述した実験例１及び実験例２では、体積平均粒径が３５［μｍ］である小径のキャリアを用いたが、本比較例１では、体積平均粒径が５５［μｍ］である小径のキャリアを用いた。そして、本比較例１では、上述した実験例２と同様に、白ポチの個数に基づいて感光体ドラム２０へのキャリア付着を評価した。
【００６８】
図１３及び図１４は、本実施形態１よりも強い磁石からなる主磁極Ｐ１ｂを用いたときの現像スリーブ表面での磁束密度の分布を示した円グラフである。なお、この円グラフから分かるように、これらの図にそれぞれ示した現像装置は、本実施形態１に係る現像装置とは異なり、合成磁束密度が単調増加する構成とはなっていない。
図１３に示した比較用の現像装置では、主磁極Ｐ１ｂにより現像スリーブ表面上に生じる最大の法線方向磁束密度Ｂｍ_１は１００．２［ｍＴ］であり、下流側補助磁極Ｐ１ｃにより現像スリーブ表面上に生じる最大の法線方向磁束密度Ｂｍ_２は９５．７［ｍＴ］であった。また、キャリア離脱領域に対応する現像スリーブ表面すなわち現像スリーブ表面上において法線方向磁束密度が０となる部分に生じる接線方向磁束密度Ｂｍ_０は８４．１［ｍＴ］であった。
図１４に示した比較用の現像装置では、主磁極Ｐ１ｂにより現像スリーブ表面上に生じる最大の法線方向磁束密度Ｂｍ_１は９４．５［ｍＴ］であり、下流側補助磁極Ｐ１ｃにより現像スリーブ表面上に生じる最大の法線方向磁束密度Ｂｍ_２は１０２．２［ｍＴ］であった。また、キャリア離脱領域に対応する現像スリーブ表面すなわち現像スリーブ表面上において法線方向磁束密度が０となる部分に生じる接線方向磁束密度Ｂｍ_０は８６．３［ｍＴ］であった。
【００６９】
図１５は、本比較例１におけるキャリア付着の評価結果を示すグラフである。この評価結果と、図１２に示した上記実験例２における比較用の現像装置の評価結果とを比較すると、本比較例１に係る２つの現像装置の白ポチ数の方が明らかに少ない。これにより、キャリアの体積平均粒径が小さくなるほどキャリア付着が起きやすくなることが確認できる。
【００７０】
また、図１６は、地肌ポテンシャルを２００Ｖと一定にし、体積平均粒径が３５［μｍ］、５０［μｍ］、６５［μｍ］の３種類のキャリアを用いた現像剤により画像を形成したときの感光体ドラムに付着したキャリア付着数を測定したときの結果を示すグラフである。なお、このキャリア付着数は、上述した実験例１と同様の方法で測定したものであり、現像剤のトナー濃度を変化させて測定した。この測定結果からもわかるように、キャリアの体積平均粒径が小さくなるほどキャリア付着が起きやすくなることが確認できる。
【００７１】
〔比較例２〕
次に、キャリアの体積平均粒径が小さくなるほど画像の粒状度が改善される点を確認するために行った実験（以下、「比較例２」という。）について説明する。なお、この実験では、体積平均粒径が３５［μｍ］、５０［μｍ］、６５［μｍ］の３種類のキャリアを用いた現像剤によりハーフトーンドット画像を形成し、その画像の明度を変化させて粒状度を測定した。この粒状度は、値が小さいほど画像上でのドット再現性が良いことを示している。
【００７２】
図１７は、本比較例２における実験結果を示すグラフである。このグラフから分かるように、体積平均粒径が小さいほど画像の粒状度が小さくなる。よって、体積平均粒径が小さいほど画像の粒状度が改善されることが確認された。
特に、キャリアの体積平均粒径が５０［μｍ］以上の場合には、明度が７０〜９０の領域においてハーフトーンドット画像の粒状度が０．３以上である。これに対し、キャリアの体積平均粒径を３５［μｍ］程度まで小さくすると、その粒状度が０．１となり、キャリアの体積平均粒径が５０［μｍ］以上の場合に比べて３倍近くも粒状度が改善される。そのため、近年においては、キャリアの小径化は避けられないものであると言える。
【００７３】
このようにキャリアの体積平均粒径が大きくなると粒状度が悪化する理由は、次のように考えられる。
すなわち、キャリアの体積平均粒径が大きくなると、感光体ドラム２０に接触する磁気ブラシの先端部分を構成するキャリアの数が少なくなり、磁気ブラシ先端部分で互いに隣り合うキャリアの間隔が広くなる。そのため、感光体ドラム２０上の静電潜像に対する磁気ブラシによる摺擦が粗く、静電潜像に付着するトナーの量のバラツキが大きいものとなる。その結果、ドット再現性が悪く、また画質にザラツキ感のある画像が形成され、粒状度が悪くなる。
【００７４】
次に、本実施形態１に用いる現像剤のキャリアについて説明する。
図１８は、本実施形態１に用いるキャリアの断面図を示す模式図である。
二成分現像剤のキャリアとしては、弾力性と強い接着力とを有するコート膜であって、膜厚よりも大きい径を有する粒子を表面に含有したコート膜で被覆したものを用いることが望ましい。キャリア９０の芯材としてフェライト９１を用いている。このフェライトは、３×１０^６／４π［Ａ／ｍ］磁場中における磁化の強さが、６２×１０^−７×４π［Ｗｂ・ｍ／ｋｇ］であるものである。このフェライト９１の表面を、アクリル等の熱可塑性樹脂とメラミン樹脂とを架橋させた樹脂成分に、帯電調整剤を含有させたコート膜９２で被覆している。このコート膜９２は弾力性と強い接着力を有している。さらに、コート膜９２の膜厚よりも大きい径の粒子、例えばアルミナ粒子９３を表面に分散している。アルミナ粒子９３はコート膜９２の強い接着力で保持されている。従来のキャリアは硬いコート膜を徐々に削りながら長寿命を得るという思想の基で構成されていたのに対し、図示のキャリア９０はコート膜９２が弾力性を有することで衝撃を吸収して膜削れを抑制する。また、膜厚よりも大きい径を有するアルミナ粒子９３をキャリア９０表面に分散することで、コート膜９２への衝撃を阻止し、しかもスペント物のクリーニングを行なうことができる。このように、コート膜の膜削れとスペント化を抑制できるので、従来のキャリアに比べ、より長寿命化を図ることができる。これにより、長期間に渡り、トナー汲上量の安定化、すなわち品質の安定化を期待できる。
【００７５】
更には、キャリア粒径を小さくして、よりドット再現性に優れた画像を形成することもできる。具体的には、キャリア粒径の大きさは、２０［μｍ］以上５０［μｍ］以下が望ましい。キャリア粒径を従来よりも小さく、さらに粒径をこのような範囲に設定することで、作像時の磁気ブラシの太さを均一に細くすることが可能になる。よって、より緻密なトナーの受け渡しをすることができる。また、現像スリーブ上の単一面積当たりにおける現像剤穂の密度も多くなるので、感光体上の潜像に隙間無くトナーの受け渡しが可能になる。これにより、よりドット再現性に優れた画像を形成することができる。なお、キャリア粒径が５０［μｍ］よりも大きすぎると、同じキャリア量で比較した場合に、キャリアの総表面積が小さくなって、トナーの保有量が少なくなる。このため、トナー濃度の低下が生じる。汲み上げ量を増やして現像能力を維持することが可能であるが、トナー固着が発生しやすくなってしまう。一方、キャリア粒径が２０［μｍ］よりも小さすぎると、マグローラによる磁力保持力が小さくなってキャリア飛散を生じ、感光体へのキャリア付着が増加してしまうので、２０［μｍ］以上が望ましい。
【００７６】
次に、本実施形態１に用いる現像剤のトナーについて説明する。
本実施形態１に用いるトナーの製造方法は特に限定されないが、粉砕法によるトナーよりも、重合法によるトナーの方が望ましい。参考までに、重合法によるトナーの製造方法の一例について説明しておく。
【００７７】
基本樹脂準備→乳化→熟成→脱溶剤→アルカリ洗浄、水洗→乾燥→外添剤処理という一連の流れが製造工程の概要である。まず、酢酸エチル等の有機溶媒に、樹脂、着色剤、ワックス、帯電制御剤等を溶解させたものを基本樹脂溶液とする。そして、界面活性剤、粘度調整剤、樹脂微粒子を含有する水系媒体に、上記基本樹脂溶液とアミン類とを加えて、せん断力により分散させて、乳化状態を生起せしめる。次いで、イソシアネート基とアミン類との作用による伸長や架橋反応を促進させるべく、反応系を加熱して熟成させる。そして、例えば、系全体を徐々に昇温して、液滴中の有機溶媒を蒸発除去するなどの方法により、脱溶剤処理を行う。次に、脱溶剤によって得られたトナー粒子表面に残存している異物（界面活性剤、粘度調整剤など）を除去すべく、アルカリ洗浄処理や水洗処理を施す。そして、濾過によってトナー粒子を回収した後、乾燥させる。更に、必要に応じて、シリカ、チタニア、アルミナ等の外添剤微粒子をミキサー等によって０．１〜５．０重量部程度の量で外添する。
【００７８】
より詳しい製造例について説明すると、例えば次のようになる。
まず、冷却管、攪拌機及び窒素導入管の付設された反応槽中にて、ビスフェノールＡエチレンオキサイド２モル付加物６９０重量部と、テレフタル酸２５６重量部とを常圧且つ２３０［℃］の環境下で８時間重縮合反応させる。そして、１０〜１５［ｍｍＨｇ］まで減圧せしめて５時間反応させてから１６０［℃］まで冷却した後、１８重量部の無水フタル酸を加えて更に２時間反応させて変性されていないポリエステル（ａ）を得る。
【００７９】
かかるポリエステル（ａ）とは別に、ベースとなるポリマー１分子中に２以上の反応基を有するプレポリマーを準備する。具体的には、ビスフェノールＡエチレンオキサイド２モル付加物８００重量部と、イソフタル酸１８０重量部と、テレフタル酸６０重量部と、ジブチルチンオキサイド２重量部とを反応槽中に入れる。そして、常圧且つ２３０［℃］の環境下で８時間重縮合反応させた後、１０〜１５［ｍｍＨｇ］まで減圧して脱水しながら５時間反応させる。次いで、１６０［℃］まで冷却してから、３２重量部の無水フタル酸を加えて更に２時間反応させる。そして、８０［℃］まで冷却して酢酸エチル中にてイソホロンジイソシアネート１７０重量部と２時間反応させて、イソシアネート基含有プレポリマー（１）を得る。
【００８０】
また、ケチミン化合物も準備する。例えば、攪拌棒及び温度計の付設された反応槽中に、イソホロンジアミン３０重量部とメチルエチルケトン７０重量部を入れて、５０［℃］で５時間反応させてケチミン化合物（２）を得る。
【００８１】
上記イソシアネート基含有プレポリマー（１）１５．４重量部と、上記ポリエステル（ａ）６０重量部と、酢酸エチル７８．６重量部とをビーカー内に入れて攪拌溶解する。そして、離型促進剤であるライスＷＡＸ（融点８３℃）１０重量部と、銅フタロシアニンブルー顔料（シアン顔料）４重量部とを入れて、ＴＫ式ホモミキサーを用いて６０［℃］の環境下にて１２０００［ｒｐｍ］の速度で攪拌して、均一に溶解、分散させる。次いで、上記ケチミン化合物（２）２．７重量部を加え溶解させて、基本樹脂溶液（３）を得る。
【００８２】
イオン交換水３０６重量部と、リン酸カルシウム１０％懸濁液２６５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、平均粒径０．２０［μｍ］のスチレン／アクリル系樹脂微粒子とをビーカー内に入れて均一に溶解する。そして、６０［℃］に昇温した後、ＴＫ式ホモミキサーを用いて１２０００［ｒｐｍ］の速度で攪拌しながら、上記基本樹脂溶液（３）を投入して１０分間攪拌する。次いで、得られた混合液を攪拌棒及び温度計の付設されたコルベンに５００［ｇ］移して４５［℃］まで昇温した後、減圧下にてウレア化反応させながら０．５時間かけて溶剤を除去する。そして、濾別、洗浄、乾燥工程を経てから風力分級して母体粒子（４）を得る。
【００８３】
上記母体粒子１００重量部と、帯電制御剤（オリエント化学社製 ボントロンＥ−８４）０．２５重量部とをＱ型ミキサー（三井鉱山社製）に入れる。そして、ミキサーのタービン型羽根の周速を５０［ｍ／ｓｅｃ］に設定して、２分間のミキシングと１分間の休止とを５セット行う。次いで、０．５重量部の疎水性シリカ（Ｈ２０００、クラリアントジャパン社製）を添加してから、周速１５［ｍ／ｓｅｃ］の設定で３０秒間のミキシングと１分間の休止とを５セット行って、トナー粒子を得る。そして、このトナー粒子１００重量部と、疎水性シリカ０．５重量部と、疎水化酸化チタン０．５重量部とをヘンシェルミキサーにて混合して、シアントナーを得る。上述の基本樹脂溶液の組成における銅フタロシアニンブルー顔料（シアン顔料）４重量部を次のように変更すれば、他色のトナーを得ることができる。
・イエロートナー：ベンジジンイエロー顔料６重量部
・マゼンタトナー：ローダミンレーキ顔料６重量部
・ブラックトナー：カーボンブラック１０重量部
【００８４】
更には、トナーは特定の形状と形状の分布を有すことが重要である。トナーの形状は平均円形度で規定することができる。トナーの平均円形度とは、トナーの形状がどのくらい真球に近いものかを示す数値であり、真球であれば円形度が「１」となる。この平均円形度は、トナーの投影画像の面積と同じ面積をもつ円の円周長をその投影画像の周囲長で割ることで得た値を個々のトナーの円形度とし、このように得た各トナーの円形度の平均値をとったものである。トナーの平均円形度は、０．９５以上０．９９以下が望ましい。より好ましくは、平均円形度が０．９６以上０．９９以下で、円形度が０．９５未満の粒子が１０％以下である。なお、実験によれば平均円形度が０．９５未満、特に０．９３より小さいと、球形から離れた不定形のトナーとなり、満足した転写性やチリのない高画質画像が得られなくなってしまう。一方、平均円形度が０．９９よりも大きいと、ブレードクリーニングなどを採用しているシステムでは、感光体上および転写ベルトなどのクリーニング不良が発生し、画像上の汚れを引き起こしてしまう。画像面積率の低い画像を出力する場合、転写残トナーが少なく、クリーニング不良が問題となることはない。しかし、例えば、カラー写真画像など画像面積率の高い画像を出力する場合、さらには、給紙不良等で未転写の状態の画像が感光体上に残ってしまった場合、クリーニング不良が発生しやすい。このようなクリーニング不良を頻発するようになると、更には、感光体を接触帯電させる帯電ローラ等を汚染してしまい、本来の帯電能力を発揮できなくなってしまう。よって、トナーの平均円形度が、０．９５以上、０．９９以下であれば、クリーニング不良を生じることなく、適正な濃度の再現性のある高精細な画像を形成することができる。
【００８５】
ここで、トナーの平均円形度は、各トナーの円形度の平均値であり、次の方法により測定したものである。各トナーの円形度の測定は、株式会社ＳＹＳＭＥＸ製フロー式粒子像分析装置ＦＰＩＡ−２１００を用いて行った。この測定では、まず、１級塩化ナトリウムを用いて、１［％］のＮａＣｌ水溶液を調整する。その後、このＮａＣｌ水溶液を０．４５のフィルターを通して５０〜１００［ｍｌ］の液を得て、これ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０．１〜５［ｍｌ］加え、更に試料を１〜１０［ｍｇ］加える。これを、超音波分散機で分散処理を１分間行い、粒子濃度を５０００〜１５０００［個／μｌ］に調整し、分散液を得る。この分散液をＣＣＤカメラで撮像し、トナーの２次元投影画像の面積と同じ面積をもつ円の円周長を、そのトナーの２次元投影画像の周囲長で割った値を、各トナーの円形度として用いた。なお、ＣＣＤの画素の精度から、トナーの２次元投影画像の面積と同じ面積をもつ円の直径（円相当径）が０．６［μｍ］以上であるトナーを有効なものとした。トナーの平均円形度は、各トナーの円形度を得た後、測定範囲内にある全トナーの円形度をすべて足し合わせ、それをトナー個数で割った値を用いたものである。
【００８６】
更に、トナーの体積平均粒径（Ｄｖ）は４［μｍ］以上８［μｍ］以下であり、この体積平均粒径Ｄｖと個数平均粒径Ｄｎとの比（Ｄｖ／Ｄｎ）が１．０５以上１．３０以下であるのが好ましい。より好ましくは、上記体積平均粒径（Ｄｖ）／個数平均粒径（Ｄｎ）のは１．１０以上１．２５以下がよい。このような粒径のトナーを用いることにより、トナーの粒度分布が狭くなるため、次のような効果を得ることができる。
すなわち、トナー粒径面での選択現像といった現象が発生しにくいため、常時、安定した画像を形成することができる。ここで、選択現像とは、画像パターンに応じた（適した）トナー粒径を持つトナー粒子が選択的に現像される現象をいう。また、トナーリサイクルシステムを搭載している場合、転写されにくい小サイズのトナー粒子が量的に多くリサイクルされることになるが、もともとトナーの粒度分布が狭いため、上述した作用を受けにくくなる。従って、この点からも常時、安定した画像を形成することができる。また、二成分現像剤においては、長期にわたるトナーの収支が行われても、現像剤中のトナー粒子径の変動が少なくなり、現像装置における長期の攪拌においても、良好で安定した現像性が得られる。
【００８７】
一般的には、トナーの粒子径は小さければ小さい程、高解像で高画質の画像を得るために有利であると言われているが、逆に転写性やクリーニング性に対しては不利である。また、トナーの体積平均粒子径Ｄｖが４［μｍ］よりも小さい場合、二成分現像剤では現像装置における長期の攪拌において磁性キャリアの表面にトナーが融着し、磁性キャリアの帯電能力を低下させてしまう。また、これらの現象は微粉の含有率が本実施形態１の範囲より多いトナーにおいても同様である。逆に、トナーの体積平均粒子径Ｄｖが８［μｍ］よりも大きい場合には、高解像で高画質の画像を得ることが難しくなると共に、現像剤中のトナーの収支が行われた場合にトナーの粒子径の変動が大きくなる場合が多い。また、上記体積平均粒径Ｄｖ／個数平均粒径Ｄｎの値が１．３０よりも大きい場合も同様であることが明らかとなった。また、上記体積平均粒径Ｄｖ／個数平均粒径Ｄｎの値が１．０５より小さい場合には、トナーの挙動の安定化、帯電量の均一化の面から好ましい面もある。しかしながら、この場合は、細線部分を小サイズ粒子で現像し、一方、ベタ画像を大サイズ粒子を中心に現像するといったトナー粒径による機能分離ができにくくなるため、かえって好ましくない。
【００８８】
上記トナーの体積平均粒径は、例えばコールターカウンター法によるトナー粒子の粒度分布の測定装置を用いて測定することができる。この測定装置としては、コールターカウンターＴＡ−ＩＩやコールターマルチサイザーＩＩ（いずれもコールター社製）が挙げられる。以下、この測定装置を用いたトナー粒径の測定方法について述べる。まず、電解水溶液１００〜１５０ｍｌ中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加える。ここで、電解液とは１級塩化ナトリウムを用いて約１％ＮａＣｌ水溶液を調製したもので、例えばＩＳＯＴＯＮ−ＩＩ（コールター社製）が使用できる。ここで、更に測定試料を２〜２０ｍｇ加える。試料を懸濁した電解液は、超音波分散器で約１〜３分間分散処理を行い、前記測定装置により、アパーチャーとして１００［μｍ］アパーチャーを用いて、トナー粒子又はトナーの体積、個数を測定して、体積分布と個数分布を算出する。得られた分布から、トナーの体積平均粒径（Ｄｖ）、個数平均粒径（Ｄｎ）を求めることができる。チャンネルとしては、２．００〜２．５２［μｍ］未満；２．５２〜３．１７［μｍ］未満；３．１７〜４．００［μｍ］未満；４．００〜５．０４［μｍ］未満；５．０４〜６．３５［μｍ］未満；６．３５〜８．００［μｍ］未満；８．００〜１０．０８［μｍ］未満；１０．０８〜１２．７０［μｍ］未満；１２．７０〜１６．００［μｍ］未満；１６．００〜２０．２０［μｍ］未満；２０．２０〜２５．４０［μｍ］未満；２５．４０〜３２．００［μｍ］未満；３２．００〜４０．３０［μｍ］未満の１３チャンネルを使用し、粒径２．００［μｍ］以上乃至４０．３０［μｍ］未満の粒子を対象とした。
【００８９】
〔実施形態２〕
次に、本発明を、上記実施形態１と同様の複写機に着脱可能なプロセスカートリッジに適用した一実施形態（以下、本実施形態を「実施形態２」という。）について説明する。
図１９は、本実施形態２に係るプロセスカートリッジの内部構造を示す概略構成図である。このプロセスカートリッジ５００は、上記実施形態１の複写機に設けられる各画像形成ユニット１８Ｙ，１８Ｃ，１８Ｍ，１８Ｋを、複写機本体に対して着脱可能に一体構成したものである。具体的には、感光体ドラム２０と、帯電装置６０、現像装置８０及び感光体クリーニング装置６３が設けられている。これらの構成については、上記実施形態１と同様であるので、説明を省略する。このように各画像形成ユニット１８Ｙ，１８Ｃ，１８Ｍ，１８Ｋをプロセスカートリッジ５００として構成すれば、これに収容された部品に寿命が到来したり、メンテナンスが必要になったりしたときには、そのプロセスカートリッジを交換すればよく、利便性が向上する。なお、プロセスカートリッジとして一体構成する部品は、少なくとも感光体ドラム２０と現像装置８０とが含まれていれば、そのほかにどのような部品を含ませてもよい。
【００９０】
以上、上記実施形態１の画像形成装置は、潜像を表面に担持して表面移動する感光体ドラム２０としての感光体ドラム２０と、この感光体ドラム２０に潜像を形成する潜像形成手段としての露光装置２１とを備えている。また、感光体ドラム２０上の潜像をトナーと磁性粒子であるキャリアとを含む現像剤により現像する現像手段としての現像装置８０も備えている。そして、感光体ドラム２０上のトナー像を記録材としての転写紙上に転位させることで、その転写紙上に画像を形成する。上記現像装置８０は、現像剤を表面に担持して表面移動する現像剤担持体としての現像スリーブ８１を備えており、この現像スリーブ８１は、その現像スリーブ表面移動方向における表面の一部が装置ケーシング８４から露出している。この現像装置８０により、露出した現像スリーブ８１の表面と感光体ドラム２０の表面とが対向する現像領域で、この現像領域に対向するように配置した現像磁極としての主磁極Ｐ１ｂにより現像スリーブ８１上の現像剤を穂立ちさせてその現像スリーブ上に磁気ブラシを形成し、感光体ドラム２０上の潜像を現像する。そして、この現像装置８０は、主磁極Ｐ１ｂから、主磁極Ｐ１ｂとは逆極性であって主磁極Ｐ１ｂと隣り合うように現像スリーブ表面移動方向下流側に配置された他の磁極としての下流側補助磁極Ｐ１ｃまでの現像スリーブ表面上で、法線方向磁束密度Ｂｚの絶対値と、接線方向磁束密度Ｂｙの絶対値との和が単調増加するように構成されている。このように単調増加していれば、上述したように、キャリア付着を効果的に抑制することができる。
また、上記実施形態１における現像装置８０は、現像スリーブ８１を現像領域で感光体ドラム２０の表面移動方向と同方向かつ感光体ドラム表面の線速よりも大きい線速で表面移動させて、現像スリーブ上の磁気ブラシを感光体ドラム２０の表面に摺擦させて現像を行う。そして、この現像装置８０は、現像スリーブ８１に担持されて現像剤規制部材としてのドクターブレード８３によって規制された後に現像領域に搬送される現像剤の量が６５［ｍｇ／ｃｍ^２］以上９５［ｍｇ／ｃｍ^２］以下に設定されている。しかも、現像領域の現像スリーブ表面外側に生じる磁束の現像スリーブ表面法線方向における磁束密度の減衰率が４０［％］以上であるので、磁気ブラシの長さが短く、かつ、感光体ドラム表面に接触するブラシ部分の密度を高くできる。よって、上述したように細線再現性の向上や後端白抜け現象の抑制を図ることができる。このような構成を有する現像装置は、細線再現性の向上や後端白抜け現象の抑制を図ることができる点で非常に有用であるが、上述したようにキャリア付着が起きやすいという欠点がある。しかし、この欠点は、上述したように法線方向磁束密度Ｂｚの絶対値と接線方向磁束密度Ｂｙの絶対値との和が単調増加するように構成することで十分に解消することが可能である。よって、本実施形態１によれば、キャリア付着を十分に抑制しつつ、細線再現性の向上や後端白抜け現象の抑制を図ることができる。
なお、この現像装置８０は、主磁極Ｐ１ｂにより現像スリーブ８１の表面上に生じるその表面の法線方向における最高磁束密度の半分の磁束密度となるその表面上の半値点を、現像領域における現像スリーブ８１の表面の曲率中心軸から見たときの現像スリーブ表面移動方向における半値点間の角度幅が２５［°］以下に設定されている。これにより、減衰率を４０［％］以上とするのと同様の効果が得られる。
また、主磁極Ｐ１ｂにより現像スリーブ表面上に生じる現像スリーブ表面の法線方向磁束密度の最大値Ｂｍ_１と、下流側補助磁極Ｐ１ｃにより現像スリーブ表面上に生じる現像スリーブ表面の法線方向磁束密度の最大値Ｂｍ_２とが、上記数７の関係式を満たすように構成すれば、上述したように、キャリア付着をより効果的に抑制することができる。
また、上記実施形態１では、現像剤中のキャリアとして、体積平均粒径が２０［μｍ］以上５０［μｍ］以下のものを用いている。よって、上述したように、よりドット再現性に優れた画像を形成することができる。このような現像装置は、ドット再現性に優れた画像を形成できる点で非常に有用であるが、このような小径のキャリアを用いる場合には上述したようにキャリア付着が起きやすいという欠点がある。しかし、この欠点は、上述したように法線方向磁束密度Ｂｚの絶対値と接線方向磁束密度Ｂｙの絶対値との和が単調増加するように構成することで十分に解消することが可能である。よって、本実施形態１によれば、キャリア付着を十分に抑制しつつ、ドット再現性に優れた画像を形成できる。
特に、上記実施形態１では、現像剤中のキャリア９０として、熱可塑性樹脂及びメラミン樹脂を架橋させた樹脂成分と帯電調整剤とを含有した樹脂コート膜９２を磁性体の芯材９１に対してコーティングしたものを用いている。よって、上述したように、長期間に渡り、トナー汲上量の安定化、すなわち品質の安定化を期待できる。
また、上記実施形態１においては、感光体ドラム２０表面と現像スリーブ８１の表面との間の最小間隔である現像キャップＰｇを、０．１［ｍｍ］以上０．４［ｍｍ］以下としている。このように現像キャップＰｇを狭い範囲とすることで、上述したように画像品質を向上させることができる。また、磁石ローラ８５によって生じる磁束密度は、全ての成分について現像スリーブ８１表面から離れるにつれて減衰していく。そのため、磁気ブラシの高さが高くなるほど、その先端部分キャリアが受ける現像スリーブ側に向かう磁力が弱まることになる。よって、現像キャップＰｇを０．１［ｍｍ］以上０．４［ｍｍ］以下という狭いものとすることで、磁気ブラシの高さを制限できる結果、キャリア付着の抑制にも有効に働く。
また、上記実施形態２のプロセスカートリッジ５００は、上記実施形態１の複写機の本体に対して着脱可能であって、少なくとも、感光体ドラム２０と現像装置８０とが一体になって構成されてので、利便性が向上する。
【００９１】
なお、上述した実施形態１及び実施形態２では、いわゆるタンデム型のカラー複写機について説明したが、これとは異なる構成を有する画像形成装置にも本発明を適用することができる。例えば、感光体ドラム上に形成したトナー像を転写紙に直接転写するモノクロの画像形成装置にも適用することができる。また、本実施形態では、画像形成装置として複写機を例に挙げたが、プリンタやファクシミリなど他の画像形成装置にも適用ことができる。
【００９２】
【発明の効果】
請求項１乃至１０の発明によれば、現像剤中のキャリアが小径であっても、そのキャリアが潜像担持体表面に付着するのを十分に抑制することが可能
できるという優れた効果がある。
【図面の簡単な説明】
【図１】実施形態１に係る複写機の現像装置に設けられる磁石ローラの各磁極により現像スリーブ表面に発生するその表面上の磁束密度の分布を示した円グラフ。
【図２】同複写機全体の概略構成図。
【図３】同複写機本体部分の構成を示す拡大図。
【図４】隣り合う２つの画像形成ユニットの構成を示す拡大図。
【図５】同現像装置を示す概略構成図。
【図６】同現像装置の現像スリーブ表面に発生するその表面の法線方向磁束密度の分布を実線で示し、現像スリーブ表面からその法線方向に１［ｍｍ］離れた位置での法線方向磁束密度を波線で示した円グラフ。
【図７】現像領域における現像スリーブ表面の曲率中心軸から見た現像スリーブ表面移動方向の半値角度幅を説明するための説明図。
【図８】同現像装置における磁石ローラに設けられる主磁極及び２つの補助磁極の配置を示す説明図。
【図９】実験例１において、実施形態１の現像装置よりも強い磁石からなる主磁極及び補助磁極を用いた比較用の現像装置の現像スリーブ表面での磁束密度の分布を示した円グラフ。
【図１０】実験例１におけるキャリア付着の評価結果を示すグラフ。
【図１１】実験例２において、実施形態１の現像装置よりも強い磁石からなる主磁極を用いた比較用の現像装置の現像スリーブ表面での磁束密度の分布を示した円グラフ。
【図１２】実験例２におけるキャリア付着の評価結果を示すグラフ。
【図１３】比較例１において、実施形態１の現像装置よりも強い磁石からなる主磁極を用いた比較用の現像装置の現像スリーブ表面での磁束密度の分布を示した円グラフ。
【図１４】比較例１において、実施形態１の現像装置よりも強い磁石からなる主磁極を用いた比較用の他の現像装置の現像スリーブ表面での磁束密度の分布を示した円グラフ。
【図１５】比較例１におけるキャリア付着の評価結果を示すグラフ。
【図１６】比較例１において、地肌ポテンシャルを一定にし、体積平均粒径が異なる３種類のキャリアを用いた現像剤により画像を形成したときの感光体ドラムに付着したキャリア付着数を測定したときの結果を示すグラフ。
【図１７】比較例２におけるキャリア付着の評価結果を示すグラフ。
【図１８】実施形態１に用いるキャリアの断面図を示す模式図。
【図１９】実施形態２に係るプロセスカートリッジの内部構造を示す概略構成図。
【図２０】（ａ）乃至（ｃ）は、後端白抜け等の画質劣化が発生するメカニズムを説明するための説明図。
【図２１】（ａ）は、現像磁極が１つの磁極からなる従来の現像装置における現像領域近傍の磁力分布を示す説明図。
（ｂ）は、その現像磁極により形成される磁界から磁力を受けて穂立ちした現像剤からなる磁気ブラシを現像スリーブの軸方向から見たときの形状を示す説明図。
【図２２】（ａ）は、現像磁極が１つの主磁極と２つの補助磁極からなるＳＬＩＣ式現像装置における現像領域近傍の磁力分布を示す説明図。
（ｂ）は、これらの３つの磁極により形成される磁界から磁力を受けて穂立ちした現像剤からなる磁気ブラシを現像スリーブの軸方向から見たときの形状を示す説明図。
【符号の説明】
１０ 中間転写ベルト
１８Ｙ，１８Ｃ，１８Ｍ，１８Ｋ 画像形成ユニット
２０Ｙ，２０Ｃ，２０Ｍ，２０Ｋ 感光体ドラム
２１ 露光装置
６０ 帯電装置
６３ 感光体クリーニング装置
８０Ｙ，８０Ｃ，８０Ｍ，８０Ｋ 現像装置
８１ 現像スリーブ
８３ ドクターブレード
８５ 磁石ローラ
９０ キャリア
９１ 芯材
９２ 樹脂コート膜
９３ アルミナ粒子
５００ プロセスカートリッジ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a developing device used in an image forming apparatus such as a copying machine, a printer, a facsimile, etc., an image forming device provided with the developing device, a process cartridge provided with the developing device, and a developer carrier used in the developing device. Things.
[0002]
[Prior art]
Generally, in an image forming apparatus such as an electrophotographic type or an electrostatic recording type, an electrostatic latent image corresponding to image information is formed on a latent image carrier such as a photosensitive drum or a photosensitive belt, and the electrostatic latent image is formed. A visible image is obtained by developing the image with a developing device. In recent years, in performing such development, from the viewpoint of transferability, reproducibility of halftone, stability of development characteristics with respect to temperature and humidity, and the like, a two-component developer composed of a toner and a carrier (hereinafter simply referred to as The mainstream is to use a two-component development system using a "agent". In a developing device using this two-component developing method, while the developer is held in a brush shape on a developer carrier, the developer is conveyed to a development area where the developer carrier and the latent image carrier are opposed to each other. . Then, in the developing area, the brush-shaped developer is brought into contact with the surface of the latent image carrier, and the toner in the developer is supplied to the electrostatic latent image portion on the latent image carrier, and the electrostatic latent image is developed. A so-called brush type development is performed.
[0003]
A developer carrier in a developing device for performing such a brush-type development generally includes a developing sleeve formed in a cylindrical shape, and a magnet roller having a plurality of magnetic poles fixedly arranged inside the developing sleeve. I have. This magnet roller is for forming a magnetic field that causes the developer to spike on the surface of the developing sleeve. As the developing sleeve moves relative to the magnet roller, the developer standing on the surface of the developing sleeve is transported. In the developing area, the developer on the developing sleeve rises along magnetic lines of force generated from the developing magnetic pole of the magnet roller. The brush-formed developer that has been raised and comes into contact with the surface of the latent image carrier so as to bend as the surface of the developing sleeve moves, and supplies toner to the electrostatic latent image.
[0004]
In such a developing device, it is known that the closer the distance between the latent image carrier and the developing sleeve in the developing area, the easier it is to obtain a high image density and the less the edge effect. For this reason, it is desirable to make the distance between the latent image carrier and the developing sleeve closer. However, if the distance is reduced, the rear end of a solid black image or a halftone solid image becomes white, causing a phenomenon called “back end white spot”, or the thin line reproducibility deteriorates. This causes a problem that image quality is deteriorated.
[0005]
The direction of movement of the surface of the developing sleeve in the developing area is a direction that follows the latent image carrier, and the linear speed is generally set to be faster than the linear speed of the latent image carrier. In this case, the magnetic brush relatively moves with respect to the electrostatic latent image such that the magnetic brush rubs while overtaking the electrostatic latent image on the latent image carrier. That is, the surface of the latent image carrier is rubbed so as to be sequentially passed by a plurality of magnetic brushes while passing through the developing area. Focusing on the electrostatic latent image portion (latent image rear end portion) on the latent image carrier corresponding to the image rear end position, a plurality of magnetic brushes that sequentially rub the latent image rear end portion are as follows. The toner supply capacity is gradually reduced.
[0006]
FIGS. 20A to 20C are explanatory diagrams for explaining a mechanism in which image quality deterioration such as a trailing edge white spot occurs in the developing device. FIGS. 20A to 20C show, in order, the movement of the tip of a brush-like developer (magnetic brush) gradually approaching the surface of the photosensitive drum 20 as a latent image carrier. it's shown. The state shown in the drawing shows a portion where the boundary between the non-latent image portion F and the latent image portion G of the solid black image is developed just in the developing region where the photosensitive drum 20 and the developing sleeve face each other. On the downstream side in the rotation direction of the photosensitive drum 20, a toner image just developed is formed. The trailing edge white spot occurs at the trailing edge portion E of the latent image. Here, the photosensitive drum 20 is actually rotating in the direction of the arrow in the figure, but since the developing sleeve is moving faster than the photosensitive drum 20 as described above, the magnetic brush is The surface of 20 is rubbed. Therefore, in FIGS. 20A to 20C, the illustration is simplified assuming that the photosensitive drum 20 is stationary.
[0007]
As shown in FIG. 20A, a magnetic brush that rubs the rear end portion E of the latent image after entering the developing area is a non-latent image positioned on the photosensitive drum 20 upstream of the photosensitive drum surface moving direction. It has been opposed to the portion F. Therefore, at the tip of the magnetic brush, the toner 3a adhering to the surface of the carrier 3b is moved toward the developing sleeve side by the electrostatic force received from the non-latent image portion F during the period facing the non-latent image portion F. A moving toner drift has occurred. The toner drift progresses as the period facing the non-latent image portion F increases. Therefore, as the magnetic brush rubs the rear end portion E of the latent image on the downstream side of the developing area in the direction of movement of the photosensitive drum surface, the period in which the magnetic brush faces the non-latent image portion F is longer, and the toner drift progresses. A small amount of the toner 3a adheres to the surface of the carrier 3b at the leading end, so that the toner supply ability is small.
When the rear end portion E of the latent image escapes from the developing area, the magnetic brush rubbing the rear end portion E of the latent image forms a toner on the surface of the carrier 3b at the front end as shown in FIG. 3a hardly exists. The magnetic brush in which the toner drift has progressed to such an extent electrostatically removes the toner adhering to the rear end portion E of the latent image on the surface of the carrier 3b at the tip of the magnetic brush to which the toner 3a does not adhere. It will be attractive. As a result, with respect to the rear end portion E of the latent image, even if the toner 3a is once supplied by the magnetic brush in the developing area, the toner 3a almost disappears on the surface of the carrier 3b before the toner 3a escapes from the developing area. It moves to the tip of another magnetic brush. It is considered that this causes a trailing edge white spot and a decrease in fine line reproducibility.
[0008]
The present applicant has proposed in Patent Literature 1, Patent Literature 2, Patent Literature 3 and the like an invention for suppressing trailing white spots and a decrease in fine line reproducibility. In the inventions proposed in these publications, the attenuation rate of the magnetic flux density in the normal direction in the developing area, the angular interval between the main magnetic pole for adhering the developer in the developing area and the adjacent magnetic pole, the half-value central angle of the main magnetic pole And the like are specified to predetermined values. As a specific configuration, for example, the above-described developing magnetic pole is constituted by a single main magnetic pole composed of an N pole and an S pole arranged so as to be close to the upstream side and the downstream side in the surface moving direction of the developing sleeve of the main magnetic pole. And two auxiliary magnetic poles. Further, the present applicant sets the developing nip and the magnetic brush density (see Patent Document 4), sets the half-value angular width (also referred to as the half-value central angle) of the main magnetic pole (see Patent Document 5), and the like, and sets the image quality. Inventions that realize improvements are also proposed.
According to these inventions, it has been confirmed that the trailing edge white spot phenomenon and the fine line reproducibility can be improved.
[0009]
The reason why the trailing edge white spot phenomenon and fine line reproducibility can be improved by the developing devices (hereinafter, referred to as “SLIC-type developing device”) of Patent Documents 1, 2, and 3 are as follows. it is conceivable that.
FIG. 21A is an explanatory diagram showing a magnetic force distribution in the vicinity of a developing area in a conventional developing device in which a developing magnetic pole includes one magnetic pole P1. FIG. 21B is an explanatory view showing the shape of the magnetic brush made of the developer which has been raised by receiving a magnetic force from the magnetic field formed by the developing magnetic pole P1 when viewed from the axial direction of the developing sleeve 81. is there.
FIG. 22A is an explanatory diagram showing a magnetic force distribution in the vicinity of the developing area in the SLIC type developing device in which the developing magnetic pole includes one main magnetic pole P1b and two auxiliary magnetic poles P1a and P1c. FIG. 22 (b) shows the shape of a magnetic brush made of developer that has been raised by receiving magnetic force from the magnetic field formed by these magnetic poles P1a, P1b, and P1c when viewed from the axial direction of the developing sleeve 81. FIG.
[0010]
In the conventional developing device shown in FIGS. 21A and 21B, the magnetic pole of the S pole adjacent to the magnetic pole of the N pole is a region located on the downstream side in the surface movement direction of the developing sleeve 81 with respect to the developing region. There is a magnetic pole P2 that forms a magnetic field for transporting the developer. There is also a magnetic pole P7 that forms a magnetic field for transporting the developer pumped onto the developing sleeve 81 to the developing area. Since these magnetic poles P2 and P7 are arranged at positions relatively distant from the developing magnetic pole P1, the magnetic force distribution of the magnetic field in the developing area is such that the magnetic force lines coming out of the developing magnetic pole P1 are as shown in FIG. It passes through a position relatively far from the sleeve surface. Then, the developer carried on the developing sleeve 81 and conveyed to the developing area rises along the lines of magnetic force as shown in FIG. 21B to form a magnetic brush.
[0011]
On the other hand, in the SLIC type developing device shown in FIGS. 22A and 22B, there are two auxiliary magnetic poles P1a and P1c as S magnetic poles adjacent to the N main magnetic pole P1b. The distance between the main magnetic pole P1b and these auxiliary magnetic poles P1a and P1c is determined by the distance between the developing magnetic pole and the two adjacent magnetic poles P2 and P7 in the conventional developing device shown in FIGS. 21 (a) and 21 (b). small. For this reason, as shown in FIG. 22A, the magnetic force distribution of the magnetic field in the developing region is different from the magnetic field distribution of the developing magnetic pole of the conventional developing device shown in FIG. It passes through a position near the surface of the developing sleeve. Further, most of the magnetic lines of force emitted from the main magnetic pole P1b are directed to two auxiliary magnetic poles P1a and P1c as adjacent magnetic poles. As a result, a conventional developing device in which the same number of magnetic lines of force (hereinafter referred to as “spring magnetic lines of force”) directed in a direction close to the normal direction of the surface of the developing sleeve involved in the formation of the magnetic brush are generated. Less than The width in the surface movement direction of the developing sleeve 81 where the magnetic field lines for the ears exist (the ear width) also becomes narrow. Therefore, as can be seen from a comparison between FIG. 21B and FIG. 22B, the spike start position of the developer conveyed to the developing area is more in the developing sleeve surface than in the conventional developing device. It approaches the center of the moving direction (hereinafter simply referred to as “center”). Similarly, the end position of the spikes of the developer passing through the developing region while being carried on the surface of the developing sleeve 81 is also closer to the center of the developing region than in the conventional developing device. That is, the developer on the developing sleeve 81 starts rising at a point closer to the center of the developing area than the conventional developing device, and ends the rising. As a result, the period during which the magnetic brush on the developing sleeve 81 approaches or contacts the photosensitive drum 20 is shorter than in the conventional developing device. In response to this, when the latent image rear end portion E moves out of the developing area by moving the photosensitive drum surface, the magnetic brush rubbing the latent image rear end portion E comes close to the non-latent image portion F until then. Alternatively, the period of contact is shorter than that of the conventional developing device. Therefore, the degree of progress of the toner drift of the magnetic brush rubbing the rear end portion E of the latent image on the photosensitive drum 20 when exiting the development area can be reduced, and the rear end white spots and fine line reproduction can be reduced as compared with the conventional developing device. Can be prevented from decreasing.
[0012]
Further, in the SLIC type developing device, the two auxiliary magnetic poles P1a and P1c of the S pole are arranged close to the main magnetic pole P1b of the N pole, so that the auxiliary magnetic poles P1a and P1c are separated from the surface of the developing sleeve 81 in the normal direction. The lines of magnetic force in the developing area at the position are sparser than in the conventional developing device. For this reason, the magnetic flux density in the normal direction in the developing area at a position away from the surface of the developing sleeve 81 in the normal direction (for example, the position where the tip of the magnetic brush exists in the conventional device) is equal to the SLIC type developing device. Is smaller than the conventional developing device. Therefore, in the SLIC-type developing device, most of the developer constituting the magnetic brush is attracted to the vicinity of the surface of the developing sleeve 81 having a high magnetic flux density, and as shown in FIG. It is shorter than the developing device.
[0013]
Furthermore, when the above-mentioned SLIC type developing device is actually used, the amount of the developer supplied to the developing area is set to be smaller than the maximum amount of the developer that can be raised while passing through the developing area. . In other words, in the SLIC type developing device, although a longer magnetic brush can be formed, the amount of the developer supplied to the developing area is made smaller and the length of the magnetic brush is controlled to be shorter. As a result, the tip of the magnetic brush is located in a region near the surface of the developing sleeve 81 where the magnetic flux density is high, and the tip of the magnetic brush has a higher brush density than the tip of the magnetic brush in the conventional developing device. Will be higher. By shortening the minimum distance (hereinafter referred to as “developing gap”) Pg between the surface of the developing sleeve 81 and the surface of the photosensitive drum 20 by an amount corresponding to the shortening of the magnetic brush, the developing sleeve 81 is made smaller than the conventional device. The photosensitive drum 20 can be rubbed with a brush portion having a high density existing in a region having a high magnetic flux density close to the surface of the photosensitive drum 20.
[0014]
In the SLIC type developing device, as described above, the start position of the developer spike and the end position of the developer spike are closer to the center of the development area than in the conventional developing device. For this reason, as shown in FIG. 22B, the width (sliding width) Pn in the developing sleeve surface moving direction of the portion where the magnetic brush rubs the latent image carrier in the developing area is smaller than that of the conventional developing device. Become. Therefore, the amount of toner supplied to the latent image portion on the photosensitive drum 20 by the rubbing by the magnetic brush is smaller than that of the conventional developing device if the brush density of the rubbing portion is the same. However, if the SLIC-type developing device is used, as described above, the brush density of the tip portion of the magnetic brush in contact with the photosensitive drum 20 can be higher than that of the conventional developing device. Therefore, it is possible to suppress the amount of toner supplied to the latent image portion on the photosensitive drum 20 from being reduced as compared with the conventional developing device.
As described above, even when the rubbing width Pn is smaller than that of the conventional developing device, the rubbing width Pn is supplied to the electrostatic latent image by increasing the linear speed ratio of the developing sleeve 81 to the linear speed of the photosensitive drum 20 in the developing area. It is possible to secure a sufficient amount of toner. Therefore, it is possible to suppress the trailing edge white spots, improve the fine line reproducibility, and provide a high quality image with high image density.
[0015]
By the way, in the above-mentioned SLIC type developing device, as described above, the developer shows a movement of rising over the magnetic brush after passing through the developing area, forming a magnetic brush. In this movement, the time required to transition from the falling state to the rising state and the time required to transition from the rising state to the falling state are shorter than those of the conventional developing device. That is, in the SLIC-type developing device, the developer rapidly rises and falls rapidly. Therefore, when shifting from the falling state to the rising state, and when shifting from the rising state to the falling state, a relatively large centrifugal force is applied to the tip of the magnetic brush in a direction away from the developing sleeve 81. work. In addition, as described above, when the surface moving speed of the developing sleeve 81 is increased in order to sufficiently secure the amount of toner supplied to the electrostatic latent image, in addition to the centrifugal force at the time of standing or falling, the developing Centrifugal force due to the surface movement of the sleeve 81 also acts. As a result, the carrier 3b constituting the tip of the magnetic brush may be detached from the developer at the time of standing up and falling down. The carrier 3b thus detached has an initial velocity toward the surface of the photosensitive drum 20 at the time of the detachment. In addition, it receives a developing electric field formed between the photosensitive drum 20 and the developing sleeve 81, an electrostatic force from a toner image already developed on the photosensitive drum, and the like, and also receives a force toward the photosensitive drum surface. As a result, the carrier 3b detached from the developer adheres to the surface of the photoconductor drum, particularly near the boundary between the non-latent image portion and the latent image portion.
[0016]
Even if the carrier 3b adheres to the surface of the photosensitive drum, if the carrier is a carrier that has been detached at the time of spikes, the carrier is recovered in the developer by being rubbed with a developer coming later. Therefore, there is no particular problem. However, the carrier 3b detached at the time of falling of the ear passes through the area rubbed by the developer before being rubbed by the developer coming later, and thus remains attached to the surface of the photosensitive drum. For this reason, there is a problem in that the image corresponding to the surface portion of the photoconductor drum on which the carrier 3b is attached causes image quality deterioration such as generation of white spots in a solid image. In addition, when the surface of the photoconductor drum is cleaned by the cleaning blade, the carrier 3b is pressed against the surface of the photoconductor drum, and the surface of the photoconductor drum is damaged. Therefore, it is an extremely important task to suppress or prevent the phenomenon that the carrier 3b adheres to the surface of the photosensitive drum (hereinafter, referred to as “carrier adhesion”).
[0017]
In addition, in recent years, there has been an increasing demand for higher image quality, and as the diameter of the toner 3a has been reduced, the particle size of the carrier 3b has also become smaller. As a result, the magnetic moment of the carrier 3b is relatively small with respect to the charge amount. The above-described carrier adhesion is due to the fact that the electrostatic force of the carrier 3b detached from the developer is attracted to the photosensitive drum side rather than the magnetic force attracted to the developing sleeve side by the magnetic field formed by the developing magnetic pole of the magnet roller in the developing sleeve. Occurs when exceeded. Therefore, in recent years in which the diameter of the carrier 3b has been reduced, the carrier is likely to adhere. In a conventional developing device that is not of the SLIC type, since the toner drift occurs at the tip of the magnetic brush as described above, the carrier constituting the tip is exposed to the surface of the photosensitive drum. Since the exposed carrier is susceptible to electrostatic force directed toward the surface of the photosensitive drum, the electrostatic force that attracts the carrier toward the photosensitive drum is more likely to be greater than the magnetic force that attracts the carrier toward the developing sleeve, and the carrier is removed from the developer. Easy to leave. On the other hand, in the SLIC type developing device, as shown in FIG. 22A, most of the magnetic lines of force coming out of the main magnetic pole P1b pass through a position close to the surface of the developing sleeve, and the region where the tip of the magnetic brush exists at the time of falling down ( Hereinafter, in the “carrier departure region”, the lines of magnetic force are sparse. Therefore, the magnetic flux density in the carrier separation region is small. In the SLIC type developing device, it is considered that the toner drift is less likely to occur, so that the electrostatic force received by the carrier to the photosensitive drum side is relatively small, but the magnetic force for attracting the carrier to the developing sleeve side is further reduced. Therefore, in the SLIC type developing device, the carrier is more easily separated from the developer than the conventional developing device, and the carrier is more likely to adhere.
[0018]
In view of this, the present applicant has proposed a developing device capable of suppressing such carrier adhesion in Patent Document 6. This developing device has a normal magnetic flux density and a tangential magnetic flux density between a main magnetic pole P1b shown in FIGS. 22A and 22B and an auxiliary magnetic pole P1c located on the downstream side in the developing sleeve surface moving direction. By setting the vector sum of the above to 85 [mT] or more, an attempt is made to suppress the carrier adhesion described above. Although not specified in Patent Document 6, the normal magnetic flux density here means the magnetic flux density on the developing sleeve surface in the normal direction of the developing sleeve surface, and the tangential magnetic flux density means , Means the magnetic flux density on the developing sleeve surface in the developing sleeve surface moving direction.
[0019]
[Patent Document 1]
JP 2000-305360 A
[Patent Document 2]
JP 2000-347506 A
[Patent Document 3]
JP 2001-5296 A
[Patent Document 4]
JP 2001-27849 A
[Patent Document 5]
JP 2001-134100 A
[Patent Document 6]
JP-A-2002-287501
[0020]
[Problems to be solved by the invention]
As described above, since it is extremely important to suppress carrier adhesion, it is very important to further enhance the suppression effect. In addition, as a result of the progress of the reduction in the carrier particle diameter as compared with the filing of the patent application according to the cited document 6, the carrier is more likely to adhere. More specifically, the diameter is further reduced as compared with the carrier having a volume average particle size of 55 [μm] assumed in the above cited document 6, and a carrier having a volume average particle size of 35 [μm] or less is realized. ing. Therefore, it is necessary to enhance the effect of suppressing the carrier adhesion to such a small diameter carrier.
[0021]
The present invention has been made in view of the above background, and an object of the present invention is to sufficiently suppress the carrier from adhering to the surface of the latent image carrier even if the carrier in the developer has a small diameter. It is an object of the present invention to provide a developing device, an image forming apparatus, a process cartridge, and a developer carrier capable of performing the above-mentioned operations.
[0022]
[Means for Solving the Problems]
In order to achieve the above object, according to the first aspect of the present invention, a part of a surface in a surface moving direction of a developer carrier that carries a developer containing toner and magnetic particles on the surface and moves on the surface is exposed from an apparatus casing. A developing region in which the surface of the latent image carrier that carries the latent image on the surface and moves and the exposed surface of the developer carrier faces the developing region, and is arranged so as to face the developing region. A developing device for developing a latent image on the latent image carrier by forming a magnetic brush on the developer carrier by causing the developer on the developer carrier to spike by a magnetic pole; On the surface of the developer carrier up to another magnetic pole having a polarity opposite to that of the developing magnetic pole and adjacent to the developing magnetic pole and arranged downstream in the direction of movement of the developer carrier, the developer carrier surface method is used. The absolute value of the magnetic flux density in the linear direction and the developer loading The sum of the absolute values of the magnetic flux density at the surface moving direction and is characterized by being configured so as to increase monotonically.
According to a second aspect of the present invention, in the developing device of the first aspect, the developer carrier is moved in the developing area in the same direction as the surface moving direction of the latent image carrier and the linear velocity of the surface of the latent image carrier. The surface is moved at a large linear velocity, and the magnetic brush on the developer carrier is rubbed against the surface of the latent image carrier to perform development. The developer carried on the developer carrier is The amount of the developer conveyed to the developing area after being regulated by the regulating member is 65 mg / cm.²] 95 [mg / cm]²The attenuation rate of magnetic flux density in the normal direction of the surface of the developer carrier of the magnetic flux generated outside the surface of the developer carrier in the development area by the developing magnetic pole is 40% or more. Is what you do.
According to a third aspect of the present invention, in the developing device of the first aspect, the developer carrier is moved in the developing area in the same direction as the surface moving direction of the latent image carrier and the linear velocity of the surface of the latent image carrier. The surface is moved at a large linear velocity, and the magnetic brush on the developer carrier is rubbed against the surface of the latent image carrier to perform development. The developer carried on the developer carrier is The amount of the developer conveyed to the developing area after being regulated by the regulating member is 65 mg / cm.²] 95 [mg / cm]²The following is a half-value point on the surface of the developer carrying member, which is generated on the surface of the developer carrying member by the developing magnetic pole and has a magnetic flux density of half of the maximum magnetic flux density in the normal direction of the surface of the developer carrying member, An angle width between half-value points in a direction of movement of the surface of the developer carrier when viewed from a central axis of curvature of the surface of the developer carrier in the development region is 25 ° or less. is there.
According to a fourth aspect of the present invention, in the developing device of the first, second or third aspect, the maximum magnetic flux density in the normal direction of the surface of the developer carrier generated on the surface of the developer carrier by the developing magnetic pole is represented by Bm.₁The maximum magnetic flux density in the normal direction of the surface of the developer carrier generated on the surface of the developer carrier by the other magnetic pole is represented by Bm.₂Where the following relational expression is satisfied.
100 [mT] ≦ Bm₁≦ Bm₂≦ 160 [mT] According to a fifth aspect of the present invention, in the developing device according to the first, second, third or fourth aspect, the magnetic particles have a volume average particle diameter of 20 μm or more and 50 μm or less. It is characterized by using a thing.
According to a sixth aspect of the present invention, in the developing device according to the fifth aspect, a resin coat film containing a resin component obtained by crosslinking a thermoplastic resin and a melamine resin as a magnetic particle and a charge controlling agent is used as the magnetic particle. It is characterized in that a material coated on a material is used.
Further, according to the present invention, a latent image carrier that carries a latent image on its surface and moves on the surface, a latent image forming unit that forms a latent image on the latent image carrier, and a latent image carrier on the latent image carrier An image forming an image on the recording material by displacing the toner image on the latent image carrier onto the recording material, comprising: developing means for developing the latent image with a developer containing toner and magnetic particles. In the forming apparatus, the developing device according to claim 1, 2, 3, 4, 5, or 6 is used as the developing unit.
According to an eighth aspect of the present invention, in the image forming apparatus of the seventh aspect, the minimum distance between the surface of the latent image carrier and the surface of the developer carrier is 0.1 mm or more and 0.4 mm or more. mm] or less.
According to a ninth aspect of the present invention, at least the latent image carrier and the developing unit are detachable from the main body of the image forming apparatus according to the seventh or eighth aspect. It is characterized by the following.
According to a tenth aspect of the present invention, a developer containing a toner and a magnetic particle for developing a latent image on a latent image carrier that carries a latent image on the surface and moves on the surface is moved on the surface. In the developer carrying member, the developing region has a developing magnetic pole for forming a magnetic field for forming a magnetic brush by raising the developer in a developing region opposed to the surface of the latent image supporting member.From the developing magnetic pole, On the surface of the developer carrier up to another magnetic pole having a polarity opposite to that of the developing magnetic pole and adjacent to the developing magnetic pole and arranged downstream in the direction of movement of the developer carrier, the developer carrier surface method is used. It is characterized in that the sum of the absolute value of the magnetic flux density in the linear direction and the absolute value of the magnetic flux density in the direction of movement of the surface of the developer carrier monotonically increases.
[0023]
The developing device according to any one of the first to sixth aspects, the image forming apparatus according to the seventh and eighth aspects, the process cartridge according to the ninth aspect, and the developer carrying member according to the tenth aspect allow the developer to stand on the magnetic brush in the development area. It has a developing magnetic pole for forming a magnetic field for forming. On the downstream side of the developing magnetic pole in the direction of movement of the surface of the developer carrier, another magnetic pole having a polarity opposite to that of the developing magnetic pole is arranged so as to be adjacent to the developing magnetic pole. Then, on the surface of the developer carrying member from the developing magnetic pole to another magnetic pole, the absolute value of the magnetic flux density (normal direction magnetic flux density) in the direction normal to the surface of the developer carrying material and the magnetic flux in the moving direction of the surface of the developer carrying member It is configured such that the sum of the density (tangential magnetic flux density) and the absolute value monotonically increases. It has been confirmed by the present inventors that this configuration makes it possible to effectively suppress carrier adhesion. The details will be described in Experimental Example 1 to be described later. If the increase is monotonous, the carrier present in the carrier detachment region (the carrier constituting the tip portion of the magnetic brush) is placed on the developer carrier side. It is considered that the attracted magnetic force is increased, so that carrier adhesion can be effectively suppressed.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
[Embodiment 1]
Hereinafter, an embodiment in which the present invention is applied to an electrophotographic copying machine (hereinafter, simply referred to as “copier”) as an image forming apparatus (hereinafter, this embodiment is referred to as “embodiment 1”) will be described. . The copying machine according to the first embodiment is a so-called tandem-type color copying machine having a photosensitive drum as a latent image carrier for each color, but is not limited thereto.
[0025]
First, the configuration of the entire copying machine according to the first embodiment will be described.
FIG. 2 is a schematic configuration diagram of the entire copying machine according to the first embodiment. The copying machine includes a copying machine main body 100, a paper feed table 200 on which the copying machine main body is placed, a scanner 300 mounted on the copying machine main body, and an automatic document feeder (ADF) mounted on an upper portion of the scanner. 400.
[0026]
FIG. 3 is an enlarged view showing the configuration of the copying machine main body 100 part. The copying machine main body 100 is provided with an intermediate transfer belt 10 which is an intermediate transfer member as an endless belt-shaped image carrier. The intermediate transfer belt 10 is driven to rotate clockwise in FIG. 3 while being stretched around three support rollers 14, 15, 16. Of the support rollers, four image forming units 18Y, 18C, 18M, and 18K of yellow, cyan, magenta, and black are arranged side by side on a belt stretched portion between the first support roller 14 and the second support roller 15. Are located. Above these image forming units 18Y, 18C, 18M, 18K, an exposure device 21 as a latent image forming means is provided, as shown in FIG. The exposure device 21 forms an electrostatic latent image on photosensitive drums 20Y, 20C, 20M, and 20K as latent image carriers provided in each image forming unit based on image information of a document read by the scanner 300. It is for doing. A secondary transfer device 22 is provided at a position of the support roller that faces the third support roller 16. The secondary transfer device 22 has a configuration in which an endless secondary transfer belt 24 is stretched between two rollers 23a and 23b. When the toner image on the intermediate transfer belt 10 is secondarily transferred onto a transfer sheet as a recording material, the secondary transfer belt 24 is pressed against the intermediate transfer belt 10 wound around the third support roller 16. To perform secondary transfer. Note that the secondary transfer device 22 does not have to use the secondary transfer belt 24 but may use a transfer roller or a non-contact transfer charger, for example. Further, a fixing device 25 for fixing the toner image transferred onto the transfer paper is provided on the downstream side of the secondary transfer device 22 in the transfer paper transport direction by the secondary transfer belt 24. The fixing device 25 has a configuration in which a pressure roller 27 is pressed against a heating roller 26. A belt cleaning device 17 is provided at a position facing the second support roller 15 among the support rollers of the intermediate transfer belt 10. The belt cleaning device 17 is for removing residual toner remaining on the intermediate transfer belt 10 after transferring the toner image on the intermediate transfer belt 10 to transfer paper as a recording material.
[0027]
Next, the configuration of the image forming units 18Y, 18C, 18M, and 18K will be described. In the following description, the image forming unit 18K that forms a black toner image will be described as an example, but the other image forming units 18Y, 18C, and 18M have the same configuration.
FIG. 4 is an enlarged view showing a configuration of two adjacent image forming units 18M and 18K. Note that, in the reference numerals in the drawings, the symbols “M” and “K” indicating the distinction of colors are omitted, and the symbols are omitted as appropriate in the following description.
In the image forming unit 18, a charging device 60, a developing device 80, and a photoconductor cleaning device 63 are provided around the photoconductor drum 20. A primary transfer device 62 is provided at a position facing the photosensitive drum 20 via the intermediate transfer belt 10.
[0028]
The charging device 60 is of a contact charging type using a charging roller, and charges the surface of the photosensitive drum 20 uniformly by contacting the photosensitive drum 20 and applying a voltage. As the charging device 60, a non-contact charging type using a non-contact scorotron charger or the like can be used.
[0029]
Further, the developing device 80 uses a two-component developer composed of a magnetic carrier as magnetic particles and a non-magnetic toner. The developing device 80 includes a casing 84 provided with an opening for allowing a part of the surface of a developing sleeve 81 as a developer carrier to face the photosensitive drum 20. Inside the casing 84, a two-component developer (hereinafter, simply referred to as "developer") is accommodated. The developing sleeve 81 is driven to rotate in the direction of rotation with the rotation of the photosensitive drum 20 with the surface of the developer carrying the developer. In the internal space of the casing 84, two transport screws 82a and 82b as transport members for transporting the developer in the rotation axis direction of the developing sleeve 81 are provided. The two conveying screws 82a and 82b rotate the fins fixed to the rotation shaft to convey the developer in a direction parallel to the rotation axis direction of the developing sleeve 81 while stirring the developer. The transport screws 82a and 82b are configured to transport the developer in opposite directions. Between the two conveying screws 82a and 82b, a partition member 84a is formed integrally with the casing 84 so as to partition at both ends in the rotation axis direction of the developing sleeve so as to communicate with each other. As a result, the developer transported to the transport end ends of one of the transport screws 82a and 82b is transferred to the transport start ends of the other transport screws 82b and 82a in both end regions of the two transport screws 82a and 82b. A movement passage for movement is formed. Therefore, when the developer is transported to the transport end by the transport screws 82a and 82b, the developer moves to the other transport screws 82b and 82a through the moving passage, and is transported in the opposite direction. The developer circulates in the internal space. The configuration and operation of the developing device 80 will be described later in detail.
[0030]
The primary transfer device 62 employs a primary transfer roller, and is installed so as to be pressed against the photosensitive drum 20 with the intermediate transfer belt 10 interposed therebetween. The primary transfer device 62 does not have to be a roller, but may be a conductive brush or a non-contact corona charger.
Further, the photoconductor cleaning device 63 includes a cleaning blade 75 made of, for example, polyurethane rubber, which is disposed so that the front end thereof is pressed against the photoconductor drum 20. In the first embodiment, a conductive fur brush 76 that comes into contact with the photosensitive drum 20 is used in order to enhance the cleaning performance. A bias is applied to the fur brush 76 from a metal electric field roller 77, and the tip of a scraper 78 is pressed against the electric field roller 77. Then, the toner removed from the photosensitive drum 20 by the cleaning blade 75 or the fur brush 76 is stored inside the photosensitive member cleaning device 63. Thereafter, the toner is brought to one side of the photoconductor cleaning device 63 by the collection screw 79, returned to the developing device 80 through a toner recycling device (not shown), and reused.
The static eliminator 64 includes a static elimination lamp, and irradiates light to initialize the surface potential of the photosensitive drum 20.
[0031]
In the image forming unit 18 having the above configuration, the charging device 60 first uniformly charges the surface of the photosensitive drum 20 as the photosensitive drum 20 rotates. Next, based on the image information read by the scanner 300, the exposure device 21 irradiates a writing light L such as a laser or an LED from the exposure device 21 to form an electrostatic latent image on the photosensitive drum 20. Thereafter, the electrostatic latent image is visualized by the developing device 80 to form a toner image. This toner image is primarily transferred onto the intermediate transfer belt 10 by the primary transfer device 62. The transfer residual toner remaining on the surface of the photoconductor drum 20 after the primary transfer is removed by the photoconductor cleaning device 63. Thereafter, the surface of the photoconductor drum 20 is neutralized by the static eliminator 64 to be ready for the next image formation. Provided.
[0032]
Next, the operation of the copying machine according to the first embodiment will be described.
When copying a document using the copying machine having the above-described configuration, first, a document is set on the document table 30 of the automatic document feeder 400 shown in FIG. Alternatively, the automatic document feeder 400 is opened, a document is set on the contact glass 32 of the scanner 300, and the automatic document feeder 400 is closed and pressed. Thereafter, when the user presses a start switch (not shown), the original is transported onto the contact glass 32 when the original is set in the automatic original transport device 400. Then, the scanner 300 is driven, and the first traveling body 33 and the second traveling body 34 start traveling. Accordingly, light from the first traveling body 33 is reflected by the original on the contact glass 32, and the reflected light is reflected by the mirror of the second traveling body 34, and is guided to the reading sensor 36 through the imaging lens 35. . Thus, the image information of the document is read.
[0033]
When a start switch is pressed by a user, a drive motor (not shown) is driven, and one of the support rollers 14, 15, 16 is rotationally driven, and the intermediate transfer belt 10 is rotationally driven. At the same time, the photosensitive drums 20Y, 20C, 20M, and 20K of the image forming units 18Y, 18C, 18M, and 18K are also driven to rotate. The details of the drive mechanism of the photosensitive drums 20Y, 20C, 20M, and 20K will be described later. Thereafter, based on the image information read by the reading sensor 36 of the scanner 300, the exposure light 21 writes the writing light L onto the photosensitive drums 20Y, 20C, 20M, and 20K of the image forming units 18Y, 18C, 18M, and 18K. Are respectively irradiated. Thus, an electrostatic latent image is formed on each of the photosensitive drums 20Y, 20C, 20M, and 20K, and is visualized by the developing devices 80Y, 80C, 80M, and 80K. Then, yellow, cyan, magenta, and black toner images are formed on the photosensitive drums 20Y, 20C, 20M, and 20K, respectively. The toner images of each color thus formed are primary-transferred by the primary transfer devices 62Y, 62C, 62M, and 62K so as to sequentially overlap the intermediate transfer belt 10. As a result, on the intermediate transfer belt 10, a combined toner image in which the toner images of the respective colors are overlapped is formed. The transfer residual toner remaining on the intermediate transfer belt 10 after the secondary transfer is removed by the belt cleaning device 17.
[0034]
When the user presses the start switch, the paper feed roller 42 of the paper feed table 200 rotates according to the transfer paper selected by the user, and the transfer paper is sent out from one of the paper feed cassettes 44. The transferred transfer paper is separated into one sheet by a separation roller 45, enters a paper feed path 46, and is transported by a transport roller 47 to a paper feed path 48 in the copying machine main body 100. The transfer paper conveyed in this way is stopped when it comes into contact with the registration rollers 49. When transfer paper not set in the paper feed cassette 44 is used, the transfer paper set in the manual feed tray 51 is sent out by the paper feed roller 50 and conveyed through the manual feed path 53. Then, it is stopped when it comes into contact with the registration roller 49.
[0035]
The registration roller 49 rotates in synchronization with the timing at which the composite toner image formed on the intermediate transfer belt 10 as described above is conveyed to the secondary transfer section facing the secondary transfer belt 24 of the secondary transfer device 22. To start. Here, the registration roller 49 is generally often used while grounded, but a bias may be applied to remove paper dust from the transfer paper. The transfer paper sent out by the registration rollers 49 is sent between the intermediate transfer belt 10 and the secondary transfer belt 24, and the secondary transfer device 22 transfers the synthetic toner image on the intermediate transfer belt 10 to the secondary transfer paper. Transcribed. Thereafter, the transfer paper is conveyed to the fixing device 25 while being attracted to the secondary transfer belt 24, and heat and pressure are applied by the fixing device 25 to perform a fixing process of the toner image. The transfer paper that has passed through the fixing device 25 is discharged to a discharge tray 57 by discharge rollers 56 and stacked. When an image is to be formed on the back side of the surface on which the toner image has been fixed, the transfer path of the transfer sheet that has passed through the fixing device 25 is switched by the switching claw 55. Then, the transfer paper is sent to a sheet reversing device 28 located below the secondary transfer device 22, where it is reversed and guided again to the secondary transfer unit.
[0036]
Next, the configuration and operation of the developing device, which is a feature of the present invention, will be described in detail. The configuration and operation of the developing devices 80Y, 80C, 80M, and 80K are the same for all of the image forming units 18Y, 18C, 18M, and 18K. explain.
[0037]
FIG. 5 is a schematic configuration diagram illustrating the developing device 80 according to the first embodiment.
Inside the developing sleeve 81, a magnet roller 85 as a magnetic field generating means having a plurality of magnets is fixedly arranged. The developing sleeve 81 rotates around the magnet roller 85. The developer conveyed and circulated in the internal space A of the casing 84 while being stirred by the two screws 82 a and 82 b is pumped to the surface of the developing sleeve 81 by the magnetic field of the magnet roller 85. Then, it is held by the magnetic force on the surface of the developing sleeve 81, is conveyed with the rotation of the developing sleeve 81, and is appropriately adjusted by a gap (doctor gap) between the tip of the doctor blade 83 as a developer regulating member and the surface of the developing sleeve 81. Is regulated to a small amount. This doctor gap affects the amount of developer supplied to the developing area. In the first embodiment, the doctor gap is set to 0.3 [mm], but may be in the range of 0.1 [mm] to 0.4 [mm]. In the first embodiment, the developer supply amount to the development region is adjusted to 65 [mg / cm] by adjusting various parameters such as the strength of the magnetic field near the doctor gap and the saturation magnetization value of the carrier.²] 95 [mg / cm]²] Are set to be as follows. The developer that has passed through the doctor gap is transported to a development area facing the photoconductor drum 20 as the development sleeve 81 rotates.
[0038]
The developer conveyed to the developing area in this way is subjected to the action of the magnetic field by the magnet roller 85, and is in a state of ears on the surface of the developing sleeve 81 to form a magnetic brush. In this developing area, a developing electric field for moving the toner in the developer to the electrostatic latent image portion on the photosensitive drum 20 is formed by the developing bias applied to the developing sleeve 81. As a result, the toner in the developer is transferred to the electrostatic latent image on the photosensitive drum 20, and the electrostatic latent image on the photosensitive drum 20 is visualized to form a toner image.
[0039]
The developer that has passed through the developing region is caused by a repulsive magnetic field formed by two adjacent magnets P2 and P3 of the same polarity that are adjacent to each other as a developer peeling unit provided on the magnet roller 85 as the developing sleeve 81 rotates. The first transport screw 82a is peeled off from the surface of the developer 81 and is taken into the developer transported by the first transport screw 82a.
[0040]
In the first embodiment, the photosensitive drum 20 having a diameter of 90 [mm] is rotationally driven so that the drum linear velocity in the developing region becomes 245 [mm / sec]. A certain developing sleeve 81 is rotationally driven so that the linear velocity of the sleeve in the developing region becomes 377 [mm / sec]. That is, in the first embodiment, the linear speed ratio of the sleeve linear speed to the drum linear speed is set to about 1.54.
In the first embodiment, the developing gap is set to 0.4 [mm]. The conventional developing gap is generally set to about 10 times the carrier particle size. For example, when the carrier particle size is 50 [μm], it is about 0.65 [mm] to 0.8 [mm]. . On the other hand, in the first embodiment, since the magnetic force of the main magnetic pole is larger than that in the related art, it can be set to about 30 times the carrier particle size. However, even in the first embodiment, if the developing gap is wider than about 30 times the carrier particle size, it becomes difficult to obtain a desired image density.
[0041]
Next, a magnetic field formed by the magnet roller 85 will be described.
The magnet roller 85 according to the first embodiment includes a main magnetic pole P1b as a developing magnetic pole that forms a magnetic field for causing the developer in the developing region to stand. Auxiliary magnetic poles P1a and P1c are arranged on the upstream side and the downstream side in the surface movement direction of the developing sleeve 81 with respect to the main magnetic pole P1b, respectively, so as to be close to the main magnetic pole P1b. Each of the magnetic poles P1a, P1b, P1c is constituted by a magnet having a small cross section. Generally, when the cross section of the magnet is reduced, the magnetic force becomes weaker. Therefore, in the first embodiment, the three magnetic poles P1a, P1b, and P1c are made of a magnet made of a rare earth metal alloy having a relatively strong magnetic force. Among the rare earth metal alloy magnets, according to a typical iron neodymium boron alloy magnet, 358 [kJ / m³] Can be obtained. According to the iron-neodymium boron alloy bonded magnet, 80 [kJ / m³] The maximum energy product before and after can be obtained. Generally, the maximum energy product is 36 [kJ / m³], Before and after 20 [kJ / m³A front and rear ferrite magnet, a ferrite bonded magnet, and the like are used. However, if a rare earth metal alloy magnet is used as in the first embodiment, a stronger magnetic force can be secured as compared with these. Therefore, even if a magnet having a small cross section is used, the magnetic force on the surface of the developing sleeve 81 can be sufficiently ensured.
[0042]
FIG. 6 is a pie chart showing the distribution of the magnetic flux density (hereinafter, referred to as “normal-direction magnetic flux density”) of the surface of the developing sleeve 81 generated by each magnetic pole of the magnet roller 85 in the normal direction of the surface by a solid line. It is. The broken line in FIG. 6 indicates the magnetic flux density in the normal direction at a position 1 [mm] away from the surface of the developing sleeve 81 in the normal direction. In order to create this pie graph, a Gauss meter (HGM-8300) manufactured by ADS and an A1 type axial probe manufactured by ADS were used, and the results measured with these were recorded by a pie chart recorder.
[0043]
In the first embodiment, the attenuation rate of the magnetic flux density in the normal direction means a value obtained by the following equation (1). At this time, “X” in Equation 1 indicates the peak value of the magnetic flux density in the normal direction generated on the surface of the developing sleeve 81, and “Y” indicates 1 [mm] from the surface of the developing sleeve 81 in the normal direction. ] Indicates the peak value of the magnetic flux density in the normal direction at a remote position. For example, the magnetic flux density in the normal direction on the surface of the developing sleeve 81 is 100 [mT], and the magnetic flux density in the normal direction at a portion 1 [mm] away from the surface of the developing sleeve 81 is 80 [mT]. At this time, the attenuation rate is 20 [%].
(Equation 1)
Attenuation rate [%] = {(XY) / X} × 100
[0044]
Next, the magnetic pole arrangement of the magnet roller 85 will be described.
The two auxiliary magnetic poles P1a and P1c are provided at positions adjacent to the main magnetic pole P1b on the upstream side and the downstream side in the surface moving direction of the developing sleeve 81, and have opposite polarities to the main magnetic pole P1b. In the first embodiment, two auxiliary magnetic poles P1a and P1c, a magnetic pole P4 for forming a magnetic field for pumping the developer onto the developing sleeve 81, and a magnetic field for passing the developer on the developing sleeve 81 to the doctor gap. Are formed of N poles. Further, the main magnetic pole P1b, the magnetic poles P2 and P3 for forming a repulsive magnetic field for peeling off the developer on the developing sleeve 81 that has passed through the developing area, and the same as the magnetic pole P4 for pumping the developer onto the developing sleeve 81. A magnetic pole P5 for forming a magnetic field and a magnetic pole P7 for forming a magnetic field for transporting the developer that has passed through the doctor gap to the developing area are configured by S poles. The polarity of each magnetic pole may be reversed. Further, the number of magnetic poles provided in the magnet roller 85 can be increased or decreased as appropriate.
[0045]
The two auxiliary magnetic poles P1a and P1c are used to adjust the distribution of the magnetic flux density in the normal direction on the surface of the developing sleeve 81 by the main magnetic pole P1c. Specifically, the angle width between the half-points in the direction of movement of the surface of the developing sleeve viewed from the center axis of curvature of the surface of the developing sleeve 81 in the developing area, that is, the center axis of the developing sleeve 81 (hereinafter, referred to as “half-value angle width”). Used to narrow. Here, the half-value angular width is the maximum value Bm of the normal direction magnetic flux density generated on the surface of the developing sleeve 81 by the main magnetic pole P1b as shown in FIG.₁The angle width θ in the surface movement direction of the developing sleeve 81 when viewed from the central axis of the developing sleeve 81 when two half-value points on the surface of the developing sleeve 81 indicating a magnetic flux density that is half of₁Say. Therefore, for example, when the maximum value of the magnetic flux density in the normal direction is 120 [mT], the half-value angle width θ₁Is the angular width when the half-value point on the surface of the developing sleeve 81 at which the magnetic flux density in the normal direction is 60 [mT], which is the half value thereof, is viewed from the central axis of the developing sleeve 81. In the first embodiment, the half-value angular width θ of the main magnetic pole P1b₁Is set to 25 ° or less, the magnetic characteristics and arrangement of the main magnetic pole P1b and the auxiliary magnetic poles P1a and P1c are set. Specifically, the width of the cross section of the magnets of the three magnetic poles P1a, P1b, and P1c constituting the developing magnetic pole in the direction of movement of the developing sleeve surface is set to 2 [mm]. As a result, the half value angular width of the main magnetic pole P1b in the first embodiment is 16 [°]. It has been confirmed that when the half value angular width of the main magnetic pole P1b exceeds 25 [°], an abnormal image such as a trailing edge white spot may occur. The half-value angular width of the auxiliary magnetic poles P1a and P1c is set to be equal to or less than 35 [°].
[0046]
FIG. 8 is an explanatory diagram showing the arrangement of the main magnetic pole P1b and the two auxiliary magnetic poles P1a and P1c provided on the magnet roller 85.
In the first embodiment, the positional relationship between the main magnetic pole P1b and each of the auxiliary magnetic poles P1a and P1c is, as shown, such that the arrangement angle width between the main magnetic pole P1b and each of the auxiliary magnetic poles P1a and P1c is 35 [°] or less. It is set as follows. This arrangement angle width is the maximum value Bm of the normal direction magnetic flux density generated on the surface of the developing sleeve 81 by the main magnetic pole P1b and the two auxiliary magnetic poles P1a and P1c.₁, Bm₂, Bm₃Are the respective angular widths in the direction of movement of the developing sleeve surface when viewed from the central axis of curvature of the surface of the developing sleeve 81 in the developing area, that is, the central axis of the developing sleeve 81. In the first embodiment, since the half angle width of the main magnetic pole P1b is 16 [°] as described above, the arrangement angle width of each of the auxiliary magnetic poles P1a and P1c with respect to the main magnetic pole P1b is set to 25 [°]. I have.
[0047]
Further, in the first embodiment, as shown in FIG. 8, the normal magnetic flux density generated on the surface of the developing sleeve 81 by the main magnetic pole P1b and the auxiliary magnetic poles P1a and P1c is 0 [mT]. The angle width between the two inflection points located on the most upstream side and the most downstream side in the surface movement direction of the developing sleeve 81 is configured to be 120 [°] or less. That is, the angle width between the inflection points existing between the two auxiliary magnetic poles P1a and P1c and the magnetic poles P2 and P7 adjacent to the respective auxiliary magnetic poles P1a and P1c is 120 [°] or less.
[0048]
The developer spikes along the lines of magnetic force generated by the magnet roller 85 having the main magnetic pole P1b and the auxiliary magnetic poles P1a and P1c as described above, and a magnetic brush is formed on the developing sleeve 81. In this magnetic brush, only the brush portion formed by the magnetic field of the main magnetic pole P1b comes into contact with the surface of the photosensitive drum 20 and contributes to visualization of the electrostatic latent image on the photosensitive drum 20. Will do. At this time, the length of the magnetic brush in the developing area is set to be about 1 [mm]. Note that the length of the magnetic brush here is the length when the photoconductor drum 20 is removed, and in actuality, the developing gap is set to 0.4 [mm]. The length of the magnetic brush becomes shorter according to its development gap.
[0049]
The reason why the length of the magnetic brush can be reduced as described above is that the attenuation rate of the magnetic flux density in the normal direction is large as described above. The reason is that the normal direction magnetic flux density on the surface of the developing sleeve 81 is high, but the attenuation rate is high, so that the normal direction magnetic flux density at a position 1 mm away from the surface of the developing sleeve 81 is abrupt. Lower. For this reason, the developer near the surface of the developing sleeve 81 is densely affected by the action of the strong magnetic field. However, the developer is unable to maintain the magnetic brush because the magnetic field is weak at a relatively distant position from the surface of the developing sleeve 81. is there. Further, in the first embodiment, as described above, the developer supply amount is set to 30 [mg / cm²] 95 [mg / cm]²] It is set to be less than the following. As a result, a longer magnetic brush can be originally formed, but the magnetic brush is restricted to be short due to a shortage of the developer supply amount. As a result of the short regulation, by setting the developing gap to 0.4 [mm], the photosensitive drum 20 is brushed with a dense brush portion near the surface of the developing sleeve 81 having a high magnetic flux density. Can be rubbed. In the first embodiment, the developing gap is set to 0.4 [mm]. However, it is preferable that the developing gap be within a shorter range. Specifically, the distance may be in a range of 0.1 [mm] or more and 0.4 [mm] or less. Within this range, the surface of the photosensitive drum 20 can be rubbed with a brush portion made of a dense developer near the surface of the developing sleeve 81 having a high magnetic flux density.
[0050]
As a result of the study by the present inventors, the absolute value of the magnetic flux density in the normal direction on the surface of the developing sleeve from the main magnetic pole P1b to the auxiliary magnetic pole P1c adjacent to the downstream side thereof, and the surface of the developing sleeve on the developing sleeve surface A configuration in which the sum of the absolute value of the magnetic flux density in the moving direction (hereinafter, referred to as “tangential magnetic flux density”) monotonically increases is effective in suppressing carrier adhesion, which is an object of the present invention. It was confirmed that. Hereinafter, a specific description will be given.
[0051]
[Experimental example 1]
First, the evaluation of the carrier adhesion is performed using the developing device 80 according to the first embodiment and the developing device using the main magnetic pole P1b and the auxiliary magnetic poles P1a and P1c having higher magnetic flux density in the normal direction than the developing device 80. An experimental example (hereinafter, this experimental example is referred to as “Experimental example 1”) will be described.
In the first experimental example, a developer using a small-diameter carrier having a volume average particle diameter of 35 [μm] was used. The two developing devices used in Experimental Example 1 have the same configuration except that the magnetic flux density generated by the main magnetic pole P1b and the auxiliary magnetic poles P1a and P1c is different.
[0052]
FIG. 1 is a circle showing the distribution of magnetic flux density on the surface of the developing sleeve 81 generated by the magnetic poles P1a, P1b, P1c, P2 to P7 of the magnet roller 85 in the developing device according to the first embodiment. It is a graph. FIG. 9 is a pie chart showing the distribution of magnetic flux density on the surface of the developing sleeve when the main magnetic pole P1b and the auxiliary magnetic poles P1a and P1c made of a stronger magnet than in the first embodiment are used. In these circular bluffs, the thin solid line indicates the normal magnetic flux density on the developing sleeve surface, the broken line indicates the tangential magnetic flux density on the developing sleeve surface, and the thick solid line indicates these normal magnetic flux density and tangential magnetic flux. The sum with the density (hereinafter, referred to as “synthetic magnetic flux density”) is shown.
In the developing device according to the first embodiment, the maximum normal direction magnetic flux density Bm generated on the surface of the developing sleeve by the main magnetic pole P1b.₁Is 87.9 [mT], and the maximum normal direction magnetic flux density Bm generated on the surface of the developing sleeve by the downstream auxiliary magnetic pole P1c.₂Was 117.0 [mT]. Further, the tangential magnetic flux density Bm generated at the portion where the normal magnetic flux density becomes 0 on the surface of the developing sleeve corresponding to the carrier separation region, that is, on the developing sleeve surface.₀Was 96.2 [mT].
On the other hand, in the comparative developing device shown in FIG. 9, the maximum normal magnetic flux density Bm generated on the developing sleeve surface by the main magnetic pole P1b.₁Is 116.3 [mT], and the maximum normal direction magnetic flux density Bm generated on the developing sleeve surface by the downstream auxiliary magnetic pole P1c.₂Was 117.9 [mT]. Further, the tangential magnetic flux density Bm generated at the portion where the normal magnetic flux density becomes 0 on the surface of the developing sleeve corresponding to the carrier separation region, that is, on the developing sleeve surface.₀Was 98.9 [mT].
[0053]
In the first experimental example, a solid image was formed on an A3-size sheet using the developing device according to the first embodiment shown in FIG. 1 and the developing device for comparison shown in FIG. The carrier on the drum surface was collected with an adhesive tape. Then, based on the number of carriers collected with the adhesive tape, the evaluation of carrier adhesion to the photosensitive drum 20 was performed. In general, the larger the difference between the potential (charging potential) of the non-latent image portion on the surface of the photosensitive drum and the potential (development potential) on the surface of the developing sleeve (hereinafter referred to as “background potential”), the worse the carrier adhesion. It is known to Therefore, in this experimental example, the background potential was adjusted, and the carrier adhesion number was measured at each of six different background potentials.
[0054]
FIG. 10 is a graph showing evaluation results of carrier adhesion in Experimental Example 1. As shown in this graph, the number of carriers adhered to the developing device according to the first embodiment shown in FIG. 1 is clearly smaller than that of the comparative developing device shown in FIG. Thus, it can be confirmed that the developing device according to the first embodiment can suppress carrier adhesion more than the developing device for comparison.
[0055]
[Experimental example 2]
Next, using the developing device 80 according to the first embodiment and the developing device using the main magnetic pole P1b and the auxiliary magnetic poles P1a and P1c having higher magnetic flux density in the normal direction than the developing device 80, evaluation of carrier adhesion is performed. (Hereinafter, this experimental example will be referred to as “Experimental Example 1”.)
In this experimental example 2, a developer using a small-diameter carrier having a volume average particle diameter of 35 [μm] was used as in the above-mentioned experimental example 1. The two developing devices used in Experimental Example 1 have the same configuration except that the magnetic flux density generated by the main magnetic pole P1b and the auxiliary magnetic poles P1a and P1c is different.
[0056]
FIG. 11 is a pie chart showing the distribution of the magnetic flux density on the developing sleeve surface when the main magnetic pole P1b made of a stronger magnet than in the first embodiment is used. In the comparative developing device shown in FIG. 11, the maximum normal magnetic flux density Bm generated on the developing sleeve surface by the main magnetic pole P1b.₁Is 105.1 [mT], and the maximum normal direction magnetic flux density Bm generated on the surface of the developing sleeve by the downstream auxiliary magnetic pole P1c.₂Was 99.2 [mT]. Further, the tangential magnetic flux density Bm generated at the portion where the normal magnetic flux density becomes 0 on the surface of the developing sleeve corresponding to the carrier separation region, that is, on the developing sleeve surface.₀Was 89.2 [mT].
[0057]
In Experimental Example 2, a solid image was formed on an A3-size sheet by using the developing device according to the first embodiment shown in FIG. 1 and the developing device for comparison shown in FIG. 11, respectively. The number of white spots present therein was counted. Since the white spots are generated by carriers adhering to the surface of the photoconductor drum, the evaluation of the carrier adhesion to the photoconductor drum 20 was performed based on the number of the white spots. Other experimental methods are the same as in Experimental Example 1.
[0058]
FIG. 12 is a graph showing evaluation results of carrier adhesion in Experimental Example 2. As shown in this graph, the number of white spots in the developing device according to the first embodiment shown in FIG. 1 is clearly smaller than that in the comparative developing device shown in FIG. Thus, it can be confirmed that the developing device according to the first embodiment can suppress carrier adhesion more than the developing device for comparison.
[0059]
Here, the experimental results in Experimental Examples 1 and 2 described above will be considered. At first glance, the developing device for comparison, in which a strong magnet is used for the main magnetic pole P1b, has a stronger magnetic force for restraining the carrier toward the developing sleeve 81 than the developing device according to the first embodiment, and suppresses carrier adhesion. Seems effective. However, as a result of the research of the present inventors, it has been found that within the range of the magnetic flux density in the normal direction due to the commonly used developing magnetic pole, even if the magnetic flux density in the normal direction is increased, the effect of suppressing the carrier adhesion is not significantly increased. did. Then, the present inventors conducted a number of experiments including the experiments described in the above-mentioned Experimental Examples 1 and 2, and as shown in FIG. 1 as in the developing device according to the first embodiment. It has been found that the effect of suppressing carrier adhesion is enhanced when the magnetic flux density is high. That is, a magnetic flux density distribution from the main magnetic pole P1b to the auxiliary magnetic pole P1c adjacent to the main magnetic pole P1b such that the composite magnetic flux density obtained by combining the normal magnetic flux density and the tangential magnetic flux density on the developing sleeve surface monotonically increases. It has been found that, if obtained, it is effective for suppressing carrier adhesion.
[0060]
The reason why the magnetic flux density distribution in which the resultant magnetic flux density monotonically increases is effective in suppressing carrier adhesion is considered as follows.
That is, as described above, the carrier that causes the carrier to adhere is a resultant force such as a centrifugal force that acts when the carrier falls down after passing through the center of the developing area and an electrostatic force that is attracted to the photosensitive drum 20 side. It is detached from the developer when the force of restraining the developing sleeve 81 by the magnetic force is exceeded. Therefore, the developing device according to the first embodiment shown in FIG. 1 has a relatively large restraining force with respect to the resultant force in the carrier detachment region as compared with the developing device for comparison shown in FIGS. it is conceivable that.
[0061]
Here, the magnetic force acting on one carrier by the magnetic field generated by the main magnetic pole P1b and the downstream auxiliary magnetic pole P1c is F, the dipole moment (magnetic moment) of the carrier is m (vector), and the carrier exists. Assuming that the magnetic flux density at the position is B (vector), the following relational expression 2 is established. In addition, the magnetic moment m is expressed by μ₀Where μ is the relative magnetic permeability of the carrier and d is the radius of the carrier, it can be obtained from the following equation (3). In the equation (3), k is a constant determined by the particle size of the carrier and its composition.
(Equation 2)

(Equation 3)

[0062]
The axial direction of the developing sleeve 81 is defined as the x direction, the tangential direction of the surface in the moving direction of the developing sleeve surface is defined as the y direction, and the normal direction of the developing sleeve surface is defined as the z direction. In the y direction, the direction in which the developing sleeve moves on the surface is defined as positive, and in the z direction, the direction away from the central axis of the developing sleeve is defined as positive. At this time, the restraining force for restraining the carrier toward the developing sleeve 81 is the z-direction component Fz of the magnetic force F acting on the carrier. Then, the x-direction component Bx of the magnetic flux density B can be approximated to zero. Therefore, the restraining force Fz for restraining the carrier toward the developing sleeve 81 is as shown in the following Expression 4 from the arithmetic expressions shown in the above Expressions 2 and 3.
(Equation 4)
Fz ≒ k ｛{(By ・ (∂Bz / ∂y) + Bz ・ (∂Bz / ∂z)}｝
[0063]
The restraining force Fz acting on the carrier in the carrier separation region is obtained from the arithmetic expression shown in the above Expression 4. This carrier separation region is a region exactly in the middle between the main magnetic pole P1b and the downstream auxiliary magnetic pole P1c, and is a portion through which the tip of the magnetic brush passes. Therefore, in the carrier separation region, the direction of the magnetic force lines is substantially parallel to the y direction, and the component of the magnetic flux density B in the z direction can be approximated to zero. Then, the restraining force Fz acting on the carrier in the carrier separation region is as shown in the following Expression 5 from the operation expression shown in the above Expression 4.
(Equation 5)
Fz ｋ k ・ By ・ (∂Bz / ∂y)
[0064]
By the way, at the center of the development area, the development gap Pg is narrower than the original length of the magnetic brush, so that the magnetic brush is kept short. Therefore, after passing through the center of the development area, the magnetic brush gradually becomes longer as it goes away from the center of the development area. Therefore, the tip portion of the magnetic brush moves away from the surface of the developing sleeve as the distance from the center of the developing area increases. Therefore, the magnetic force acting on the carrier constituting the tip portion of the magnetic brush weakens as the distance from the center of the developing area increases. Note that the surface of the photosensitive drum 20 also moves away from the surface of the developing sleeve as the distance from the center of the developing area increases, so that the electrostatic force acting on the carrier decreases as the distance from the center of the developing area decreases. However, the degree of weakening of the electrostatic force is smaller than the degree of weakening of the magnetic force acting on the magnetic brush. Therefore, the carrier tends to separate from the developer as the distance from the center of the development area increases. Therefore, in order to suppress carrier detachment from the developer, it is necessary to increase the restraining force Fz as the distance from the center of the developing area increases.
[0065]
From the arithmetic expression shown in Equation 5, in order to increase the constraint force Fz in the carrier separation region, it is necessary to increase the y-direction component By of the magnetic flux density B in the carrier separation region. Further, in the carrier detachment region, the absolute value of the magnetic flux density B in the carrier detachment region can be obtained from the following equation (6), and the value can be approximated to the absolute value of the y-direction component By. In consideration of the fact that it is necessary to increase the binding force Fz as the distance from the center of the development region increases in order to suppress the carrier detachment from the developer as described above, the absolute value of the magnetic flux density B in the carrier detachment region is The larger the distance from the center of the region, the more the carrier can be prevented from detaching from the developer. Since the carrier separation region is located at an intermediate point between the main magnetic pole P1b and the downstream auxiliary magnetic pole P1c, it is a portion where the magnetic flux density B becomes the smallest between the main magnetic pole P1b and the downstream auxiliary magnetic pole P1c. Therefore, the fact that the absolute value of the magnetic flux density B in the carrier separation region increases as the distance from the center of the developing region increases indicates that the absolute value of the magnetic flux density B always increases from the main magnetic pole P1b toward the downstream auxiliary magnetic pole P1c. Means that. The absolute value of the magnetic flux density B can be approximated to the absolute value of the combination of the y-direction component Bx and the z-direction component Bz since the x-direction component Bx can approximate 0. The fact that the absolute value of the magnetic flux density B constantly increases is equivalent to the fact that the sum of the absolute value of the y-direction component Bx and the absolute value of the z-direction component Bz monotonically increases. Further, such monotonic increase is synonymous with monotonically increasing the sum of the absolute value of the y-direction component Bx and the absolute value of the z-direction component Bz of the magnetic flux density on the developing sleeve surface. Therefore, a magnetic flux density distribution is obtained such that the sum of the absolute value of the normal magnetic flux density and the absolute value of the tangential magnetic flux density on the surface of the developing sleeve monotonically increases from the main magnetic pole P1b to the downstream auxiliary magnetic pole P1c. Accordingly, it is considered that the separation of the carrier from the developer can be suppressed, and the adhesion of the carrier to the photosensitive drum 20 can be suppressed.
(Equation 6)
B = (Bx²+ By²+ Bz²)^1/2
≒ | By |
[0066]
As a result of the study of the present inventors, the maximum normal direction magnetic flux density generated on the developing sleeve surface by the main magnetic pole P1b is represented by Bm.₁The maximum normal magnetic flux density generated on the surface of the developing sleeve by the downstream auxiliary magnetic pole P1c is represented by Bm.₂Then, it was also found that the carrier adhesion can be more effectively suppressed if the following relational expression 7 is satisfied.
(Equation 7)
100 [mT] ≦ Bm₁  ≤ Bm₂  ≤ 160 [mT]
[0067]
[Comparative Example 1]
Next, a comparative experiment (hereinafter, referred to as “Comparative Example 1”) performed to confirm that the smaller the volume average particle diameter of the carrier is, the more likely the carrier is to adhere is described. In Comparative Example 1, the carrier adhesion was evaluated using a carrier having a smaller volume average particle diameter than the carriers used in Experimental Examples 1 and 2 described above. Specifically, in the above-described Experimental Examples 1 and 2, a small-diameter carrier having a volume average particle diameter of 35 [μm] was used. In Comparative Example 1, the volume average particle diameter was 55 [μm]. Was used. Then, in Comparative Example 1, similarly to Experimental Example 2 described above, the carrier adhesion to the photosensitive drum 20 was evaluated based on the number of white dots.
[0068]
FIGS. 13 and 14 are pie charts showing the distribution of magnetic flux density on the surface of the developing sleeve when the main magnetic pole P1b made of a stronger magnet than in the first embodiment is used. As can be seen from this pie chart, the developing devices shown in these figures do not have a configuration in which the composite magnetic flux density monotonically increases, unlike the developing device according to the first embodiment.
In the comparative developing device shown in FIG. 13, the maximum normal magnetic flux density Bm generated on the surface of the developing sleeve by the main magnetic pole P1b.₁Is 100.2 [mT], and the maximum normal direction magnetic flux density Bm generated on the surface of the developing sleeve by the downstream auxiliary magnetic pole P1c.₂Was 95.7 [mT]. Further, the tangential magnetic flux density Bm generated at the portion where the normal magnetic flux density becomes 0 on the surface of the developing sleeve corresponding to the carrier separation region, that is, on the developing sleeve surface.₀Was 84.1 [mT].
In the comparative developing device shown in FIG. 14, the maximum magnetic flux density Bm in the normal direction generated on the surface of the developing sleeve by the main magnetic pole P1b.₁Is 94.5 [mT], and the maximum normal direction magnetic flux density Bm generated on the developing sleeve surface by the downstream auxiliary magnetic pole P1c.₂Was 102.2 [mT]. Further, the tangential magnetic flux density Bm generated at the portion where the normal magnetic flux density becomes 0 on the surface of the developing sleeve corresponding to the carrier separation region, that is, on the developing sleeve surface.₀Was 86.3 [mT].
[0069]
FIG. 15 is a graph showing evaluation results of carrier adhesion in Comparative Example 1. Comparing this evaluation result with the evaluation result of the developing device for comparison in Experimental Example 2 shown in FIG. 12, the number of white spots in the two developing devices according to Comparative Example 1 is clearly smaller. Thus, it can be confirmed that the smaller the volume average particle diameter of the carrier, the more likely the carrier adhesion occurs.
[0070]
FIG. 16 shows a case where an image was formed with a developer using three types of carriers having a background potential of 200 V and a volume average particle diameter of 35 μm, 50 μm, and 65 μm. 9 is a graph showing the result when the number of carriers attached to the photosensitive drum is measured. Note that the carrier adhesion number was measured by the same method as in Experimental Example 1 described above, and was measured by changing the toner concentration of the developer. As can be seen from the measurement results, it can be confirmed that the smaller the volume average particle diameter of the carrier, the easier the carrier adhesion occurs.
[0071]
[Comparative Example 2]
Next, an experiment (hereinafter, referred to as “Comparative Example 2”) performed to confirm that the granularity of an image is improved as the volume average particle diameter of the carrier decreases is described. In this experiment, a halftone dot image was formed with a developer using three types of carriers having a volume average particle diameter of 35 μm, 50 μm, and 65 μm, and the brightness of the image was changed. Then, the granularity was measured. This granularity indicates that the smaller the value, the better the dot reproducibility on the image.
[0072]
FIG. 17 is a graph showing experimental results in Comparative Example 2. As can be seen from this graph, the smaller the volume average particle size, the smaller the granularity of the image. Therefore, it was confirmed that the smaller the volume average particle size, the more improved the granularity of the image.
In particular, when the volume average particle size of the carrier is 50 [μm] or more, the granularity of the halftone dot image is 0.3 or more in the region where the brightness is 70 to 90. On the other hand, when the volume average particle size of the carrier is reduced to about 35 [μm], the granularity becomes 0.1, which is almost three times as large as the case where the volume average particle size of the carrier is 50 [μm] or more. The granularity is improved. For this reason, in recent years, it can be said that the diameter of the carrier is inevitable.
[0073]
The reason why the granularity is deteriorated when the volume average particle diameter of the carrier is increased as described above is considered as follows.
That is, as the volume average particle diameter of the carrier increases, the number of carriers constituting the tip portion of the magnetic brush in contact with the photosensitive drum 20 decreases, and the interval between adjacent carriers at the tip portion of the magnetic brush increases. Therefore, the magnetic brush rubs the electrostatic latent image on the photoconductive drum 20 with a coarse brush, and the amount of toner adhering to the electrostatic latent image varies greatly. As a result, an image having poor dot reproducibility and roughness in image quality is formed, and the granularity is deteriorated.
[0074]
Next, the developer carrier used in the first embodiment will be described.
FIG. 18 is a schematic diagram illustrating a cross-sectional view of the carrier used in the first embodiment.
As the carrier of the two-component developer, it is desirable to use a coat film having elasticity and strong adhesiveness, which is coated with a coat film containing particles having a diameter larger than the film thickness on the surface. Ferrite 91 is used as a core material of the carrier 90. This ferrite is 3 × 10⁶The magnetization intensity in a / 4π [A / m] magnetic field is 62 × 10^-7× 4π [Wb · m / kg]. The surface of the ferrite 91 is covered with a coat film 92 containing a charge controlling agent in a resin component obtained by crosslinking a thermoplastic resin such as acrylic resin and a melamine resin. This coat film 92 has elasticity and strong adhesive strength. Further, particles having a diameter larger than the thickness of the coat film 92, for example, alumina particles 93 are dispersed on the surface. The alumina particles 93 are held by the strong adhesive force of the coat film 92. While the conventional carrier has been constructed based on the idea of obtaining a long life while gradually scraping a hard coat film, the illustrated carrier 90 absorbs an impact due to the elasticity of the coat film 92 so that the film can absorb the impact. Reduce scraping. Further, by dispersing alumina particles 93 having a diameter larger than the film thickness on the surface of the carrier 90, it is possible to prevent the impact on the coat film 92 and clean the spent material. As described above, since the coating film can be prevented from being scraped and spent, the life can be further extended as compared with the conventional carrier. Thereby, stabilization of the toner pumping amount, that is, stabilization of quality can be expected over a long period of time.
[0075]
Further, an image having more excellent dot reproducibility can be formed by reducing the carrier particle size. Specifically, the size of the carrier particle size is desirably 20 [μm] or more and 50 [μm] or less. By setting the particle size of the carrier smaller than the conventional one and setting the particle size in such a range, the thickness of the magnetic brush at the time of image formation can be reduced uniformly. Therefore, more precise delivery of the toner can be achieved. Further, since the density of the developer spikes per unit area on the developing sleeve is increased, the toner can be transferred without any gap between the latent images on the photoconductor. As a result, an image having more excellent dot reproducibility can be formed. If the carrier particle size is larger than 50 [μm], the total surface area of the carrier becomes smaller and the toner holding amount becomes smaller when compared with the same carrier amount. For this reason, the toner density is reduced. Although the developing capacity can be maintained by increasing the pumping amount, toner sticking is likely to occur. On the other hand, if the carrier particle size is smaller than 20 [μm], the magnetic force holding force by the mag roller becomes small, causing carrier scattering and increasing the carrier adhesion to the photoreceptor. .
[0076]
Next, the developer toner used in the first embodiment will be described.
The method for producing the toner used in the first embodiment is not particularly limited, but a toner produced by a polymerization method is more preferable than a toner produced by a pulverization method. For reference, an example of a method for producing a toner by a polymerization method will be described.
[0077]
A series of flows of basic resin preparation → emulsification → aging → solvent removal → alkali washing, water washing → drying → external additive treatment is an outline of the manufacturing process. First, a basic resin solution is prepared by dissolving a resin, a coloring agent, a wax, a charge control agent, and the like in an organic solvent such as ethyl acetate. Then, the basic resin solution and the amines are added to an aqueous medium containing a surfactant, a viscosity modifier, and resin fine particles, and dispersed by a shear force to generate an emulsified state. Next, the reaction system is heated and aged to promote the elongation and crosslinking reaction due to the action of the isocyanate group and the amines. Then, for example, the entire system is gradually heated to remove the solvent by evaporating and removing the organic solvent in the droplets. Next, an alkali washing treatment or a water washing treatment is performed to remove foreign substances (such as a surfactant and a viscosity modifier) remaining on the surface of the toner particles obtained by removing the solvent. After collecting the toner particles by filtration, the toner particles are dried. Further, if necessary, external additive fine particles such as silica, titania, and alumina are externally added by a mixer or the like in an amount of about 0.1 to 5.0 parts by weight.
[0078]
A more detailed manufacturing example will be described below, for example.
First, 690 parts by weight of bisphenol A ethylene oxide 2 mol adduct and 256 parts by weight of terephthalic acid were placed in a reaction vessel provided with a cooling pipe, a stirrer, and a nitrogen introduction pipe under normal pressure and 230 [° C.] environment. For 8 hours. Then, the pressure was reduced to 10 to 15 [mmHg], the reaction was carried out for 5 hours, and after cooling to 160 [° C.], 18 parts by weight of phthalic anhydride was added, and the reaction was further carried out for 2 hours. Get)
[0079]
Aside from the polyester (a), a prepolymer having two or more reactive groups in one molecule of a base polymer is prepared. Specifically, 800 parts by weight of a bisphenol A ethylene oxide 2 mol adduct, 180 parts by weight of isophthalic acid, 60 parts by weight of terephthalic acid, and 2 parts by weight of dibutyltin oxide are put in a reaction vessel. After a polycondensation reaction for 8 hours under an environment of normal pressure and 230 [° C.], the reaction is carried out for 5 hours while decompressing to 10 to 15 [mmHg] and dehydrating. Next, after cooling to 160 [° C.], 32 parts by weight of phthalic anhydride are added and the reaction is further performed for 2 hours. Then, the mixture is cooled to 80 [° C.] and reacted with 170 parts by weight of isophorone diisocyanate in ethyl acetate for 2 hours to obtain an isocyanate group-containing prepolymer (1).
[0080]
Also, a ketimine compound is prepared. For example, 30 parts by weight of isophoronediamine and 70 parts by weight of methyl ethyl ketone are placed in a reaction vessel provided with a stirring bar and a thermometer, and reacted at 50 ° C. for 5 hours to obtain a ketimine compound (2).
[0081]
15.4 parts by weight of the isocyanate group-containing prepolymer (1), 60 parts by weight of the polyester (a), and 78.6 parts by weight of ethyl acetate are put in a beaker and dissolved by stirring. Then, 10 parts by weight of rice WAX (melting point: 83 ° C.) as a mold release accelerator and 4 parts by weight of copper phthalocyanine blue pigment (cyan pigment) were added, and the mixture was heated to 60 ° C. using a TK homomixer. At 12000 [rpm] to uniformly dissolve and disperse. Next, 2.7 parts by weight of the ketimine compound (2) is added and dissolved to obtain a basic resin solution (3).
[0082]
306 parts by weight of ion-exchanged water, 265 parts by weight of a 10% calcium phosphate suspension, 0.2 parts by weight of sodium dodecylbenzenesulfonate, and styrene / acrylic resin fine particles having an average particle size of 0.20 [μm] were beaked. Dissolve uniformly in the inside. Then, after the temperature is raised to 60 ° C., the above basic resin solution (3) is charged and stirred for 10 minutes while stirring at a speed of 12000 [rpm] using a TK homomixer. Next, 500 g of the obtained mixed solution was transferred to a kolben equipped with a stir bar and a thermometer, and the temperature was raised to 45 ° C .. Remove the solvent. After filtering, washing and drying steps, the particles are classified by wind power to obtain base particles (4).
[0083]
100 parts by weight of the base particles and 0.25 part by weight of a charge control agent (Bontron E-84 manufactured by Orient Chemical Co., Ltd.) are put into a Q-type mixer (manufactured by Mitsui Mining Co., Ltd.). Then, the peripheral speed of the turbine type blade of the mixer is set to 50 [m / sec], and two sets of mixing for two minutes and one set of pause for one minute are performed. Next, 0.5 parts by weight of hydrophobic silica (H2000, manufactured by Clariant Japan Co., Ltd.) was added, and then mixing was performed for 30 seconds at a peripheral speed of 15 [m / sec] and paused for 1 minute for 5 sets. Thus, toner particles are obtained. Then, 100 parts by weight of the toner particles, 0.5 parts by weight of hydrophobic silica, and 0.5 parts by weight of hydrophobic titanium oxide are mixed with a Henschel mixer to obtain a cyan toner. By changing 4 parts by weight of the copper phthalocyanine blue pigment (cyan pigment) in the composition of the above basic resin solution as follows, a toner of another color can be obtained.
Yellow toner: 6 parts by weight of benzidine yellow pigment
・ Magenta toner: 6 parts by weight of rhodamine lake pigment
・ Black toner: 10 parts by weight of carbon black
[0084]
Furthermore, it is important that the toner has a specific shape and shape distribution. The shape of the toner can be defined by the average circularity. The average circularity of the toner is a numerical value indicating how close the toner shape is to a true sphere. If the toner is a true sphere, the circularity is “1”. The average circularity was obtained by dividing the circumference of a circle having the same area as the area of the projected image of the toner by the perimeter of the projected image as the circularity of each toner. The average value of the circularity of each toner is obtained. The average circularity of the toner is desirably 0.95 or more and 0.99 or less. More preferably, particles having an average circularity of 0.96 or more and 0.99 or less and a circularity of less than 0.95 are 10% or less. According to the experiment, if the average circularity is less than 0.95, particularly less than 0.93, the toner becomes an irregular toner separated from a spherical shape, and it becomes impossible to obtain satisfactory transferability and high quality images without dust. . On the other hand, if the average circularity is greater than 0.99, in a system employing blade cleaning or the like, poor cleaning of the photoreceptor and the transfer belt occurs, causing stains on the image. When outputting an image having a low image area ratio, the amount of untransferred toner is small, and there is no problem of poor cleaning. However, for example, when an image having a high image area ratio such as a color photographic image is output, or when an image in an untransferred state remains on the photoconductor due to a paper feeding defect or the like, a cleaning defect is likely to occur. . If such cleaning failure occurs frequently, the charging roller for contact-charging the photoconductor is contaminated, and the original charging ability cannot be exhibited. Therefore, when the average circularity of the toner is 0.95 or more and 0.99 or less, a high-definition image with proper density and reproducibility can be formed without causing cleaning failure.
[0085]
Here, the average circularity of the toner is an average value of the circularity of each toner, and is measured by the following method. The circularity of each toner was measured using a flow particle image analyzer FPIA-2100 manufactured by SYSMEX CORPORATION. In this measurement, first, a 1% aqueous NaCl solution is prepared using primary sodium chloride. Thereafter, this NaCl aqueous solution was passed through a 0.45 filter to obtain a liquid of 50 to 100 [ml], and a surfactant, preferably an alkylbenzene sulfonate, of 0.1 to 5 [ml] was added thereto as a dispersant. Further, 1 to 10 [mg] of the sample is added. This is subjected to a dispersion treatment for 1 minute by an ultrasonic disperser, and the particle concentration is adjusted to 5000 to 15000 [particles / μl] to obtain a dispersion. This dispersion is imaged with a CCD camera, and the value obtained by dividing the circumference of a circle having the same area as the area of the two-dimensional projected image of the toner by the perimeter of the two-dimensional projected image of the toner is obtained. Used as a degree. Note that, from the accuracy of the pixels of the CCD, a toner having the same area as the area of the two-dimensional projected image of the toner and having a diameter (equivalent circle diameter) of 0.6 [μm] or more was regarded as effective. The average circularity of the toner is obtained by obtaining the circularity of each toner, adding all the circularities of all toners within the measurement range, and dividing the sum by the number of toners.
[0086]
Further, the volume average particle diameter (Dv) of the toner is 4 μm or more and 8 μm or less, and the ratio (Dv / Dn) of the volume average particle diameter Dv to the number average particle diameter Dn is 1.05 or more. It is preferably 1.30 or less. More preferably, the ratio of the volume average particle diameter (Dv) / the number average particle diameter (Dn) is preferably from 1.10 to 1.25. By using a toner having such a particle size, the particle size distribution of the toner is narrowed, and the following effects can be obtained.
That is, since a phenomenon such as selective development on the toner particle size surface hardly occurs, a stable image can be always formed. Here, the selective development refers to a phenomenon in which toner particles having a toner particle diameter corresponding to (suitable for) an image pattern are selectively developed. Further, when a toner recycling system is installed, small-sized toner particles that are difficult to transfer are quantitatively recycled in large quantities. However, since the particle size distribution of the toner is narrow from the beginning, the above-described effects are less likely to occur. Therefore, a stable image can be always formed from this point. Further, in the case of the two-component developer, even when the toner balance is performed for a long time, the fluctuation of the toner particle diameter in the developer is reduced, and good and stable developability is obtained even in the long-term stirring in the developing device. Can be
[0087]
In general, it is said that the smaller the particle size of the toner, the more advantageous it is to obtain a high-resolution and high-quality image, but it is disadvantageous for transferability and cleaning performance. is there. When the volume average particle diameter Dv of the toner is smaller than 4 [μm], in the case of a two-component developer, the toner fuses to the surface of the magnetic carrier in a long-term stirring in the developing device, and the charging ability of the magnetic carrier is reduced. Would. Further, these phenomena are the same in the toner having a fine powder content higher than the range of the first embodiment. Conversely, when the volume average particle diameter Dv of the toner is larger than 8 [μm], it becomes difficult to obtain a high-resolution image with high resolution, and when the toner in the developer is balanced. In many cases, the fluctuation of the particle diameter of the toner becomes large. It was also found that the same applies when the value of the volume average particle diameter Dv / the number average particle diameter Dn is larger than 1.30. When the value of (volume average particle diameter Dv) / (number average particle diameter Dn) is smaller than 1.05, there are some aspects which are preferable in terms of stabilizing the behavior of the toner and making the charge amount uniform. However, in this case, it is difficult to separate functions by the toner particle diameter, such as developing the thin line portion with small-sized particles and developing a solid image mainly with large-sized particles.
[0088]
The volume average particle diameter of the toner can be measured using, for example, a particle size distribution measuring device using a Coulter counter method. Examples of the measuring device include Coulter Counter TA-II and Coulter Multisizer II (both manufactured by Coulter Inc.). Hereinafter, a method for measuring the particle size of the toner using this measuring device will be described. First, 0.1 to 5 ml of a surfactant (preferably an alkylbenzene sulfonate) is added as a dispersant to 100 to 150 ml of the electrolytic aqueous solution. Here, the electrolyte is prepared by preparing an approximately 1% NaCl aqueous solution using primary sodium chloride, and for example, ISOTON-II (manufactured by Coulter Inc.) can be used. Here, 2 to 20 mg of the measurement sample is further added. The electrolytic solution in which the sample is suspended is subjected to dispersion treatment for about 1 to 3 minutes using an ultrasonic disperser, and the volume and number of toner particles or toner are measured using the measuring apparatus with a 100 [μm] aperture. Then, the volume distribution and the number distribution are calculated. From the obtained distribution, the volume average particle diameter (Dv) and the number average particle diameter (Dn) of the toner can be obtained. As channels, 2.00 to less than 2.52 [μm]; 2.52 to less than 3.17 [μm]; 3.17 to less than 4.00 [μm]; 4.00 to 5.04 [μm] Less than 5.04 to 6.35 [μm]; 6.35 to less than 8.00 [μm]; 8.00 to less than 10.08 [μm]; 10.08 to less than 12.70 [μm]; 12.70 to less than 16.00 [μm]; 16.00 to less than 20.20 [μm]; 20.20 to less than 25.40 [μm]; 25.40 to less than 32.00 [μm]; Thirteen channels having a particle diameter of from 00 to 40.30 [μm] were used, and particles having a particle diameter of 2.00 [μm] or more and less than 40.30 [μm] were targeted.
[0089]
[Embodiment 2]
Next, an embodiment in which the present invention is applied to a process cartridge that can be attached to and detached from a copying machine similar to that of the first embodiment (hereinafter, this embodiment is referred to as “second embodiment”) will be described.
FIG. 19 is a schematic configuration diagram illustrating an internal structure of the process cartridge according to the second embodiment. In the process cartridge 500, the image forming units 18Y, 18C, 18M, and 18K provided in the copying machine according to the first embodiment are integrally formed so as to be detachable from the copying machine main body. Specifically, the photosensitive drum 20, a charging device 60, a developing device 80, and a photosensitive member cleaning device 63 are provided. Since these configurations are the same as those in the first embodiment, description thereof will be omitted. If each of the image forming units 18Y, 18C, 18M, and 18K is configured as the process cartridge 500, the process cartridge is replaced when the life of the components contained in the image cartridge reaches the end of its life or when maintenance is required. And the convenience is improved. Note that the components integrally configured as a process cartridge may include any other components as long as at least the photosensitive drum 20 and the developing device 80 are included.
[0090]
As described above, the image forming apparatus according to the first embodiment includes the photosensitive drum 20 serving as the photosensitive drum 20 that moves while carrying the latent image on the surface, and a latent image forming unit that forms the latent image on the photosensitive drum 20. And an exposure device 21 as a device. Further, a developing device 80 as a developing unit that develops the latent image on the photosensitive drum 20 with a developer containing toner and a carrier that is a magnetic particle is also provided. Then, an image is formed on the transfer paper by displacing the toner image on the photosensitive drum 20 onto a transfer paper as a recording material. The developing device 80 includes a developing sleeve 81 as a developer carrying member that carries the developer on the surface and moves on the surface. The developing sleeve 81 has a part of the surface in the developing sleeve surface moving direction. It is exposed from the casing 84. By the developing device 80, the exposed surface of the developing sleeve 81 and the surface of the photosensitive drum 20 are opposed to each other in a developing region, and the main magnetic pole P <b> 1 b as a developing magnetic pole disposed so as to face the developing region is applied to the developing sleeve 81. And a magnetic brush is formed on the developing sleeve to develop the latent image on the photosensitive drum 20. The developing device 80 is provided with a downstream auxiliary as another magnetic pole from the main magnetic pole P1b to the downstream side in the developing sleeve surface moving direction so as to be opposite in polarity to the main magnetic pole P1b and adjacent to the main magnetic pole P1b. On the surface of the developing sleeve up to the magnetic pole P1c, the sum of the absolute value of the magnetic flux density Bz in the normal direction and the absolute value of the magnetic flux density By in the tangential direction monotonically increases. If the increase is monotonous, carrier adhesion can be effectively suppressed as described above.
Further, the developing device 80 according to the first embodiment moves the developing sleeve 81 in the developing area in the same direction as the surface moving direction of the photosensitive drum 20 and at a linear velocity higher than the linear velocity of the photosensitive drum surface. The magnetic brush on the sleeve is rubbed against the surface of the photosensitive drum 20 to perform development. In the developing device 80, the amount of the developer carried on the developing sleeve 81 and regulated to the developing area after being regulated by the doctor blade 83 as a developer regulating member is 65 [mg / cm].²] 95 [mg / cm]²] Are set as follows. In addition, since the attenuation rate of the magnetic flux density of the magnetic flux generated outside the developing sleeve surface in the developing area in the normal direction of the developing sleeve surface is 40% or more, the length of the magnetic brush is short, and The density of the brush portion in contact can be increased. Therefore, as described above, it is possible to improve the reproducibility of fine lines and suppress the trailing edge white spot phenomenon. The developing device having such a configuration is very useful in that the reproducibility of fine lines can be improved and the trailing edge white spot phenomenon can be suppressed, but there is a disadvantage that carrier adhesion easily occurs as described above. . However, this disadvantage can be sufficiently solved by configuring the sum of the absolute value of the normal magnetic flux density Bz and the absolute value of the tangential magnetic flux density By monotonically increasing as described above. . Therefore, according to the first embodiment, it is possible to improve the reproducibility of fine lines and suppress the trailing edge white spot phenomenon while sufficiently suppressing the carrier adhesion.
The developing device 80 sets a half-value point on the surface of the developing sleeve 81 which is half the maximum magnetic flux density in the normal direction of the surface generated on the surface of the developing sleeve 81 by the main magnetic pole P1b to the developing sleeve in the developing region. The angle width between the half-value points in the direction of movement of the developing sleeve surface when viewed from the central axis of curvature of the surface of the surface 81 is set to 25 [°] or less. Thereby, the same effect as when the attenuation rate is set to 40% or more can be obtained.
The maximum value Bm of the magnetic flux density in the normal direction of the surface of the developing sleeve generated on the surface of the developing sleeve by the main magnetic pole P1b.₁And the maximum value Bm of the magnetic flux density in the normal direction of the developing sleeve surface generated on the developing sleeve surface by the downstream auxiliary magnetic pole P1c.₂Is satisfied so as to satisfy the relational expression of the above Expression 7, the carrier adhesion can be more effectively suppressed as described above.
In the first embodiment, a carrier having a volume average particle diameter of 20 μm or more and 50 μm or less is used as a carrier in the developer. Therefore, as described above, an image having more excellent dot reproducibility can be formed. Such a developing device is very useful in that an image having excellent dot reproducibility can be formed, but has a drawback in that when such a small-diameter carrier is used, carrier adhesion easily occurs as described above. . However, this disadvantage can be sufficiently solved by configuring the sum of the absolute value of the normal magnetic flux density Bz and the absolute value of the tangential magnetic flux density By monotonically increasing as described above. . Therefore, according to the first embodiment, it is possible to form an image with excellent dot reproducibility while sufficiently suppressing carrier adhesion.
In particular, in the first embodiment, as the carrier 90 in the developer, a resin coat film 92 containing a resin component obtained by crosslinking a thermoplastic resin and a melamine resin and a charge control agent is applied to the magnetic core 91. The coated one is used. Therefore, as described above, the stabilization of the toner pumping amount, that is, the stabilization of the quality can be expected for a long period of time.
In the first embodiment, the developing cap Pg, which is the minimum distance between the surface of the photosensitive drum 20 and the surface of the developing sleeve 81, is set to 0.1 mm or more and 0.4 mm or less. By setting the developing cap Pg in such a narrow range, the image quality can be improved as described above. Further, the magnetic flux density generated by the magnet roller 85 attenuates as the distance from the surface of the developing sleeve 81 increases for all components. For this reason, as the height of the magnetic brush increases, the magnetic force of the carrier at the tip end of the magnetic brush toward the developing sleeve decreases. Therefore, by setting the developing cap Pg as narrow as not less than 0.1 [mm] and not more than 0.4 [mm], the height of the magnetic brush can be limited, which effectively works to suppress carrier adhesion.
Further, the process cartridge 500 according to the second embodiment is detachable from the main body of the copying machine according to the first embodiment, since at least the photosensitive drum 20 and the developing device 80 are integrally formed. , Convenience is improved.
[0091]
In the first and second embodiments described above, a so-called tandem-type color copying machine has been described. However, the present invention can be applied to an image forming apparatus having a different configuration. For example, the present invention can be applied to a monochrome image forming apparatus that directly transfers a toner image formed on a photosensitive drum to transfer paper. In the present embodiment, a copying machine has been described as an example of an image forming apparatus. However, the present invention can be applied to other image forming apparatuses such as a printer and a facsimile.
[0092]
【The invention's effect】
According to the first to tenth aspects, even if the carrier in the developer has a small diameter, it is possible to sufficiently suppress the carrier from adhering to the surface of the latent image carrier.
There is an excellent effect that it can be done.
[Brief description of the drawings]
FIG. 1 is a pie chart showing a distribution of a magnetic flux density on a developing sleeve surface generated by each magnetic pole of a magnet roller provided in a developing device of a copying machine according to a first embodiment.
FIG. 2 is a schematic configuration diagram of the entire copying machine.
FIG. 3 is an enlarged view showing a configuration of a main body of the copying machine.
FIG. 4 is an enlarged view illustrating a configuration of two adjacent image forming units.
FIG. 5 is a schematic configuration diagram showing the developing device.
FIG. 6 is a solid line showing the distribution of magnetic flux density in the normal direction of the surface of the developing sleeve of the developing device, and the normal direction at a position 1 mm away from the surface of the developing sleeve in the normal direction. A pie chart showing the magnetic flux density by wavy lines.
FIG. 7 is an explanatory diagram for explaining a half-value angular width in a developing sleeve surface moving direction as viewed from a central axis of curvature of the developing sleeve surface in a developing area.
FIG. 8 is an explanatory diagram showing an arrangement of a main magnetic pole and two auxiliary magnetic poles provided on a magnet roller in the developing device.
FIG. 9 is a pie chart showing the distribution of magnetic flux density on the developing sleeve surface of a comparative developing device using a main magnetic pole and an auxiliary magnetic pole made of a stronger magnet than the developing device of the first embodiment in Experimental Example 1.
FIG. 10 is a graph showing evaluation results of carrier adhesion in Experimental Example 1.
FIG. 11 is a pie chart showing the distribution of magnetic flux density on the surface of a developing sleeve of a comparative developing device using a main magnetic pole made of a stronger magnet than the developing device of the first embodiment in Experimental Example 2.
FIG. 12 is a graph showing evaluation results of carrier adhesion in Experimental Example 2.
FIG. 13 is a pie chart showing the distribution of magnetic flux density on the surface of a developing sleeve of a comparative developing device using a main magnetic pole made of a stronger magnet than the developing device of the first embodiment in Comparative Example 1.
FIG. 14 is a pie chart showing the distribution of the magnetic flux density on the surface of the developing sleeve of another comparative developing device using a main magnetic pole made of a stronger magnet than the developing device of the first embodiment in Comparative Example 1.
FIG. 15 is a graph showing evaluation results of carrier adhesion in Comparative Example 1.
FIG. 16 shows the results of measurement of the number of carriers attached to the photosensitive drum when an image was formed with a developer using three types of carriers having different volume average particle diameters while keeping the background potential constant in Comparative Example 1. The graph which shows the result of.
FIG. 17 is a graph showing evaluation results of carrier adhesion in Comparative Example 2.
FIG. 18 is a schematic view showing a cross-sectional view of a carrier used in the first embodiment.
FIG. 19 is a schematic configuration diagram illustrating an internal structure of a process cartridge according to a second embodiment.
FIGS. 20A to 20C are explanatory diagrams for explaining a mechanism in which image quality deterioration such as a trailing edge white spot occurs.
FIG. 21A is an explanatory diagram showing a magnetic force distribution in the vicinity of a developing area in a conventional developing device in which a developing magnetic pole includes one magnetic pole.
(B) is an explanatory view showing the shape of the magnetic brush made of the developer that has been raised by receiving magnetic force from the magnetic field formed by the developing magnetic pole when viewed from the axial direction of the developing sleeve.
FIG. 22A is an explanatory diagram showing a magnetic force distribution in the vicinity of a development region in an SLIC type developing device in which a development magnetic pole includes one main magnetic pole and two auxiliary magnetic poles.
FIG. 3B is an explanatory diagram illustrating a shape of a magnetic brush made of developer that has been raised by receiving magnetic force from a magnetic field formed by these three magnetic poles when viewed from the axial direction of the developing sleeve.
[Explanation of symbols]
10 Intermediate transfer belt
18Y, 18C, 18M, 18K image forming unit
20Y, 20C, 20M, 20K Photoconductor drum
21 Exposure equipment
60 Charging device
63 Photoconductor cleaning device
80Y, 80C, 80M, 80K developing device
81 Developing sleeve
83 Doctor Blade
85 magnet roller
90 career
91 core material
92 resin coat film
93 Alumina particles
500 process cartridge

Claims (10)

A portion of the surface of the developer carrier that moves and carries the developer containing the toner and the magnetic particles on the surface is exposed from the apparatus casing, and carries the latent image on the surface and moves on the surface. In the developing area where the surface of the latent image carrier and the exposed surface of the developer carrier face each other, the developer on the developer carrier is raised by a developing magnetic pole arranged to face the developing area. Forming a magnetic brush on the developer carrier, and developing a latent image on the latent image carrier,
On the developer carrier surface from the developing magnetic pole to another magnetic pole which has the opposite polarity to the developing magnetic pole and is located downstream of the developer carrier moving direction so as to be adjacent to the developing magnetic pole, A developing device characterized in that the sum of the absolute value of the magnetic flux density in the direction normal to the surface of the developer carrier and the absolute value of the magnetic flux density in the direction of movement of the surface of the developer carrier monotonically increases. The developing device according to claim 1,
The developer carrier is moved in the developing area in the same direction as the surface moving direction of the latent image carrier and at a linear velocity greater than the linear velocity of the surface of the latent image carrier, and the developer carrier is moved on the developer carrier. A magnetic brush is rubbed against the surface of the latent image carrier to perform development,
An amount of the developer carried on the developer carrier and conveyed to the developing area after being regulated by the developer regulating member is 65 [mg / cm ² ] or more and 95 [mg / cm ² ] or less;
A developing device, characterized in that a magnetic flux density attenuated in a direction normal to the surface of the developer carrier of the developer carrier in the developing region outside the surface of the developer carrier by the developing magnetic pole is 40% or more. The developing device according to claim 1,
The developer carrier is moved in the developing area in the same direction as the surface moving direction of the latent image carrier and at a linear velocity greater than the linear velocity of the surface of the latent image carrier, and the developer carrier is moved on the developer carrier. A magnetic brush is rubbed against the surface of the latent image carrier to perform development,
An amount of the developer carried on the developer carrier and conveyed to the developing area after being regulated by the developer regulating member is 65 [mg / cm ² ] or more and 95 [mg / cm ² ] or less;
The half-value point on the surface of the developer carrier, which is half the maximum magnetic flux density in the normal direction of the surface of the developer carrier generated on the surface of the developer carrier by the developing magnetic pole, A developing device, wherein an angle width between half-value points in a surface movement direction of the developer carrier when viewed from a central axis of curvature of the surface of the developer carrier is 25 ° or less. The developing device according to claim 1, 2 or 3,
Said by the developing magnetic pole maximum magnetic flux density in the normal line direction of the developer carrying member surface occurring to the developer carrying member surface and Bm _1, developer carrying member surface caused the developer carrying member surface by the other magnetic pole Wherein the maximum magnetic flux density in the normal direction is defined as Bm _2, and the following relational expression is satisfied.
100 [mT] ≦ Bm ₁ ≦ Bm ₂ ≦ 160 [mT] The developing device according to claim 1, 2, 3, or 4,
A developing device, wherein the magnetic particles have a volume average particle diameter of 20 μm or more and 50 μm or less. The developing device according to claim 5,
A developing device, wherein the magnetic particles are obtained by coating a magnetic material core with a resin coat film containing a resin component obtained by crosslinking a thermoplastic resin and a melamine resin and a charge controlling agent. . A latent image carrier that carries the latent image on the surface and moves on the surface,
Latent image forming means for forming a latent image on the latent image carrier,
Developing means for developing the latent image on the latent image carrier with a developer containing toner and magnetic particles,
An image forming apparatus that forms an image on a recording material by displacing a toner image on the latent image carrier on a recording material,
7. An image forming apparatus comprising the developing device according to claim 1, 2, 3, 4, 5 or 6. The image forming apparatus according to claim 7,
An image forming apparatus, wherein a minimum distance between the surface of the latent image carrier and the surface of the developer carrier is 0.1 mm or more and 0.4 mm or less. 9. A process cartridge detachably mountable to a main body of the image forming apparatus according to claim 7 or 8, wherein at least the latent image carrier and the developing unit are integrally formed. In a developer carrying member carrying a developer containing toner and magnetic particles on the surface to develop a latent image on a latent image carrying member carrying a latent image on the surface and moving on the surface,
Having a developing magnetic pole for forming a magnetic field for forming a magnetic brush by causing the developer to stand in a developing region opposed to the surface of the latent image carrier;
On the surface of the developer carrier from the developing magnetic pole to another magnetic pole which has a polarity opposite to that of the developing magnetic pole and is located downstream of the direction of movement of the surface of the developer carrier so as to be adjacent to the developing magnetic pole, A developer carrier characterized in that the sum of the absolute value of the magnetic flux density in the direction normal to the surface of the developer carrier and the absolute value of the magnetic flux density in the direction of movement of the surface of the developer carrier monotonically increases.

Images