JP2004037677A - Developing device and image forming apparatus having the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing device in which sufficient supply of developer to a plurality of developer carriers is possible by a simple structure without increasing its size and cost even in a development system having the developer carriers using a magnetic single-component development system, and to provide an image forming apparatus having the developing device. <P>SOLUTION: A magnetic pole A facing the second developer carrier 13 of the first developer carrier 12 and a magnetic pole C at a downstream side of the magnetic pole A in the direction of the rotation of the first developer carrier 12 are made the same in polarity. The magnetic pole B facing the first developer carrier 12 of the second developer carrier 13 is made different in polarity from the magnetic poles A and C. Magnetic force lines connecting the magnetic poles A and B and magnetic poles B and C are formed, thereby dividing the flow of developer into a flow in the direction toward the first developer carrier 12 and a flow in the direction toward the second developer carrier 13. <P>COPYRIGHT: (C)2004,JPO

Description

【００６０】
【表２】

【００６１】
また、図５及び図６に示す下流側の現像ローラ１３の内部に固定配置された上４極のマグネット１５は、同図において反時計回りに、下Ｎ２、図４においてＢ極に相当する下Ｎ１、下Ｓ２極を有すると共に表３に示すように、現像ローラ表面での磁力が７０〜１００ｍＴの磁場パターンを備えている。なお、表中の角度は各磁極の位置と下流側の現像ローラ１３の中心軸１３ａを結ぶ線と２つの現像ローラ１２，１３の中心軸１２ａ，１３ａを結ぶ線の交差角で表し、２つの現像ローラ１２，１３の中心軸１２ａ，１３ａを結ぶ線を基準（０°）、感光ドラム側を＋、現像容器内部側を−と定義する。
【００６２】
【表３】

【００６３】
ここで、本実施の形態においては、図６に示す上Ｎ２（磁極Ａ）、下Ｓ１（磁極Ｂ）及び上Ｎ３（磁極Ｃ）の位置関係を、上Ｎ２と上Ｎ３の位置と上流側の現像ローラ１２の中心軸１２ａをそれぞれ結ぶ線と、第１及び第２現像ローラ１２，１３の中心軸１２ａ，１３ａを結ぶ線の交差角をそれぞれａ，ｃ、下Ｓ１の位置と第２現像ローラ１３の中心軸１３ａを結ぶ線と、第１及び第２現像ローラ１２，１３の中心軸１２ａ，１３ａを結ぶ線の交差角をｂとし、交差角ａ，ｂ，ｃの関係がａ＞ｂ＞ｃとなるようにしている。
【００６４】
下記の表４は、図５に示す構成（上４極、下４極）と、図６に示す構成（上５極、下４極）の性能を比較したものである。ここでは、磁性１成分現像剤を用いた現像装置３の動作中の上流側の現像ローラ１２の現像剤乗り量、画像濃度（現像バイアス同設定）（Ｍａｃｂｅｔｈ　ＲＤ９１４を用いて測定）、文字再現性（画像濃度同濃になるよう現像バイアス調整時）、現像剤消費量（画像濃度同濃になるよう現像バイアス調整時）を比較した。なお、図７の（ａ）及び（ｂ）は、文字再現性の判定の基準となる画像を示している。
【００６５】
【表４】

【００６６】
ここで、この表４に示すように、（上４極、下４極）の場合は上流側の現像ローラ１２の現像剤担持量が５ｇ／ｃｍ^２と極端に少なくなっているの対し、（上５極、下４極）の場合、即ち交差角ａ，ｂ，ｃの関係がａ＞ｂ＞ｃとなるようにしている場合は、上流側の現像ローラ１２の現像剤担持量が１２ｇ／ｃｍ^２と（上４極、下４極）よりも増えている。
【００６７】
ここで、磁性１成分現像構成を使用した場合、通常の現像ローラ上のトナー乗り量は７〜１４ｇ／ｍ^２であり、本実施例の場合、通常の磁性１成分現構成を使用した場合の現像ローラ上の乗り量の範囲に納まっていることがわかる。なお、トナー乗り量が７ｇ／ｍ^２以下では現像バイアスを強化しても適切な画像濃度が得られず、また１４ｇ／ｍ^２以上では現像ローラ上のトナーコーティングが不均一になり、画像ムラが生じる。
【００６８】
次に、マグネット１４，１５の発する磁界と現像ローラの回転による搬送力による磁性１成分現像剤の現像容器内での流れ方を説明する。
【００６９】
（上５極、下４極）の構成においては、図６に示す攪拌部材１７，１８により現像剤ホッパー直下から上流及び下流側の現像ローラ近傍まで運ばれてきた現像剤は、まずは下流側の現像ローラ１３の回転による搬送力を受け磁極下Ｎ２から下Ｓ１の間の位置で下流側の現像ローラ表層に供給される。
【００７０】
そして、下流側の現像ローラ上のトナーは上流及び下流側の現像ローラ１２，１３の対向位置手前の、図４に示すＤ領域において磁極Ａと磁極Ｂとを結ぶ磁力線及び磁極Ｂと磁極Ｃとを結ぶ磁力線の２方向に延びる磁力線の束にそれぞれ現像剤がトラップされ、上流側の現像ローラ１２に供給される現像剤と下流側の現像ローラ１３に供給される現像剤に分配される。つまり、上流及び下流側の現像ローラ両方に現像剤が分配されるようになる。これにより、上側の現像ローラ１２の現像剤乗り量を確保することができる。
【００７１】
一方、（上４極、下４極）の構成では、図５に示す攪拌部材１７，１８により下流側の現像ローラ１３までは現像剤が供給されるが、２方向に延びる磁力線が形成されないため上流側の現像ローラ１４に現像剤を供給することができず、このため図８に示すように上流側の現像ローラ上の現像剤乗り量が極端に減少してしまうのである。
【００７２】
そして、このような上流側の現像ローラ１２にほとんど現像剤が担持されてない状態で、高速機対応の現像を行った場合、下流側の現像ローラ１本で現像している状態にほぼ等しいため画像濃度が得にくいことが表４から確認できる。
【００７３】
また、このような画像濃度が得にくい状態で現像バイアスを上げることにより強引に画像濃度を確保した場合は表４及び図７の（ｂ）に示すように画像の再現性が悪化する。これは、上流側の現像ローラ１２で通常の現像を行った後、下流側の現像ローラ１３で感光ドラム上のトナー像を再調整する効果が得られていないことに起因する。
【００７４】
ここで、下流側の現像ローラ１３のトナー像の再調整効果について詳しく説明する。上流及び下流側の現像ローラ１２，１３に十分な現像剤が担持されている場合は、上流側の現像ローラ１２で通常の現像が行われ、感光ドラム上にトナー像が形成される。そして、感光ドラム上のトナー像が下流側の現像ローラ１３を通過する際に潜像上にのり損ねたミストナー（かぶりトナー、文字画像近傍の飛び散りトナー）を回収しつつ、潜像上のトナーが乗るべきところにトナーが乗っていない、いわゆる抜け部分にトナーを補給する能力を有する。
【００７５】
なお、これはＤＣ＋ＡＣ現像バイアスが印加されていることが必須条件であり、ＡＣ成分による現像電界が現像ニップに印加されることにより、この部分でのトナーの往復運動が起きており、そのために潜像上に乗っていない感光ドラム１との静電的拘束力の弱いトナー（かぶり、飛び散り）は現像ローラ上に戻され、潜像上だがトナーが乗っていない、いわゆる抜け部分にはトナーが補給されるのである。従って、（上５極、下４極）の構成においては、（上４極、下４極）の構成や単一像ローラ構成よりも画像が良化する。
【００７６】
又、表４中の現像剤消費量の欄によると、（上４極、下４極）の構成よりも（上５極、下４極）の構成の方が同画像濃度を得るためのトナー消費量を低減することができることが判る。これは、上述した下流側の現像ローラ１３で感光ドラム上のトナー像を再調整する現象が現れたものである。
【００７７】
即ち、従来の一本現像ローラ方式や（上４極、下４極）の構成の上流側の現像ローラ１２のトナーが不十分である状態の現像の場合におこる、図９の（ａ）に示すようなトナーの上にトナーがのって無駄なトナー消費がおこなわれるような状態が、上流側の現像ローラ１２で通常現像、下流側の現像ローラ１３でトナーの上のトナーを剥ぎ取ることにより、図９の（ｂ）に示すような薄層高密度現像が可能となる効果が現れたものである。
【００７８】
このように、磁極Ａと磁極Ｃとを同極、磁極Ｂを磁極Ａ，Ｃと異極とすると共に、磁極Ａ、Ｂ，Ｃの交差角ａ，ｂ，ｃの関係をａ＞ｂ＞ｃとなるようにすることにより、磁極Ａと磁極Ｂ及び磁極Ｂと磁極Ｃとを結ぶ磁力線を形作り、現像剤の流れを上流側の現像ローラ１２へ向かう流れと、下流側の現像ローラ１３へ向かう流れに分割することができる。
【００７９】
これにより、磁性１成分現像方式を用いた少なくとも２つの現像ローラ１２，１３を有する現像方式においても、装置の大型化及びコスト増を招くことなく簡易な構成で２つの現像ローラ１２，１３への現像剤の充分な供給が可能となる。また、高速機における画像濃度確保、高画質、現像剤消費量低減を含む低ランニングコスト化を一挙に達成することができる。
【００８０】
ところで、画像形成装置が高速化すると、画像濃度確保のために現像ローラのトナー搬送量を増大させる必要が生じ、これに伴い現像ローラの回転速度は速くなる。そして、このように現像ローラの回転速度が速くなると、現像容器内での昇温が生じる。そこで、このような現像容器内での昇温を抑えるために、例えば現像ローラの直径を大径化するようにする。
【００８１】
次に、このような現像容器内での昇温を抑えるため現像ローラの直径を大径化するようにした本発明の第２の実施の形態について説明する。
【００８２】
図１０は本実施の形態に係る現像装置の要部拡大図である。なお、同図において、図５と同一符号は、同一又は相当部分を示している。
【００８３】
同図において、１２Ａ，１３Ａは直径が大径化された上流及び下流側の現像ローラである。なお、本実施の形態においては、画像形成装置のプロセススピードが４５０ｍｍ／ｓｅｃから６００ｍｍ／ｓｅｃに増大し、これと共に上流及び下流側の現像ローラ１２Ａ，１３Ａの直径は、それぞれφ２０からφ３０に大径化されている。
【００８４】
そして、このように現像ローラ１２Ａ，１３Ａの直径を大径化することにより、現像剤の搬送能力をＵＰさせることができると共に、現像ローラ１２Ａ，１３Ａの回転数を低減あるいは維持することができるので、現像容器内での昇温を抑えることができる。
【００８５】
ところで、このように現像ローラ１２Ａ，１３Ａを大径化した場合、既述した第１の実施の形態に係るマグネットパターン（磁極の角度と強さを規定したもの）を用いると、上流側の現像ローラ１２Ａの現像剤担持量が若干減少する方向に変化することが検討の結果、明らかになった。
【００８６】
これは、本実施の形態のように上流側の現像ローラ１２Ａが大径化すると、磁極Ｂと磁極Ｃの距離が極端に開いてしまい、このように磁極Ｂと磁極Ｃの距離が極端に開いてしまうと、磁極Ｂ及び磁極Ｃ間の磁界が弱まり上流側の現像ローラ１２Ａへの現像剤の受け渡し機能が低下し、このため上流側の現像ローラ１２Ａの現像剤担持量が低下したと予想される。
【００８７】
そこで、本実施の形態では、上流及び下流側の現像ローラ内のマグネット１４，１５のなかで現像剤を上流側の現像ローラ１２Ａへ導く役割を持つ磁極Ｃ（上Ｎ３極）について、その位置に関するラチチュード検討をおこなった。
【００８８】
具体的には、磁極Ｃの最適位置を調べるために、磁極Ｃの位置を変えて検討を行った。このとき既述した第１の実施の形態で説明した、磁極Ａ，Ｂ，Ｃの位置関係、即ち磁極Ａ，Ｂ，Ｃの交差角度ａ，ｂ，ｃの関係（ａ＞ｂ＞ｃ）を守るべく磁極Ｂの位置をｂが（ａ＋ｃ）／２となる位置として検討した。
【００８９】
なお、この検討の際、磁極Ａの位置をａ＝０°、磁力を７０ｍＴとし、磁極Ｂ及びＣの磁力も同様にそれぞれ、８０ｍＴ、６０ｍＴとした。下記の表５は、その結果を示すものである。
【００９０】
【表５】

【００９１】
そして、同表から明らかなように磁極Ｃの角度ｃが−１１°≧ｃ≧−７３°では上流側の現像ローラ１２Ａの現像剤担持量は磁性１成分現構成を使用した場合の通常の現像ローラ上の乗り量である７〜１４ｇ／ｍ^２内に収まる事がわかる。
【００９２】
しかし、磁極Ｃの角度が、−１１°より大きい場合においては、上流側の現像ローラ１２Ａの現像剤担持量が少なすぎることがわかる。これは、磁極Ａと磁極Ｃが近すぎるため、これら２極が１つの極のように振る舞い磁極Ｂから延びる磁力線を二つにわけることができないからである。よって、このような場合には、（上４極、下４極）の場合と同様に上流側の現像ローラ１２Ａへ現像剤を分配できなくなる。
【００９３】
ここで、磁極Ａと磁極Ｃが近すぎるということに関してより本質的な要素は、磁極Ａ−Ｃ間の交差角ではなく磁極Ａ−Ｃ間の距離（Ｄ_ＡＣ）であり、本検討より磁極Ａ−Ｃ間の距離（Ｄ_ＡＣ）を３ｍｍ以上にすることが上流側の現像ローラ１２Ａに現像剤を供給するためには重要であることがわかる。このことは、φ２０で行われた別の検討でも実証されている。
【００９４】
同様に同表から、磁極Ｃの角度が、−７３°以下でも上流側の現像ローラ１２Ａの現像剤担持量が少なすぎることがわかる。これは、磁極Ｂと磁極Ｃが遠すぎるため、この２極間の磁界が弱まり、上流側の現像ローラへ現像剤を供給できなくなっているからである。
【００９５】
ここで、磁極Ｂと磁極Ｃが遠すぎるということに関してより本質的な要素は、磁極Ｂ−Ｃ間の交差角ではなく磁極Ｂ−Ｃ間の距離（Ｄ_ＢＣ）であり、本検討より磁極Ｂ−Ｃ間の距離（Ｄ_ＢＣ）を１５ｍｍ以下にすることが上流側の現像ローラ１２Ａに現像剤を供給するために重要なことがわかる。
【００９６】
又、本実施の形態では、磁極Ｃの最適磁力についても合わせて検討した。表６は磁極Ｃの磁力を０〜１００ｍＴの間で変化させたときの、上流側の現像ローラ１２Ａの現像剤担持量を測定したものである。
【００９７】
ここでは、既述した第１の実施の形態とで定めるように、磁極Ａ，Ｂ，Ｃの交差角度ａ，ｂ，ｃの関係をａ＞ｂ＞ｃとし、磁極Ａ−Ｃ間及び、Ｂ−Ｃ間の距離をＤ_ＡＣ　＝８．２ｍｍ、Ｄ_ＢＣ　＝５．３ｍｍ（表５の左から３番目の構成）で検討をおこなった。下記の表６はその結果を示すものである。
【００９８】
【表６】

【００９９】
ここで、この表６により磁極Ｃの磁力が２０ｍＴ以下になると、上流側の現像ローラ１２Ａの現像剤担持量が極端に少なくなることがわかる。また、現像ローラ径φ２０のものを用いた同様の検討でも磁極Ｃの磁力が２０ｍＴ以下になると、極端に上流側の現像ローラ１２Ａの現像剤担持量が減少することが判る。これは、磁極Ｃの磁力が２０ｍＴ以下では、上流側の現像ローラ１２Ａが現像剤を担持する力よりも、重力による現像剤の落下が支配的となり、上流側の現像ローラ１２Ａに現像剤を搬送できないことを示している。
【０１００】
以上の二つの検討の結果を踏まえて、本実施の形態において、上流側の現像ローラ１２Ａのマグネット１４は、上流側の現像ローラ１２Ａの現像剤担持量を多く確保できるように、図１０において時計回りに上Ｎ１、上Ｓ２、Ｃ極に相当する上Ｎ３、Ａ極に相当する上Ｎ２、上Ｓ１極を有し、表７に示すように、磁力６０〜１００ｍＴの磁極を配置した。
【０１０１】
なお、Ｃ極の磁力に関しては、８０ｍＴとした方がトナー担持量は増えるがマグネットに希土類磁石を採用する必要があり、コストアップするため、本実施の形態では、Ｃ極磁力を６０ｍＴとしている。
【０１０２】
【表７】

【０１０３】
又、下流側の現像ローラ１３Ａのマグネット１５は図１０において反時計回りに下Ｎ２、Ｂ極に相当する下Ｓ１、下Ｎ１、下Ｓ２極を有し、表８に示すように、磁力７０〜１００ｍＴの磁極を配置した。
【０１０４】
【表８】

【０１０５】
また、下記の表９はφ３０現像ローラを用いる本実施の形態において、既述した第１の実施の形態のマグネットパターンを使用した系と表７、表８を用いた系の上現像ローラの現像剤担持量を比較して表したものである。
【０１０６】
【表９】

【０１０７】
そして、この表９から明らかなように、磁極Ａと磁極Ｃとの距離Ｄ_ＡＣ及び磁極Ｂと磁極Ｃとの距離Ｄ_ＢＣを、上流側の現像ローラ１２Ａへの現像剤の供給が可能となる距離、即ちＤ_ＡＣ≧３ｍｍ、かつＤ_ＢＣ≦１５ｍｍとすることにより、また磁極Ｃの磁力を上流側の現像ローラ１２Ａによる現像剤担持力が重力による現像剤の落下を防ぐような大きさとなるような大きさ、即ち２０ｍＴ以上とすることにより、現像ローラ１３Ａの径が大きい場合でも、現像剤の流れを上流側の現像ローラ１２Ａへ向かう流れと、下流側の現像ローラ１３Ａへ向かう流れに分割することができる。
【０１０８】
なお、本実施の形態では、磁極Ａと磁極Ｃとの距離Ｄ_ＡＣ及び磁極Ｂと磁極Ｃとの距離Ｄ_ＢＣをＤ_ＡＣ≧３ｍｍ、かつＤ_ＢＣ≦１５ｍｍとし、さらに磁極Ｃの磁力を２０ｍＴ以上としたが、現像剤の流れを分割できるならば、この２つの条件のうちどちらか一方、満足させるようにしても良い。
【０１０９】
さらに、これまでの説明においては、現像ローラを２本備えた現像装置について述べてきたが、本発明は、これに限られるものではなく、現像ローラを３本以上備えた現像装置についても適用することができるのは言うまでもない。
【０１１０】
【発明の効果】
以上説明したように本発明によれば、第１現像剤担持体の第２現像剤担持体に面する磁極Ａと、磁極Ａよりも第１現像剤担持体の回転方向下流側の磁極Ｃとを同極とし、第２現像剤担持体の第１現像剤担持体に面する磁極Ｂを磁極Ａ，Ｃと異極として磁極Ａと磁極Ｂ及び磁極Ｂと磁極Ｃとを結ぶ磁力線を形作ることにより、現像剤の流れを第１現像剤担持体へ向かう流れと、第２現像剤担持体へ向かう流れに分割するようにすることができる。
【０１１１】
これにより、磁性１成分現像方式を用いた複数の現像剤担持体を有する現像方式においても、大型化及びコスト増を招くことなく簡易な構成で複数の現像剤担持体への現像剤の充分な供給が可能となる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態に係る画像形成装置の一例であるデジタル複写機の画像形成部の概略構成を示す図。
【図２】上記デジタル複写機の画像形成部の現像装置の模式的縦断面図。
【図３】上記現像装置の上流及び下流側の現像ローラの構成を示す側面図。
【図４】上記上流及び下流側の現像ローラの磁極配置と磁力線の延び方を示す模式的縦断面図。
【図５】本実施の形態の比較例に係る（上４極、下４極）の構成を説明する断面図。
【図６】本実施の形態の実施例に係る（上５極、下４極）の構成を説明する断面図。
【図７】本実施の形態の文字再現性の判定の基準となる画像を示す図。
【図８】上記本実施の形態の比較例に係る複数の現像ローラを備えた磁性１成分現像方式で従来の単一現像ローラ用搬送部材を用いた場合の現像剤の流れ方を示す模式的縦断面図。
【図９】本実施の形態の感光ドラム上の現像後のトナーの乗り方を示す模式図。
【図１０】本発明の第２の実施の形態に係る現像装置の模式的縦断面図。
【図１１】従来の画像形成装置の画像形成部の構成を示す図。
【図１２】上記従来の画像形成部の複数の現像ローラを備えた２成分現像装置の２成分現像剤の流れ方を示す模式的縦断面図。
【図１３】上記従来の画像形成部の複数の現像ローラを備えた２成分現像装置に磁性１成分現像剤を適用した場合の現像剤の流れを示す模式的縦断面図。
【図１４】上記従来の画像形成部の複数の現像ローラを備えた磁性１成分現像装置で上流及び下流側の現像ローラの両方に現像剤を供給可能にするため、上下独立の搬送部材を有する現像装置の模式的縦断面図。
【符号の説明】
１　　　　　感光ドラム
３　　　　　現像装置
１１　　　　現像容器
１２　　　　上流側の現像ローラ
１２Ａ　　　上流側の現像ローラ
１３　　　　下流側の現像ローラ
１３Ａ　　　下流側の現像ローラ
１４　　　　（上流側の現像ローラの）マグネット
１５　　　　（下流側の現像ローラの）マグネット
Ａ　　　　　第１の磁極
Ｂ　　　　　対向磁極
Ｃ　　　　　第２の磁極[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a developing device and an image forming apparatus provided with the same, and more particularly to a device for visualizing an electrostatic image on an image carrier with a magnetic one-component developer.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine, a laser beam printer, a facsimile, and a printing apparatus using an electrophotographic method or an electrostatic recording method, after uniformly charging an image carrier surface, for example, a semiconductor Image exposure is performed by a laser or an LED to form an electrostatic latent image on the image carrier, and the electrostatic latent image is visualized as a developer image by a developing device. An image forming apparatus is known in which an image is transferred onto a transfer material and then fixed on a transfer material by a fixing device and output.
[0003]
FIG. 11 is a diagram showing a configuration of an image forming unit of such a conventional image forming apparatus. In FIG. 1, reference numeral 1A denotes a photosensitive drum which is an example of an image carrier. This photosensitive drum 1A has a photoconductive layer such as OPC or a-Si on its surface and rotates in the direction of the arrow.
[0004]
At the time of image formation, first, the surface of the photosensitive drum 1A is uniformly charged to, for example, -700 V by the primary charger 2, and then, according to image signal information, the photosensitive drum 1A is exposed by image exposure L using, for example, a semiconductor laser or an LED array. The surface potential of the drum 1A is attenuated to -200 V, and a latent image corresponding to the image signal of the image is formed on the photosensitive drum.
[0005]
Next, the latent image is developed by a developing device 3 using a one-component developer, and is visualized as a toner (developer) image. Here, the developing device 3 using a dry type one-component developer has a high durability and a long life because it does not require replacement of a carrier or the like, and performs development by jumping development using a magnetic one-component toner. Then, for example, a negatively charged black toner is used as a developer, and a DC bias of about -500 V is applied as a developing bias to the developing rollers 12 and 13 which are developer carrying members at the time of development, so that the latent image is developed in reverse. I do.
[0006]
Next, since a photosensitive drum 1A having a long durable life, such as a high-speed machine, has a large outer diameter and cannot separate the curvature of the transfer paper from the photosensitive drum 1A, a pre-transfer process is performed using the post-charging device 4. . The post-charging device 4 normally performs a pre-transfer treatment by applying a corona by DC or AC, or by combining light removal.
[0007]
Next, the toner image is transferred by the transfer charger 5 onto the transfer material P supplied to the photosensitive drum 1A. Thereafter, the transfer material P is separated from the photosensitive drum 1A by the separation charger 6 and sent to the fixing device 8, where the toner image is fixed. Finally, the transfer residual toner on the photosensitive drum is removed by the cleaning device 7 to prepare for the next image formation.
[0008]
By the way, in a conventional developing device, as described in Japanese Patent Application Laid-Open No. 03-204084, a developing device using a two-component magnetic brush is provided with a plurality of developing rollers in order to increase the speed of the image forming apparatus. As described in Japanese Patent Application Laid-Open No. 02-188778, a plurality of developing sleeves (developing rollers) are provided, and the distance between the developing sleeve and the photosensitive drum is set closer to the downstream developing sleeve so that the distance from the developing sleeve is reduced. There is a printer in which the toner supply amount is made uniform.
[0009]
Japanese Patent Publication No. 3-5579 discloses a developing device having a plurality of miniaturized developing rollers, and Japanese Patent Application Laid-Open No. 9-80919 and Registered Patent No. 3017514 discloses a developing device using a plurality of two-component developing methods. A configuration relating to the magnetic force of the roller has been proposed.
[0010]
[Problems to be solved by the invention]
However, in the developing method using a magnetic one-component developer that does not require replacement of a carrier, which can reduce the durability and can be simplified compared with a conventional developing method using a two-component developer, for example, In a developing device having a configuration in which the body is arranged in close proximity, although low running cost and space saving are possible, as in the case of using a two-component developer having a carrier, upstream and downstream developing rollers are not used. It was difficult to coat both with a sufficient developer, and none of them were actually commercialized or reported at academic conferences.
[0011]
This is for the following reason. That is, in the conventional two-component developing apparatus using a carrier, the upstream developing roller 12 is coated with the developer G supplied to the upstream developing roller 12 as shown in FIG. Most of the developed developer layer is formed by the magnetic force of the magnets 14 and 15 in the upstream and downstream developing rollers disposed near the opposing portions of the upstream and downstream developing rollers 12 and 13. 13, the coating is applied to the developing roller 13 on the downstream side, so that the so-called co-rotation of the developer G on the so-called upstream and downstream developing rollers 12, 13 is unlikely to occur.
[0012]
On the other hand, in a developing method in which a magnetic substance uses a magnetic one-component developer contained in the toner and supplies the developer only to the upstream developing roller, as shown in FIG. Since the reflection force with the developing rollers 12 and 13 is larger than the magnetic restraining force received from the developing roller 15, the developer is entrained on the upstream and downstream developing rollers 12 and 13. For this reason, the supply of the developing material to the developing roller 13 on the downstream side is insufficient, and the problem that the image density is low occurs.
[0013]
As described above, in the conventional two-component developing method, if the toner is supplied only to the upstream developing roller 12, the developer is supplied to the upstream and downstream developing rollers 12 and 13. In the case of the developing method using a one-component developer, a transport structure capable of supplying the developer to both the upstream and downstream developing rollers 12 and 13 is essential.
[0014]
That is, in the case of the developing method using the magnetic one-component developer, the toner can be transported to the vicinity of the developing roller by one transport mechanism, but the toner is transferred to both the upstream and downstream developing rollers 12 and 13. Could not be distributed and supplied.
[0015]
Incidentally, FIG. 14 shows an upper and lower independent transport mechanism so that the developer can be supplied to both the upstream and downstream developing rollers 12 and 13 by the magnetic one-component developing system configured while ignoring an increase in the cost of the developing device. This shows a configuration having 50 and 51, and in such a configuration,
1. Ensuring image density in high-speed machines
2. High image quality
3. Reduction of developer consumption
It was confirmed that the three advantages were born.
[0016]
Note that 1 is due to an increase in developability due to the two developing rollers 12 and 13. The second problem is due to the effect that after the normal development is performed by the upstream developing roller 12, the toner image on the photosensitive drum is readjusted by the downstream developing roller 13.
[0017]
In the case of (3), as a result of the readjustment of the image described in (2), the toner on the photosensitive drum is faithfully formed into a thin layer and uniform to the latent image. This prevents the state in which wasteful toner consumption occurs.
[0018]
However, in order to obtain such advantages, as described above, a transport member that supplies the developer to both the upstream and downstream developing rollers 12 and 13 is required, and if this is done, the developing device becomes complicated. At the same time, the size and cost of the device are increased. Further, when the developer is supplied only to the vicinity of the downstream developing roller, the supply of the toner of the upstream developing roller is insufficient. In the developing state, it was also difficult to obtain the above three advantages.
[0019]
Accordingly, the present invention has been made in view of such a situation, and even in a developing system having a plurality of developing rollers (developer bearing members) using a magnetic one-component developing system, the size and cost are increased. It is an object of the present invention to provide a developing device capable of sufficiently supplying a developer to a plurality of developing rollers (developer bearing members) with a simple configuration without inviting, and an image forming apparatus including the developing device.
[0020]
[Means for Solving the Problems]
The present invention is directed to a first developer carrier, which is rotatably opposed to a rotating image carrier, in a rotation direction upstream of the image carrier, and a second developer, which is downstream in a rotation direction of the image carrier. A developing device comprising a developer carrier for visualizing an electrostatic image formed on the image carrier by a magnetic one-component developer carried by the first and second developer carriers; A magnetic pole A of the first developer carrier facing the second developer carrier, and a magnetic pole of the first developer carrier located downstream of the magnetic pole A in the rotation direction of the first developer carrier. C, and a magnetic pole B of the second developer carrier facing the first developer carrier, wherein the magnetic pole A and the magnetic pole C are of the same polarity, and the magnetic pole B is defined by the magnetic poles A and C. Lines of magnetic force connecting the magnetic poles A and B and the magnetic poles B and C are formed as different poles, and the flow of the developer is changed to the first current. A flow directed to the carrier, it is characterized in that so as to divide the flow toward the second developer carrying member.
[0021]
Also, the present invention provides a method for controlling the positional relationship between the magnetic poles A, B, and C so as to divide the flow of the developer, a line connecting the position of the magnetic poles A, C and a central axis of the first developer carrying member, The intersection angles of the lines connecting the central axes of the first and second developer carriers are a and c, respectively, and the line connecting the position of the magnetic pole B and the central axis of the second developer carrier, The intersection angle of the line connecting the central axes of the second developer carrying member is set to b, and the line connecting the central axes of the first and second developer carrying members is set as a reference (0 °), and the image carrying from the reference is performed. When the body side is defined as + and the inner side is defined as −, the relationship between the intersection angles a, b, and c is such that a>b> c.
[0022]
Further, according to the present invention, the distance between the magnetic poles A and C and the distance between the magnetic poles B and C can be set so that the developer can be supplied to the first developer carrier so as to divide the flow of the developer. It is characterized by having a certain distance.
[0023]
Further, the present invention provides that the distance between the magnetic pole A and the magnetic pole C is D _AC , The distance between the magnetic pole B and the magnetic pole C is D _BC Then D _AC ≧ 3mm and D _BC ≦ 15 mm.
[0024]
Further, according to the present invention, the magnetic force of the magnetic pole C is divided so as to divide the flow of the developer so that the developer carrying force of the first developer carrier is large enough to prevent the developer from dropping due to gravity. This is a feature of the present invention.
[0025]
Further, the present invention is characterized in that the magnetic force of the magnetic pole C is 20 mT or more.
[0026]
Further, the present invention is characterized in that a bias of AC and DC lines is applied to the first and second developer carriers.
[0027]
According to another aspect of the invention, an image forming apparatus includes the developing device described above.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0029]
FIG. 1 is a diagram illustrating a schematic configuration of an image forming unit of a digital copying machine, which is an example of an image forming apparatus according to a first embodiment of the present invention. This digital copying machine has a process speed of 450 mm / sec and is a black-and-white digital copying machine with 85 sheets / min.
[0030]
In FIG. 1, reference numeral 1 denotes a photosensitive drum serving as an image bearing member. As the photosensitive drum 1, a highly durable a-Si photosensitive member having a diameter of 108 (mm) is used. Here, the a-Si photoreceptor has higher durability than the organic photoreceptor, and has a life of 3 million or more sheets and is suitable for a high-speed machine. Note that the present invention does not limit the photoreceptor to this. Further, the image carrier such as an electrophotoreceptor and a dielectric may be in a belt shape or a sheet shape in addition to the drum shape.
[0031]
When forming an image, first, the photosensitive drum 1 is driven to rotate in the direction of the arrow a, and the surface of the photosensitive drum 1 is uniformly charged to, for example, +400 V by the charger 2, and then the image exposure L is performed at 600 dpi. Done. Here, the image exposure L is a laser beam modulated by an image signal using a semiconductor laser as a light source. This laser beam is polarized by a polygon mirror rotating at a constant rotation speed by a motor, and is turned back through an imaging lens. After being reflected by the mirror, the photosensitive drum 1 is raster-scanned, and the surface potential of the exposed portion is attenuated to, for example, +100 V, thereby forming an electrostatic latent image. The wavelength of the image exposure L (laser beam) is 680 nm.
[0032]
Thereafter, development is performed by a developing device 3 using a magnetic one-component developer t, and a total current of -200 μA (AC + DC) is passed by a post charger 4 to charge a developer image, and the photosensitive drum 1 and the developer t are The adhesive force during the transfer to facilitate transfer and separation.
[0033]
Next, a developer image formed on the photosensitive drum 1 by being developed by the developing device 3 is transferred by the transfer charger 5 to the transfer material P traveling in the direction of the arrow, and thereafter, by the action of the separation charger 6 The transfer material P is separated and sent to the fixing device 8, and the unfixed toner image transferred by heating and pressing in the fixing device 8 is fixed on the transfer material P. After the transfer of the toner image, the transfer residual toner remaining on the surface of the photosensitive drum 1 is removed by the cleaning device 7, whereby the photosensitive drum 1 is repeatedly used for image formation.
[0034]
By the way, as shown in the figure, a plurality of developing devices 3 are provided to enable high-speed operation. In the present embodiment, two developing rollers 12 and 13 are used to cover the exposed area of the photosensitive drum 1 with latent images. Reversal development, which is a development method for adhering toner charged to the same polarity as the image and visualizing the toner, is performed.
[0035]
The developing device 3 is a highly durable, black magnetic one-component developer (toner) requiring no maintenance up to 2,000 k images, which is the life of the developing rollers 12 and 13 as a developer carrier. Is adopted.
[0036]
Here, the toner constituting the developer t has positive chargeability and magnetism with a weight average particle diameter of 7.3 μm, and contains a styrene-butadiene copolymer in a binder resin. In addition, a silicone resin, polyester or the like may be contained in a ratio smaller than the content of the binder resin component.
[0037]
Further, it is preferable to contain a triphenylmethane compound as a charge control agent and a wax to give the toner releasability. Examples of the colorant for the toner include, for example, carbon black, aniline black, and acetylene black as pigments.
[0038]
The magnetic material can also serve as a coloring agent. As such a magnetic material, iron oxide such as magnetite, maghemite, ferrite, or the like is used, and 90 parts by weight of a magnetic material is contained. Further, in order to improve charging stability, developability, fluidity and durability, it is preferable to add silica fine powder.
[0039]
The effect of this silica is to improve the fluidity, reduce friction by interposing between the toner particles and the sleeve, prevent the toner from aggregating, and replace the toner in contact with the toner in contact with the sleeve. There is to promote. Further, as another additive, for example, it is preferable to add a lubricant powder such as Teflon (registered trademark) powder, zinc stearate powder and polyvinylidene fluoride powder, and particularly a fluorine-containing polymer such as polyvinylidene fluoride. Here, the fluorine-containing polymer such as polyvinylidene fluoride has a function of reducing the release of silica adhering to the toner from the toner, and has the effect of increasing the charging stability.
[0040]
Alternatively, abrasives such as cerium oxide powder, silicon carbide powder, and strontium titanate powder, among which strontium titanate is preferred. Here, strontium titanate or the like plays the role of an abrasive for the drum, and as a result, has the effect of polishing and removing toner adhering to the drum in a thin layer.
[0041]
As shown in FIG. 2, the developing device 3 is provided in a developing container (main body of the developing device) 11 for accommodating toner and an opening formed at a position of the developing container 11 facing the photosensitive drum 1. An upstream developing roller 12 located upstream with respect to the rotation direction of the photosensitive drum 1, a downstream developing roller 13 located downstream, and a developer hopper 19 are disposed below the developer hopper 19 to transfer the developer t upstream and downstream. Agitating members 17, 18 and the like for transporting to the vicinity of the developing rollers 12, 13 on the side.
[0042]
Here, the upstream and downstream developing rollers 12 and 13 which are the first and second developer carriers are disposed close to each other and are provided on both side walls of the developing container 11 as shown in FIG. The shafts 12a and 13a of 12 and 13 are supported via bearings (not shown), and are thereby rotatable.
[0043]
When the toner carried to the photosensitive drum 1 rotating clockwise is carried out of the developing container 11, the upstream and downstream developing rollers 12 and 13 transfer the toner to the upstream and downstream sides. Are rotated in the rotation direction in which they are carried out from above the developing rollers 12 and 13, that is, counterclockwise.
[0044]
Here, the developing roller 12 on the upstream side is subjected to blast treatment with FGB # 300 (grain shape specified in JIS Z8801) on aluminum A2017 having an outer diameter of 20 mm, which is a non-magnetic member, and then phenol is applied to the surface thereof. A film in which a resin, crystalline graphite, and carbon were mixed at a ratio of 100: 36: 4 was formed. By forming such a film, a ghost image can be prevented and the durability of the surface of the developing roller 12 can be increased.
[0045]
The final surface roughness of the developing roller 12 subjected to such a process is Rz 3 μm. Here, a contact type surface roughness meter (Surfcoder SE-3300, Kosaka Laboratory Co., Ltd.) was used for the measurement of Rz. The measurement conditions are a cutoff value of 0.8 mm, a measurement length of 2.6 mm, a feed speed of 0.1 mm / sec, and a magnification of 5,000.
[0046]
The upstream developing roller 12 rotates at a peripheral speed of 150% of the peripheral speed of the photosensitive drum 1. Further, the amount of toner on the developing roller 12 is adjusted such that the layer thickness is regulated by a magnetic blade 16 which is a layer thickness regulating member provided close to the developing roller 12 at the opening of the developing container 11. Has become. Here, the distance S-Bgap between the developing roller 12 and the magnetic blade 16 is 230 μm, and the distance S-Dgap between the developing roller 12 and the photosensitive drum 1 is 215 μm.
[0047]
Further, a DC bias of +300 V and an AC bias of a rectangular wave of Vpp 1000 V and a frequency of 2.7 kHz are applied to the developing roller 12, thereby performing magnetic one-component non-contact development. I have. In this case, the developing contrast is 200 V in the flying direction of the toner, and the fogging contrast is 100 V.
[0048]
On the other hand, the downstream developing roller 13 has a film of about 10 μm having the same configuration as that of the upstream developing roller 12 formed on aluminum A2017 having an outer diameter of 20 mm, which is a non-magnetic member. The downstream developing roller 13 rotates at a peripheral speed of 150% of the peripheral speed of the photosensitive drum 1. Further, the thickness of the toner on the downstream developing roller 13 is regulated by the upstream roller 12. Here, the distance Ggap between the upstream developing roller 12 and the photosensitive drum 1 is 400 μm, and the distance S-Dgap between the downstream developing roller 13 and the photosensitive drum 1 is 215 μm.
[0049]
Like the developing roller 12 on the upstream side, the developing roller 13 on the downstream side also applies a DC bias of +300 V and an AC bias of Vpp 1000 V and a rectangular wave having a frequency of 2.7 kHz to perform magnetic one-component non-contact development. Is supposed to do it.
[0050]
By the way, in the present embodiment, the toner conveying force due to the rotation of the upstream and downstream developing rollers 12, 13 and the force received from the magnetic field formed by the magnetic poles of the magnets 14, 15 in the upstream and downstream developing rollers are used. Then, the toner conveyed to the vicinity of the downstream developing roller is once supplied to the downstream developing roller 13, and the toner is further distributed (divided) to the upstream developing roller 12 and the downstream developing roller 13. ing.
[0051]
Next, the magnetic pole arrangement of the magnets 14 and 15 fixedly arranged in the developing rollers 12 and 13 will be described. After that, the magnetic one-component developer is transferred by the magnetic field generated by the magnets 14 and 15 and the conveying force by the rotation of the developing roller. The flow in the developing container will be described.
[0052]
FIG. 4 is a diagram showing the magnetic pole arrangement of the magnets 14 and 15 fixedly arranged in the developing rollers 12 and 13. In FIG. 4, A faces the developing roller 13 on the downstream side of the developing roller 12 on the upstream side. The magnetic pole B is a magnetic pole facing the developing roller 12 on the upstream side of the developing roller 13 on the downstream side, and the magnetic pole B is fixedly disposed inside the developing container of the upstream developing roller 12 and is located upstream of the magnetic pole A. Is a magnetic pole adjacent to the magnetic pole A on the downstream side in the rotation direction of.
[0053]
Here, the magnetic pole C adjacent to the magnetic pole A has the same polarity, and the magnetic pole B has a different polarity from the magnetic poles A and C. The magnetic pole B is located between the magnetic poles A and C. With such a configuration, it is possible to form a bundle of magnetic lines of force extending in two directions, a magnetic line of force connecting the magnetic poles A and B and a magnetic line of force connecting the magnetic poles B and C.
[0054]
As a result, the toner conveyed to the vicinity of the downstream developing roller can be once supplied to the downstream developing roller 13, and the toner can be further distributed (divided) to the upstream developing roller 12 and the downstream developing roller 13. .
[0055]
Next, in order to confirm such an effect, in a conventional magnetic one-component developing system as shown in FIG. 5, a system in which a standard four-pole magnet is used for both the upstream and downstream developing rollers 12 and 13 (upper and lower). And four poles having the same poles A and C as the upstream magnets 14 according to the example of the present embodiment shown in FIG. A system having a 4-pole configuration (upper 5 poles, lower 4 poles) was compared and studied.
[0056]
The upper four-pole magnet 14 fixedly disposed inside the upstream-side developing roller 12 shown in FIG. 5 has an upper N1 and an upper S2 clockwise in FIG. , And a magnetic field pattern having a magnetic force on the surface of the developing roller of 80 to 100 mT as shown in Table 1 below.
[0057]
The angle in the table is represented by the intersection angle of a line connecting the position of each magnetic pole with the central axis 12a of the upstream developing roller 12 and a line connecting the central axes 12a and 13a of the two developing rollers 12, 13. A line connecting the central axes 12a and 13a of the developing rollers 12 and 13 is defined as a reference (0 °), the photosensitive drum side is defined as +, and the inner side of the developing container is defined as −. Further, in the figure, reference numeral 40 denotes a potential sensor.
[0058]
Also, in the case of the upper five-pole magnet 14 fixedly arranged inside the upstream developing roller 12 shown in FIG. 6, it corresponds to the upper N1, upper S2 in the clockwise direction in FIG. It has an upper N3, an upper N2 and an upper S1 pole corresponding to the A pole in FIG. 4, and as shown in Table 2, has a magnetic field pattern with a magnetic force on the developing roller surface of 60 to 100 mT.
[0059]
[Table 1]

[0060]
[Table 2]

[0061]
Further, the upper four-pole magnet 15 fixed and disposed inside the downstream developing roller 13 shown in FIGS. 5 and 6 has a lower N2 counterclockwise in FIG. As shown in Table 3, it has a magnetic field pattern of 70 to 100 mT on the surface of the developing roller. The angle in the table is represented by the intersection angle of the line connecting the position of each magnetic pole with the central axis 13a of the downstream developing roller 13 and the line connecting the central axes 12a, 13a of the two developing rollers 12, 13. A line connecting the central axes 12a and 13a of the developing rollers 12 and 13 is defined as a reference (0 °), the photosensitive drum side is defined as +, and the inner side of the developing container is defined as −.
[0062]
[Table 3]

[0063]
Here, in the present embodiment, the positional relationship between the upper N2 (magnetic pole A), the lower S1 (magnetic pole B) and the upper N3 (magnetic pole C) shown in FIG. Intersecting angles of a line connecting the central axis 12a of the developing roller 12 and a line connecting the central axes 12a and 13a of the first and second developing rollers 12 and 13 are a and c, respectively, and the position of the lower S1 and the second developing roller. The intersection angle between a line connecting the central axis 13a of the first and second developing rollers 12 and 13a and the line connecting the central axes 12a and 13a of the first and second developing rollers 12 and 13 is represented by b, and the relationship between the intersection angles a, b and c is a>b>. c.
[0064]
Table 4 below compares the performance of the configuration shown in FIG. 5 (upper 4-pole, lower 4-pole) with the configuration shown in FIG. 6 (upper 5-pole, lower 4-pole). Here, during the operation of the developing device 3 using the magnetic one-component developer, the amount of the developer loaded on the upstream developing roller 12, the image density (the same setting of the developing bias) (measured using Macbeth RD914), and the character reproducibility A comparison was made between the amount of developer consumed (when the developing bias was adjusted so that the image density was the same) and the amount of developer consumed (when the developing bias was adjusted so that the image density was the same). FIGS. 7A and 7B show images serving as references for determining the character reproducibility.
[0065]
[Table 4]

[0066]
Here, as shown in Table 4, in the case of (upper 4 poles, lower 4 poles), the developer carrying amount of the upstream developing roller 12 is 5 g / cm. ² On the other hand, in the case of (upper 5 poles, lower 4 poles), that is, in the case where the relationship between the intersection angles a, b, and c is such that a>b> c, the upstream side The developer carrying amount of the developing roller 12 is 12 g / cm. ² (Upper 4 poles, lower 4 poles).
[0067]
Here, when the magnetic one-component developing configuration is used, the amount of toner on the normal developing roller is 7-14 g / m. ² In the case of the present embodiment, it can be seen that the value falls within the range of the riding amount on the developing roller when the normal magnetic one-component current configuration is used. Note that the toner riding amount is 7 g / m ² Below, even if the developing bias is strengthened, an appropriate image density cannot be obtained, and 14 g / m ² Above, the toner coating on the developing roller becomes non-uniform, and image unevenness occurs.
[0068]
Next, the flow of the magnetic one-component developer in the developing container by the magnetic field generated by the magnets 14 and 15 and the conveying force by the rotation of the developing roller will be described.
[0069]
In the configuration (upper 5 poles, lower 4 poles), the developer transported from immediately below the developer hopper to the vicinity of the upstream and downstream developing rollers by the stirring members 17 and 18 shown in FIG. It receives the conveying force due to the rotation of the developing roller 13 and is supplied to the downstream surface of the developing roller at a position between the lower magnetic pole N2 and the lower magnetic pole S1.
[0070]
Then, the toner on the downstream-side developing roller has a line of magnetic force connecting the magnetic pole A and the magnetic pole B and the magnetic pole B and the magnetic pole C in a region D shown in FIG. The developer is trapped in a bundle of magnetic lines of force extending in two directions of magnetic lines of force, and is distributed to the developer supplied to the developing roller 12 on the upstream side and the developer supplied to the developing roller 13 on the downstream side. That is, the developer is distributed to both the upstream and downstream developing rollers. As a result, the amount of developer loaded on the upper developing roller 12 can be ensured.
[0071]
On the other hand, in the (upper 4-pole, lower 4-pole) configuration, the developer is supplied to the developing roller 13 on the downstream side by the stirring members 17 and 18 shown in FIG. 5, but the lines of magnetic force extending in two directions are not formed. The developer cannot be supplied to the upstream developing roller 14, so that the amount of developer loaded on the upstream developing roller is extremely reduced as shown in FIG.
[0072]
And, when the development corresponding to the high-speed machine is performed in such a state that the developer is hardly carried on the upstream development roller 12, it is almost equal to the state where the development is performed by one downstream development roller. Table 4 shows that it is difficult to obtain an image density.
[0073]
Further, when the image density is forcibly secured by raising the developing bias in a state where it is difficult to obtain the image density, the reproducibility of the image deteriorates as shown in Table 4 and FIG. 7B. This is because the effect of re-adjusting the toner image on the photosensitive drum with the downstream developing roller 13 after performing normal development with the upstream developing roller 12 has not been obtained.
[0074]
Here, the effect of readjusting the toner image on the downstream developing roller 13 will be described in detail. When sufficient developer is carried on the upstream and downstream developing rollers 12, 13, normal development is performed on the upstream developing roller 12, and a toner image is formed on the photosensitive drum. Then, while collecting the mistoners (fogging toner, toner scattered in the vicinity of the character image) that are missed on the latent image when the toner image on the photosensitive drum passes the developing roller 13 on the downstream side, the toner on the latent image is removed. It has the ability to replenish toner in so-called missing parts where toner is not on where it should ride.
[0075]
This is an essential condition that a DC + AC developing bias is applied. When a developing electric field due to the AC component is applied to the developing nip, reciprocating movement of toner occurs in this portion. The toner (fogging, scattering) having a weak electrostatic binding force with the photosensitive drum 1 not on the image is returned to the developing roller, and the toner is supplied to the so-called missing portion on the latent image but without the toner. It is done. Therefore, in the (upper 5 poles, lower 4 poles) configuration, the image is better than the (upper 4 poles, lower 4 poles) configuration or the single image roller configuration.
[0076]
According to the column of developer consumption in Table 4, the toner composition for obtaining the same image density is higher in the (upper 5 pole, lower 4 pole) configuration than in the (upper 4 pole, lower 4 pole) configuration. It can be seen that the consumption can be reduced. This is due to the phenomenon that the toner image on the photosensitive drum is readjusted by the downstream developing roller 13 described above.
[0077]
That is, FIG. 9A shows a case in which the developing is performed in a state where the toner of the upstream developing roller 12 of the conventional single developing roller system or the (upper 4 pole, lower 4 pole) configuration is insufficient. The state in which the toner is put on the toner and wasteful toner consumption occurs as shown in the figure is that the normal development is performed by the upstream developing roller 12 and the toner on the toner is peeled off by the downstream developing roller 13. As a result, the effect of enabling thin layer high-density development as shown in FIG.
[0078]
As described above, the magnetic pole A and the magnetic pole C have the same polarity, the magnetic pole B has the different magnetic poles from the magnetic poles A and C, and the relationship between the intersection angles a, b, and c of the magnetic poles A, B, and C is a>b> c. Thus, the lines of magnetic force connecting the magnetic poles A and B and the magnetic poles B and C are formed, and the flow of the developer is directed toward the upstream developing roller 12 and toward the downstream developing roller 13. Can be divided into streams.
[0079]
As a result, even in a developing system having at least two developing rollers 12 and 13 using the magnetic one-component developing system, it is possible to apply the simple configuration to the two developing rollers 12 and 13 without increasing the size and cost of the apparatus. Sufficient supply of the developer becomes possible. Further, it is possible to achieve at once a reduction in running cost including securing of image density, high image quality, and reduction of developer consumption in a high-speed machine.
[0080]
By the way, when the speed of the image forming apparatus is increased, it is necessary to increase the amount of toner transported by the developing roller in order to secure image density, and accordingly, the rotation speed of the developing roller increases. When the rotation speed of the developing roller increases, the temperature inside the developing container rises. Therefore, in order to suppress the temperature rise in the developing container, for example, the diameter of the developing roller is increased.
[0081]
Next, a description will be given of a second embodiment of the present invention in which the diameter of the developing roller is increased in order to suppress the temperature rise in the developing container.
[0082]
FIG. 10 is an enlarged view of a main part of the developing device according to the present embodiment. In the figure, the same reference numerals as those in FIG. 5 indicate the same or corresponding parts.
[0083]
In the figure, reference numerals 12A and 13A denote upstream and downstream developing rollers whose diameters are increased. In this embodiment, the process speed of the image forming apparatus is increased from 450 mm / sec to 600 mm / sec, and the diameters of the upstream and downstream developing rollers 12A and 13A are respectively increased from φ20 to φ30. Has been
[0084]
By increasing the diameters of the developing rollers 12A and 13A in this manner, the developer carrying capacity can be increased, and the rotation speed of the developing rollers 12A and 13A can be reduced or maintained. In addition, the temperature rise in the developing container can be suppressed.
[0085]
By the way, when the diameters of the developing rollers 12A and 13A are increased in this way, if the magnet pattern according to the first embodiment described above (the one defining the angle and strength of the magnetic pole) is used, the upstream developing As a result of investigation, it has been clarified that the developer carrying amount of the roller 12A changes in a slightly decreasing direction.
[0086]
This is because if the diameter of the upstream developing roller 12A increases as in this embodiment, the distance between the magnetic poles B and C becomes extremely wide, and thus the distance between the magnetic poles B and C becomes extremely wide. If this occurs, the magnetic field between the magnetic poles B and C is weakened, and the function of transferring the developer to the upstream developing roller 12A is reduced. Therefore, it is expected that the developer carrying amount of the upstream developing roller 12A is reduced. You.
[0087]
Therefore, in the present embodiment, the position of the magnetic pole C (upper N3 pole) having a role of guiding the developer to the upstream developing roller 12A among the magnets 14 and 15 in the upstream and downstream developing rollers is determined. A latitude study was conducted.
[0088]
Specifically, in order to investigate the optimal position of the magnetic pole C, the position of the magnetic pole C was changed and examined. At this time, the positional relationship between the magnetic poles A, B, and C, that is, the relationship between the intersection angles a, b, and c (a>b> c) of the magnetic poles A, B, and C, described in the first embodiment, is described. In order to protect, the position of the magnetic pole B was considered as a position where b becomes (a + c) / 2.
[0089]
In this study, the position of the magnetic pole A was set to 0 °, the magnetic force was set to 70 mT, and the magnetic forces of the magnetic poles B and C were similarly set to 80 mT and 60 mT, respectively. Table 5 below shows the results.
[0090]
[Table 5]

[0091]
As is clear from the table, when the angle c of the magnetic pole C is −11 ° ≧ c ≧ −73 °, the developer carrying amount of the upstream developing roller 12A is smaller than the normal developing amount when the magnetic one-component current configuration is used. 7 to 14 g / m, which is the riding amount on the roller ² You can see that it fits inside.
[0092]
However, when the angle of the magnetic pole C is larger than −11 °, the developer carrying amount of the upstream developing roller 12A is too small. This is because the magnetic pole A and the magnetic pole C are too close, and these two poles behave like a single pole, and the magnetic force lines extending from the magnetic pole B cannot be divided into two. Therefore, in such a case, the developer cannot be distributed to the upstream developing roller 12A as in the case of (upper 4 poles, lower 4 poles).
[0093]
Here, a more essential factor regarding the fact that the magnetic poles A and C are too close is not the intersection angle between the magnetic poles AC but the distance between the magnetic poles AC (D _AC ), The distance between the magnetic poles A and C (D _AC It is understood that it is important to supply the developer to the developing roller 12A on the upstream side by setting the value of (3) to 3 mm or more. This has been demonstrated in another study performed at φ20.
[0094]
Similarly, from the table, it can be seen that the developer carrying amount of the upstream developing roller 12A is too small even when the angle of the magnetic pole C is −73 ° or less. This is because the magnetic pole B and the magnetic pole C are too far apart, the magnetic field between the two poles is weakened, and the developer cannot be supplied to the upstream developing roller.
[0095]
Here, a more essential factor regarding that the magnetic poles B and C are too far apart is not the intersection angle between the magnetic poles BC but the distance between the magnetic poles BC (D _BC ), The distance between the magnetic poles B and C (D _BC ) Is 15 mm or less, which is important for supplying the developer to the upstream developing roller 12A.
[0096]
In the present embodiment, the optimum magnetic force of the magnetic pole C was also examined. Table 6 shows the measurement of the amount of developer carried on the upstream developing roller 12A when the magnetic force of the magnetic pole C was changed between 0 and 100 mT.
[0097]
Here, as defined in the first embodiment described above, the relationship between the crossing angles a, b, and c of the magnetic poles A, B, and C is a>b> c, and the distance between the magnetic poles A and C and B -Distance between C and D _AC = 8.2 mm, D _BC = 5.3 mm (third configuration from the left in Table 5). Table 6 below shows the results.
[0098]
[Table 6]

[0099]
Here, from Table 6, it can be seen that when the magnetic force of the magnetic pole C becomes 20 mT or less, the developer carrying amount of the upstream developing roller 12A becomes extremely small. Similar studies using a developing roller having a diameter of φ20 also show that when the magnetic force of the magnetic pole C is 20 mT or less, the amount of developer carried on the upstream developing roller 12A is extremely reduced. This is because when the magnetic force of the magnetic pole C is 20 mT or less, the fall of the developer due to gravity becomes more dominant than the force that the upstream developing roller 12A carries the developer, and the developer is transported to the upstream developing roller 12A. Indicates that it is not possible.
[0100]
Based on the results of the above two examinations, in the present embodiment, the magnet 14 of the upstream developing roller 12A is shown in FIG. 10 so that a large amount of developer carried on the upstream developing roller 12A can be secured. It has an upper N1, an upper S2, an upper N3 corresponding to the C pole, an upper N2 and an upper S1 pole corresponding to the A pole, and magnetic poles having a magnetic force of 60 to 100 mT as shown in Table 7.
[0101]
In addition, as for the magnetic force of the C pole, when the magnetic force is set to 80 mT, the amount of toner carried increases, but it is necessary to use a rare earth magnet as the magnet.
[0102]
[Table 7]

[0103]
Further, the magnet 15 of the downstream developing roller 13A has a lower S2, a lower N1, and a lower S2 corresponding to the lower N2 and the B pole in a counterclockwise direction in FIG. A magnetic pole of 100 mT was arranged.
[0104]
[Table 8]

[0105]
Table 9 below shows the development of the upper developing roller using the magnet pattern of the first embodiment and the system using Tables 7 and 8 in the present embodiment using the φ30 developing roller. This is a comparison between the amounts of supported agents.
[0106]
[Table 9]

[0107]
As is apparent from Table 9, the distance D between the magnetic pole A and the magnetic pole C is _AC And the distance D between the magnetic pole B and the magnetic pole C _BC Is the distance at which the developer can be supplied to the upstream developing roller 12A, that is, D _AC ≧ 3mm and D _BC By setting the magnetic force of the magnetic pole C to a size such that the developer carrying force by the upstream developing roller 12A prevents the developer from falling due to gravity, that is, 20 mT or more. Even when the diameter of the developing roller 13A is large, the flow of the developer can be divided into a flow toward the upstream developing roller 12A and a flow toward the downstream developing roller 13A.
[0108]
In this embodiment, the distance D between the magnetic pole A and the magnetic pole C is D. _AC And the distance D between the magnetic pole B and the magnetic pole C _BC To D _AC ≧ 3mm and D _BC ≤ 15 mm and the magnetic force of the magnetic pole C is set to 20 mT or more. If the flow of the developer can be divided, either one of the two conditions may be satisfied.
[0109]
Further, in the above description, a developing device having two developing rollers has been described. However, the present invention is not limited to this, and is also applicable to a developing device having three or more developing rollers. It goes without saying that you can do it.
[0110]
【The invention's effect】
As described above, according to the present invention, the magnetic pole A of the first developer carrier facing the second developer carrier, and the magnetic pole C downstream of the magnetic pole A in the rotation direction of the first developer carrier. And the magnetic pole B facing the first developer carrier of the second developer carrier is different from the magnetic poles A and C to form lines of magnetic force connecting the magnetic pole A and the magnetic pole B and the magnetic pole B and the magnetic pole C. Thereby, the flow of the developer can be divided into a flow toward the first developer carrier and a flow toward the second developer carrier.
[0111]
As a result, even in a developing system having a plurality of developer carriers using the magnetic one-component developing system, a sufficient amount of the developer can be sufficiently supplied to the plurality of developer carriers with a simple configuration without increasing the size and the cost. Supply becomes possible.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a schematic configuration of an image forming unit of a digital copying machine that is an example of an image forming apparatus according to a first embodiment of the present invention.
FIG. 2 is a schematic longitudinal sectional view of a developing device of an image forming section of the digital copying machine.
FIG. 3 is a side view showing a configuration of a developing roller on the upstream and downstream sides of the developing device.
FIG. 4 is a schematic longitudinal sectional view showing the arrangement of magnetic poles of the upstream and downstream developing rollers and how magnetic lines of force extend.
FIG. 5 is a cross-sectional view illustrating a configuration (upper four poles, lower four poles) according to a comparative example of the present embodiment.
FIG. 6 is a cross-sectional view illustrating a configuration (upper 5 poles, lower 4 poles) according to an example of the present embodiment.
FIG. 7 is a diagram illustrating an image serving as a reference for determining character reproducibility according to the embodiment;
FIG. 8 is a schematic diagram illustrating a flow of a developer when a conventional single developing roller transport member is used in a magnetic one-component developing system including a plurality of developing rollers according to a comparative example of the embodiment. Longitudinal section.
FIG. 9 is a schematic diagram showing how toner after development on the photosensitive drum according to the embodiment is applied;
FIG. 10 is a schematic longitudinal sectional view of a developing device according to a second embodiment of the present invention.
FIG. 11 is a diagram illustrating a configuration of an image forming unit of a conventional image forming apparatus.
FIG. 12 is a schematic vertical sectional view showing a flow of a two-component developer in a two-component developing device having a plurality of developing rollers of the conventional image forming section.
FIG. 13 is a schematic longitudinal sectional view showing a flow of a developer when a magnetic one-component developer is applied to a conventional two-component developing device having a plurality of developing rollers in an image forming unit.
FIG. 14 shows a conventional magnetic one-component developing apparatus having a plurality of developing rollers in the image forming section, which has a vertically independent conveying member so that the developer can be supplied to both the upstream and downstream developing rollers. FIG. 2 is a schematic longitudinal sectional view of the developing device.
[Explanation of symbols]
1 Photosensitive drum
3 Developing device
11 Developing container
12 Upstream developing roller
12A Upstream developing roller
13 Downstream developing roller
13A Downstream developing roller
14 Magnet (for upstream developing roller)
15 Magnet (for downstream developing roller)
A First magnetic pole
B Opposing magnetic pole
C Second magnetic pole

Claims (8)

A first developer carrier upstream of the image carrier in the rotational direction and a second developer carrier downstream of the image carrier in the rotational direction, the first developer carrier being rotatably arranged facing the rotating image carrier; A developing device for visualizing an electrostatic image formed on the image carrier by a magnetic one-component developer carried by the first and second developer carriers,
A magnetic pole A of the first developer carrier facing the second developer carrier,
A magnetic pole C of the first developer carrier, which is located downstream of the magnetic pole A in the rotation direction of the first developer carrier,
A magnetic pole B of the second developer carrier facing the first developer carrier,
With
The magnetic pole A and the magnetic pole C have the same polarity, and the magnetic pole B is different from the magnetic poles A and C to form magnetic force lines connecting the magnetic pole A and the magnetic pole B and the magnetic pole B and the magnetic pole C. A developing device, wherein the flow is divided into a flow toward the first developer carrier and a flow toward the second developer carrier. The positions of the magnetic poles A, B, and C are divided so as to divide the flow of the developer, the lines connecting the positions of the magnetic poles A and C and the central axis of the first developer carrying member, and the first and second lines. The intersection angles of the lines connecting the central axes of the developer carrying members are a and c, respectively, the line connecting the position of the magnetic pole B and the central axis of the second developer carrying member, and the first and second developer carrying members. The intersection angle of the line connecting the central axes of the bodies is b, the line connecting the central axes of the first and second developer carriers is a reference (0 °), and the image carrier side from the reference is +, 2. The developing device according to claim 1, wherein the relationship between the intersection angles a, b, and c satisfies a> b> c when the side is negative. The distance between the magnetic pole A and the magnetic pole C and the distance between the magnetic pole B and the magnetic pole C are set such that the developer can be supplied to the first developer carrier so as to divide the flow of the developer. The developing device according to claim 1, wherein: The distance between the magnetic pole A and the magnetic pole C is D _AC , and the distance between the magnetic pole B and the magnetic pole C is D _BC , wherein D _AC ≧ 3 mm and D _BC ≦ 15 mm. Developing device. The magnetic force of the magnetic pole C is set so that the magnetic force of the magnetic pole C is so large that the developer carrying force of the first developer carrying member prevents the developer from dropping due to gravity. The developing device according to claim 1, wherein: The developing device according to claim 5, wherein the magnetic force of the magnetic pole C is 20 mT or more. The developing device according to claim 1, wherein a bias of an alternating current and a direct current is applied to the first and second developer carriers. An image forming apparatus comprising the developing device according to claim 1.

Images