JP2004114377A - Inkjet recording device and ink used for the device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the trouble of bleeding of ink to a record medium, drying failure of the ink itself, cockling of the record medium by the ink, and the like and to enable recording of highly fine images by jetting superfine ink droplets. <P>SOLUTION: The inkjet recording device has a recording head 20 equipped with nozzles 21 for jetting the ink as droplets. A desired image is recorded to a base K by jetting ink droplets from the nozzles 21 to the base K. In the inkjet recording device, a droplet amount per one droplet of ink droplets jetted from the nozzles 21 is 1-400 fl, and an ink adhesion amount per one color to the base K is 0.2-5.6 ml/m<SP>2</SP>. <P>COPYRIGHT: (C)2004,JPO

Description

で表される流域において駆動することが好ましい。
ただし、γ：液体の表面張力、ε_０：真空の誘電率、ｒ：ノズル半径、ｈ：ノズル−基板間距離、ｋ：ノズル形状に依存する比例定数（１．５＜ｋ＜８．５）とする。
（５）上記各請求項の構成、上記（１）、（２）、（３）又は（４）の構成において、印加する任意波形電圧が１０００Ｖ以下であることが好ましい。
（６）上記各請求項の構成、上記（１）、（２）、（３）、（４）又は（５）の構成において、印加する任意波形電圧が５００Ｖ以下であることが好ましい。
基材を導電性または絶縁性の基材ホルダーに裁置ことが好ましい。
（７）上記各請求項の構成、上記（１）〜（６）いずれかの構成において、ノズルと基板との距離が５００［μｍ］以下とすることが、ノズル径を微細にした場合でも高い着弾精度を得ることができるので好ましい。
（８）上記各請求項の構成、上記（１）〜（７）いずれかの構成において、ノズル内の溶液に圧力を印加するように構成することが好ましい。
（９）上記各請求項の構成、上記（１）〜（８）いずれかの構成において、単一パルスによって吐出する場合、
【数２】

により決まる時定数τ以上のパルス幅Δｔを印加する構成としても良い。ただし、ε：流体の誘電率、σ：導電率とする。
【００４５】
【発明の実施の形態】
以下の実施形態で説明するインクジェット記録装置のノズル径（内部直径）は、３０［μｍ］以下であることが好ましく、さらに好ましくは２０［μｍ］未満、さらに好ましくは８［μｍ］以下、さらに好ましくは４［μｍ］以下とすることが好ましい。また、ノズル径は、０．２［μｍ］より大きいことが好ましい。
【００４６】
（インクジェット記録装置の全体構成）
以下、インクジェット記録装置について図１に基づいて説明する。
図１はインクジェット記録装置の概略構成図であり、このインクジェット記録装置１００は、所定の基材Ｋを支持するためのプラテン１０２と、このプラテン１０２の上方に配置された記録ヘッド２０と、プラテン１０２の基材Ｋの搬送方向上流側および下流側に配置された搬送ローラ１０３ａ，１０３ｂと、これら各搬送ローラ１０３ａ，１０３ｂに圧接されて基材Ｋを搬送ローラ１０３ａ，１０３ｂとの間で挟持する圧接ローラ１０４ａ，１０４ｂとを備えている。
【００４７】
（記録ヘッド及び記録ヘッドに係る部材の構成）
以下、記録ヘッド２０及び記録ヘッド２０に係る部材について図２及び図３に基づいて説明する。図２はノズル２１に沿った記録ヘッド２０及び記録ヘッド２０に係る部材の断面図であり、図３はインクの吐出動作とインクに印加される電圧との関係を示す説明図であって、図３（Ａ）は吐出を行わない状態であり、図３（Ｂ）は吐出状態を示す。
【００４８】
図１及び図２に示す通り、記録ヘッド２０には、帯電可能なインクのインク滴をその先端部から吐出する超微細径のノズル２１が設けられている。ノズル２１は、プラテン１０２に指向されている。記録ヘッド２０のノズル２１の下方には、対向電極２３がノズル２１に対向するように設けられている。対向電極２３は、プラテン１０２に一体的に形成されており、ノズル２１の先端部に対向する対向面を有すると共にその対向面でインク滴の着弾を受ける基材Ｋを支持する。また、記録ヘッド２０には、ノズル２１内の流路２２にインクを供給するインク供給手段と、ノズル２１内のインクに吐出電圧を印加する吐出電圧印加手段２５とが、つながれている。
【００４９】
なお、上記ノズル２１とインク供給手段の一部の構成と吐出電圧印加手段２５の一部の構成はノズルプレート２６により一体的に形成されている。
【００５０】
上記記録ヘッド２０は、図示しない駆動機構により、基材Ｋの搬送方向（図１中左方向）に対して直交する方向に走査自在とされた走査型の記録ヘッドであって、複数のノズル２１を備えている。本実施形態において、この記録ヘッド２０には、イエロー（Ｙ）、マゼンダ（Ｍ）、シアン（Ｃ）、ブラック（Ｋ）等の色材を含む各プロセスカラーインクがインク供給手段の後述するインクタンクから供給され、記録ヘッド２０は、各プロセスカラーインクを滴として各ノズル２１から吐出する。
【００５１】
（ノズル）
上記ノズル２１は、後述するノズルプレート２６の下面層２６ｃと共に一体的に形成されており、当該ノズルプレート２６の平板面上から垂直に立設されている。さらに、ノズル２１にはその先端部からその中心線に沿って貫通するノズル内流路２２が形成されている。
【００５２】
ノズル２１についてさらに詳説する。ノズル２１は、前述の通り、超微細径で形成されている。詳しくは、ノズル２１の先端部における内径は、３０μｍ以下であるが、２０μ未満であってもよいし、８μｍ以下であってもよいし、４μｍ以下であってもよい。具体的な各部の寸法の一例を挙げると、ノズル内流路２２の内部直径は、１［μｍ］、ノズル２１の先端部における外部直径は２［μｍ］、ノズル２１の根元の直径は５［μｍ］、ノズル２１の高さは１００［μｍ］に設定されており、その形状は限りなく円錐形に近い円錐台形に形成されている。また、ノズル２１はその全体がノズルプレート２６の下面層２６ｃと共に絶縁性の樹脂材により形成されている。
【００５３】
なお、ノズルの各寸法は上記一例に限定されるものではない。特にノズル内径については、後述する電界集中の効果によりインク滴の吐出を可能とする吐出電圧が１０００［Ｖ］未満を実現する範囲であって、例えば、ノズル直径７０［μｍ］以下であり、より望ましくは、直径２０［μｍ］以下であって、現行のノズル形成技術によりインクを通す貫通穴を形成することが実現可能な範囲である直径をその下限値とする。
【００５４】
（インク供給手段）
インク供給手段は、ノズルプレート２６の内部であってノズル２１の根元となる位置に設けられると共にノズル内流路２２に連通するインク室２４と、図示しない外部のインクタンクからインク室２４にインクを導くインク供給路２７と、インク室２４へのインクの供給圧力を付与する図示しない供給ポンプとを備えている。
上記インクタンクには、Ｙ、Ｍ、Ｃ、Ａの各プロセスカラーインクが貯留されており、上記供給ポンプは、ノズル２１の先端部まで各プロセスカラーインクを供給し、当該先端部からこぼれ出さない範囲の供給圧力を維持しながら、記録ヘッド２０に対して各プロセスカラーインクの供給を行う（図３（Ａ）参照）。
【００５５】
（吐出電圧印加手段）
吐出電圧印加手段２５は、ノズルプレート２６の内部であってインク室２４とノズル内流路２２との境界位置に設けられた吐出電圧印加用の吐出電極２８と、この吐出電極２８に常時直流のバイアス電圧を印加するバイアス電源３０と、吐出電極２８にバイアス電圧に重畳して吐出に要する電位とするパルス電圧を印加する吐出電圧電源２９と、を備えている。
【００５６】
上記吐出電極２８は、インク室２４内部においてインクに直接接触し、インクを帯電させると共に吐出電圧を印加する。
バイアス電源３０によるバイアス電圧は、インクの吐出が行われない範囲で常時電圧印加を行うことにより、吐出時に印加すべき電圧の幅を予め低減し、これによる吐出時の反応性の向上を図っている。
【００５７】
吐出電圧電源２９は、インクの吐出を行う際にのみパルス電圧をバイアス電圧に重畳させて印加する。このときの重畳電圧Ｖは次式の条件を満たすようにパルス電圧の値が設定されている。
【数３】

但し、γ：インクの表面張力、ε_０：真空の誘電率、ｒ：ノズル半径、ｋ：ノズル形状に依存する比例定数（１．５＜ｋ＜８．５）とする。
一例を挙げると、バイアス電圧はＤＣ３００［Ｖ］で印加され、パルス電圧は１００［Ｖ］で印される。従って、吐出の際の重畳電圧は４００［Ｖ］となる。
【００５８】
（ノズルプレート）
ノズルプレート２６は、図２において最も上層に位置するベース層２６ａと、その下に位置するインクの供給路を形成する流路層２６ｂと、この流路層２６ｂのさらに下に形成される下面層２６ｃとを備え、流路層２６ｂと上面層２６ｃとの間には前述した吐出電極２８が介挿されている。
【００５９】
上記ベース層２６ａは、シリコン基板或いは絶縁性の高い樹脂又はセラミックにより形成され、その上に溶解可能な樹脂層を形成すると共に供給路２７及びインク室２４のパターンに従う部分のみを残して除去し、除去された部分に絶縁樹脂層を形成する。この絶縁樹脂層が流路層２６ｂとなる。そして、この絶縁樹脂層の下面に導電素材（例えばＮｉＰ）のメッキにより吐出電極２８を形成し、さらにその下から絶縁性のレジスト樹脂層を形成する。このレジスト樹脂層が下面層２６ｃとなるので、この樹脂層はノズル２１の高さを考慮した厚みで形成される。そして、この絶縁性のレジスト樹脂層を電子ビーム法やフェムト秒レーザにより露光し、ノズル形状を形成する。ノズル内流路２２もレーザ加工により形成される。そして、供給路２７及びインク室２４のパターンに従う溶解可能な樹脂層を除去し、これら供給路２７及びインク室２４が開通してノズルプレートが完成する。
【００６０】
（対向電極）
対向電極２３は、ノズル２１に垂直な対向面を備えており、かかる対向面に沿うように基材Ｋの支持を行う。ノズル２１の先端部から対向電極２３の対向面までの距離は、一例としては１００［μｍ］に設定される。
また、この対向電極２３は接地されているため、常時接地電位を維持している。従って、パルス電圧の印加時にはノズル２１の先端部と対向面との間に生じる電界による静電力により吐出されたインク滴を対向電極２３側に誘導する。
【００６１】
なお、インクジェット記録装置２０は、ノズル２１の超微細化による当該ノズル２１の先端部での電界集中により電界強度を高めることでインク滴の吐出を行うことから、対向電極２３による誘導がなくともインク滴の吐出を行うことは可能ではあるが、ノズル２１と対向電極２３との間での静電力による誘導が行われた方が望ましい。また、帯電したインク滴の電荷を対向電極２３の接地により逃がすことも可能である。
【００６２】
（インク）
インクとしては、通常のインクジェット記録装置に用いられるタイプのインク（具体的には、水性インク、油性インク、溶剤インク、紫外線硬化型インク、ソリッドインク等）が適用可能である。インクは、色材の濃度が８〜９０ｗｔ％であり、直径０．３μｍ以上の粒子を含まないとすることが好ましい。
【００６３】
さらに詳しく説明すると、インクとしては、粘度が０．１〜１０００ｍＰａ・ｓ（好ましくは、１〜１００ｍＰａ・ｓ）であり、表面張力が２０〜７０ｍＮ／ｍ（好ましくは、２５〜５０ｍＮ／ｍ）であるインクが適用可能である。インクの粘度が０．１ｍＰａ・ｓ未満又は１０００ｍＰａ・ｓよりも大きい場合には、ノズル２１からのインクの吐出が不安定なものとなる。また、インクの表面張力が２０ｍＮ／ｍ未満である場合には、ノズル２１から吐出されたインク滴が基材Ｋに滲みやすい。インクの表面張力が７０ｍＮ／ｍよりも大きい場合には、ノズル２１から吐出されたインク滴により基材Ｋの各画素を完全に埋めることができず、基材Ｋに対する一色当たりのインク付着量を多くしなければならない。結局、この場合、基材へのインクの滲み、インク自体の乾燥不良、基材に付着したインクによる基材のコックリング等の従来からの弊害を抑制できない。
【００６４】
なお、インクとして紫外線硬化型インクを適用する場合には、基材Ｋに着弾したインク滴に紫外線を照射できる位置（例えば、ノズル２１近傍）に紫外線照射手段を設け、基材Ｋに着弾した各インク滴に紫外線を照射させてこれら各インク滴を硬化させる必要がある。紫外線照射手段の「光源」としては、低圧水銀ランプ、紫外線レーザー、キセノンフラッシュランプ、捕虫灯、ブラックライト、殺菌灯、冷陰極管、ＬＥＤ（Ｌｉｇｈｔ　Ｅｍｉｔｔｉｎｇ　Ｄｉｏｄｅ）、ＬＥＤ高圧水銀ランプ、メタルハライドランプ、エキシマーランプ、無電極紫外線ランプ等が適用可能であり、これら以外の光源を適用してもよい。
【００６５】
（基材）
基材Ｋとしては、アート紙、コート紙、上質紙等の各種紙、各種布地、各種不織布が適用可能であって、さらに樹脂、金属、ガラス等の材質からなるものも適用可能である。基材Ｋの形態としては、ロール状、カットシート状、板状等のように記録面が二次元的に広がりを有していてもよいし、さらに円筒状、円錐状、多角状等のように記録面が三次元的に広がりを有していてもよい。
【００６６】
（インクジェット記録装置による画像記録動作）
次に、図１によりインクジェット記録装置１００の動作説明を行う。
まず、搬送ローラ１０３ａを回転駆動させることにより、所定の基材Ｋを搬送ローラ１０３ａと圧接ローラ１０４ａとにより挟持しながらプラテン１０２に搬送する。
そして、基材Ｋをプラテン１０２により支持した状態で搬送ローラ１０３ａと圧接ローラ１０４ａとにより基材Ｋを間欠的に搬送するとともに、基材Ｋの搬送方向に直交する方向に走査型の記録ヘッド２０を走査しながら記録ヘッド２０から各プロセスカラーインクを吐出させて、基材Ｋに所望の画像を記録する。画像記録後の基材Ｋは、搬送ローラ１０３ｂと圧接ローラ１０４ｂとにより挟持されてプラテン１０２から取り除かれる。
【００６７】
（記録ヘッドによる微小インク滴の吐出動作）
図２及び図３により記録ヘッド２０の動作説明を行う。
インク供給手段の供給ポンプによりノズル内流路２２にはインクが供給された状態にあり、かかる状態でバイアス電源３０により吐出電極２８を介してバイアス電圧がインクに印加されている。かかる状態で、インクは帯電すると共に、ノズル２１の先端部においてインクによる凹状に窪んだメニスカスが形成される（図３（Ａ））。
そして、吐出電圧電源２９によりパルス電圧が印加されると、ノズル２１の先端部では集中された電界の電界強度による静電力によりインクがノズル２１の先端側に誘導され、外部に突出した凸状メニスカスが形成されると共に、かかる凸状メニスカスの頂点により電界が集中し、ついにはインクの表面張力に抗して微小インク滴が対向電極２３側に吐出される（図３（Ｂ））。
【００６８】
この場合、ノズル２１からは、一滴当たり１〜４００ｆｌの滴量のインク滴が吐出され、また、ノズル２１から吐出されて基材Ｋに着弾したインク滴の一色当たりのインク付着量は、０．２〜５．６ｍｌ／ｍ^２となっている。
【００６９】
以上のようなインクジェット記録装置１００では、記録ヘッド２０は、微小径のノズル２１によりインク滴の吐出を行うので、ノズル内流路２２内で帯電した状態のインクにより電界が集中され、電界強度が高められる。このため、電界の集中化が行われない構造のノズル（例えば内径１００［μｍ］）では吐出に要する電圧が高くなり過ぎて事実上吐出不可能とされていた微細径でのノズルによるインクの吐出をより低電圧で行うことが出来るようになっている。
【００７０】
また、インクジェット記録装置１００では、微細径であるがために、ノズルコンダクタンスの低さによりその単位時間あたりの吐出流量を低減する制御を容易に行うことができると共に、パルス幅を狭めることなく十分に小さなインク滴（一滴当たり１〜４００ｆｌの滴量のインク滴）によるインクの吐出を実現している。
【００７１】
さらに、吐出されるインク滴は帯電されているので、微小のインク滴であっても蒸気圧が低減され、蒸発を抑制することからインク滴の質量の損失を低減し、飛翔の安定化を図り、インク滴の着弾精度の低下を防止している。
【００７２】
そして、これらのことより、上記の通りに、ノズル２１から吐出されるインク滴の一滴当たりの滴量を１〜４００ｆｌとすることができ、また、ノズル２１から吐出されて基材Ｋに着弾したインク滴の一色当たりの付着量を０．２〜５．６ｍｌ／ｍ^２に抑えることができるので、基材Ｋに付着したインク滴の各ドット径を大幅に減少させることができる。従って、従来において課題とされていた、基材Ｋへのインクの滲み、インク自体の乾燥不良、基材Ｋに付着したインクによる基材Ｋのコックリング等の弊害を抑制でき、ひいては複数の超微細なドットによる高精細な画像を基材Ｋに対して記録することができる。また、この場合、基材Ｋには超微細なドットが形成されるから、ハイライト部及び混色部に対して、滑らかな階調表現を行うこともできるようになる。
【００７３】
なお、上記実施形態において、ノズル２１にエレクトロウェッティング効果を得るために、ノズル２１の外周に電極を設けるか、また或いは、ノズル内流路２２の内面に電極を設け、その上から絶縁膜で被覆してもよい。そして、この電極に電圧を印加することで、吐出電極２８により電圧が印加されているインクに対して、エレクトロウェッティング効果によりノズル内流路２２の内面のぬれ性を高めることができ、ノズル内流路２２へのインクの供給を円滑に行うことができ、良好に吐出を行うと共に、吐出の応答性の向上を図ることが可能となる。
【００７４】
また、上記実施形態では、吐出電圧印加手段２５ではバイアス電圧を常時印加すると共にパルス電圧をトリガーとしてインク滴の吐出を行っているが、吐出に要する振幅で常時交流又は連続する矩形波を印加すると共にその周波数の高低を切り替えることで吐出を行う構成としてもよい。インク滴の吐出を行うためにはインクの帯電が必須であり、インクの帯電する速度を上回る周波数で吐出電圧を印加していても吐出が行われず、インクの帯電が十分に図れる周波数に替えると吐出が行われる。従って、吐出を行わないときには吐出可能な周波数より大きな周波数で吐出電圧を印加し、吐出を行う場合にのみ吐出可能な周波数帯域まで周波数を低減させる制御を行うことで、インクの吐出を制御することが可能となる。かかる場合、インクに印加される電位自体に変化はないので、より時間応答性を向上させると共に、これによりインク滴の着弾精度を向上させることが可能となる。
【００７５】
さらに、上記した実施形態では、図４（ａ）に示すように、基材Ｋの搬送方向に直交する方向に記録ヘッド２０を走査させながら記録を行う記録方式の例を示したが、インクジェット記録装置１００では、図４（ａ）に示す記録方式を以下に示す各記録方式に変更してもよい。なお、図４（ａ）〜図４（ｇ）は、種々の各記録方式を概略的に図示したものである。
【００７６】
すなわち、図４（ｂ）に示す通り、記録ヘッド２０を所定の方向に間欠的に移動させるとともに、記録ヘッド２０の移動方向と直交する方向に基材Ｋを往復移動させる記録方式でもよい。また、図４（ｃ）に示す通り、走査型の記録ヘッド２０に代えて、基材Ｋの搬送方向に対して直交する方向に多数のノズル２１を配列してなるライン型のラインヘッド２０ａを適用する記録方式でもよい。
【００７７】
また、図４（ｄ）に示す通り、基材Ｋを外面に巻き付けた円筒状のドラム４０を記録ヘッド２０に対向するように配置し、ドラム４０を所定の方向に回転させながら記録ヘッド２０を走査する記録方式でもよい。また、図４（ｅ）に示す通り、図４（ｄ）に示す記録方式での記録ヘッド２０を、図４（ｃ）に示すようなライン型のラインヘッド２０ａに代える記録方式でもよい。
【００７８】
また、図４（ｆ）に示す通り、記録ヘッド２０と基材Ｋとの間に、所定方向に回転するドラム状の中間転写体４１を配置し、記録ヘッド２０から中間転写体４１に向けてインクを吐出し、中間転写体４１に付着したインクを基材Ｋに転写する記録方式でもよい。この場合の中間転写体４１は、記録ヘッド２０から吐出された各インク滴を自己の表層で保持しながら搬送する機能と搬送した各インク滴を基材Ｋに加圧転写する機能とを具備する。また、図４（ｇ）に示す通り、図４（ｆ）に示す記録方式での記録ヘッド２０を、図４（ｃ）及び図４（ｅ）に示すようなライン型のラインヘッド２０ａに代える記録方式でもよい。
【００７９】
【実施例】
［実施例１］
以下に示す水性インクＡを作製した。
（水性インクＡ）
各色材の濃度：２０ｗｔ％
色材（Ｙ）：ウォーターイエロー６（オリエント化学工業（株）製）
色材（Ｍ）：チューガノール−ファーストレッド３Ｂ（中外化成（株）製）
色材（Ｃ）：カヤラスターコイス−ブルーＧＬ（日本化薬（株）製）
色材（Ｋ）：スペシャル・ブラック７９８４（バイエル製）
エチレングリコール濃度：２０ｗｔ％
水の濃度：６０ｗｔ％
【００８０】
そして、上記水性インクＡを用いて上記インクジェット記録装置２０により、所定の画像（各インク滴の一滴当たりの滴量：１００ｆｌ，解像度：５０００ｄｐｉ）をアート紙に記録した。この場合、アート紙の記録領域のうち、各画素が全てインク滴により埋め尽くされた所定領域の一色当たりのインク付着量は３．９ｍｌ／ｍ^２であった。また、この場合、アート紙へのインクの滲み及びアート紙のコックリングは非常に軽微なものであり、アート紙に付着したインクの各インク滴は、ドライヤーによる乾燥によりアート紙に素早く定着した。
【００８１】
［実施例１に対する比較例］
以下に示す水性インクＢを作製した。
（水性インクＢ）
各色材の濃度：３．３ｗｔ％
各色材：実施例１と同様
エチレングリコール濃度：２０ｗｔ％
水の濃度：７６．７ｗｔ％
【００８２】
そして、上記水性インクＢを用いて通常のピエゾ素子方式のインクジェット記録装置により、所定の画像（各インク滴の一滴当たりの滴量：２ｐｌ，解像度：１８００ｄｐｉ）をアート紙に記録した。この場合、アート紙の記録領域のうち、各画素が全てインク滴により埋め尽くされた所定領域の一色当たりのインク付着量は１０ｍｌ／ｍ^２であった。また、この場合、混色部において、各インク滴がアート紙に滲み、アート紙のコックリングが確認され、更にはアート紙に付着した各インク滴の乾燥に所定の時間を要した。
【００８３】
［実施例２］
以下に示す水性インクＣを作製した。
（水性インクＣ）
各色材の濃度：５０ｗｔ％
各色材：実施例１と同様
水の濃度：５０ｗｔ％
【００８４】
そして、上記水性インクＣを用いて上記インクジェット記録装置２０により、所定の画像（各インク滴の一滴当たりの滴量：１５ｆｌ，解像度：７０００ｄｐｉ）をアート紙に記録した。この場合、アート紙の記録領域のうち、各画素が全てインク滴により埋め尽くされた所定領域の一色当たりのインク付着量は１．１ｍｌ／ｍ^２であった。また、この場合、アート紙へのインクの滲み及びアート紙のコックリングは発生せず、アート紙に付着したインクの各インク滴は、即座に乾燥した。
【００８５】
［実施例３］
以下の表１に示す組成比を有する各色のインクをサンドグラインダーにて８時間分散させ、その後、これら各色のインクを０．３μｍのメンブレンフィルタに通し、紫外線硬化型のインクＤを作製した。
【００８６】
【表１】

なお、表１において、分散剤：Ａｖｅｃｉａ社製ソルスパース２４０００，ＯＸＴ２２１：東亜合成（株）製アロンオキセタンＯＸＴ２２１，２０２１Ｐ：ダイセル製セロキサイド２０２１Ｐ，ＵＶＩ６９９０：ダウケミカル製光酸発生剤である。
【００８７】
そして、上記インクＤを用いて上記インクジェット記録装置２０により、所定の画像（各インク滴の一滴当たりの滴量：３００ｆｌ，解像度：３０００ｄｐｉ）をＰＥＴ（ＰｏｌｙＥｔｈｙｌｅｎｅ　Ｔｅｒｅｐｈｔｈａｌａｔｅ）フィルムに記録した。その後、この画像に対して、短波長紫外線（波長約２８０ｎｍ以下の紫外線）と長波長紫外線（波長約３２０〜約４００ｎｍの紫外線）とを同時に照射し、画像を構成する各インク滴を硬化させてＰＥＴフィルムに定着させた。この場合、ＰＥＴフィルムの記録領域のうち、各画素が全てインク滴により埋め尽くされた所定領域の一色当たりのインク付着量は５．６ｍｌ／ｍ^２であった。また、この場合、ＰＥＴフィルムへのインクの滲みは無く、更には通常の紫外線硬化型インクに特有の基材上での凹凸はほとんどみられず、画像の質感は良好だった。
【００８８】
（印加電圧低下および微少インク滴量の安定吐出実現の方策）
本発明では、静電吸引型インクジェット方式において果たすノズルの役割を再考察し、
【数４】

即ち、
【数５】

或いは
【数６】

という従来吐出不可能として試みられていなかった領域において、マクスウェル力などを利用することで、微細インク滴を形成することができる。
【００８９】
このような駆動電圧低下および微少量吐出実現の方策のための吐出条件等を近似的に表す式を導出したので以下に述べる。
以下の説明は、上記各本発明の実施形態で説明したインクジェット記録装置に適用可能である。
いま、半径ｒのノズルに導電性溶液を注入し、基材としての無限平板導体からｈの高さに垂直に位置させたと仮定する。この様子を図５に示す。このとき、ノズル先端部に誘起される電荷は、ノズル先端の半球部に集中すると仮定し、以下の式で近似的に表される。
【数７】

ここで、Ｑ：ノズル先端部に誘起される電荷、ε_０：真空の誘電率、ε：基板の誘電率、ｈ：ノズル−基板間距離、ｒ：ノズル内径の半径、Ｖ：ノズルに印加する電圧である。α：ノズル形状などに依存する比例定数で、１〜１．５程度の値を取り、特にｒ＜＜ｈのときほぼ１程度となる。
【００９０】
また、基材としての基板が導体基板の場合、基板内の対称位置に反対の符号を持つ鏡像電荷Ｑ’が誘導されると考えられる。基板が絶縁体の場合は、誘電率によって定まる対称位置に同様に反対符号の映像電荷Ｑ’が誘導される。
ところで、ノズル先端部に於ける電界強度Ｅ_ｌｏｃ．は、先端部の曲率半径をＲと仮定すると、
【数８】

で与えられる。ここでｋ：比例定数で、ノズル形状などにより異なるが、１．５〜８．５程度の値をとり、多くの場合５程度と考えられる。（Ｐ．　Ｊ．　Ｂｉｒｄｓｅｙｅ　ａｎｄ　Ｄ．Ａ．Ｓｍｉｔｈ，　Ｓｕｒｆａｃｅ　Ｓｃｉｅｎｃｅ，　２３　（１９７０）　１９８−２１０）。今簡単のため、ｒ＝Ｒとする。これは、ノズル先端部に表面張力で導電性溶液がノズル径ｒと同じ半径を持つ半球形状に盛り上がっている状態に相当する。
ノズル先端の液体に働く圧力のバランスを考える。まず、静電的な圧力は、ノズル先端部の液面積をＳとすると、
【数９】

（８）、（９）、（１０）式よりα＝１とおいて、
【数１０】

と表される。
【００９１】
一方、ノズル先端部に於ける液体の表面張力をＰｓとすると、
【数１１】

ここで、γ：表面張力、である。
静電的な力により流体の吐出が起こる条件は、静電的な力が表面張力を上回る条件なので、
【数１２】

となる。十分に小さいノズル径ｒをもちいることで、静電的な圧力が、表面張力を上回らせる事が可能である。
この関係式より、Ｖとｒの関係を求めると、
【数１３】

が吐出の最低電圧を与える。すなわち、式（７）および式（１４）より、
【数１４】

が、本発明の動作電圧となる。
【００９２】
ある半径ｒのノズルに対し、吐出限界電圧Ｖｃの依存性を前述した図６に示す。この図より、微細ノズルによる電界の集中効果を考慮すると、吐出開始電圧は、ノズル径の減少に伴い低下する事が明らかになった。つまり、ノズル径が２０μｍ未満であることにより、電界を集中させることができ、微細液滴を安定に吐出でき、かつ吐出開始電圧を低減できる。図６より、ノズル径が８μｍ以下であることにより、さらに電界を集中させることができ、微細液滴をさらに安定に吐出でき、かつさらに吐出開始電圧を低減できることがわかる。また、さらにノズル径は、４μｍ以下であることが好ましい。
従来の電界に対する考え方、すなわちノズルに印加する電圧と対向電極間の距離によって定義される電界のみを考慮した場合では、微小ノズルになるに従い、吐出に必要な電圧は増加する。一方、局所電界強度に注目すれば、微細ノズル化により吐出電圧の低下が可能となる。
【００９３】
静電吸引による吐出は、ノズル端部における流体の帯電が基本である。帯電の速度は誘電緩和によって決まる時定数程度と考えられる。
【数１５】

ここで、ε：流体の比誘電率、σ：流体の導電率である。流体の比誘電率を１０、導電率を１０^−６　Ｓ／ｍ　を仮定すると、τ＝１．８５４×１０^−５ｓｅｃとなる。あるいは、臨界周波数をｆｃとすると、
【数１６】

となる。このｆｃよりも早い周波数の電界の変化に対しては、応答できず吐出は不可能になると考えられる。上記の例について見積もると、周波数としては１０　ｋＨｚ程度となる。このとき、ノズル半径２μｍ、電圧５００Ｖ弱の場合、Ｇは１０^−１３ｍ^３／ｓと見積もることができるが、上記の例の液体の場合、１０ｋＨｚでの吐出が可能なので、１周期での最小吐出量は１０ｆｌ（フェムトリットル、１ｆｌ：１０^−１５　ｌ）程度を達成できる。
【００９４】
なお、各上記本実施の形態においては、図５に示したようにノズル先端部に於ける電界の集中効果と、対向基板に誘起される鏡像力の作用を特徴とする。このため、先行技術のように基板または基板支持体を導電性にしたり、これら基板または基板支持体に電圧を印加する必要はない。すなわち、基板として絶縁性のガラス基板、ポリイミドなどのプラスチック基板、セラミックス基板、半導体基板などを用いることが可能である。
また、上記各実施形態において電極への印加電圧はプラス、マイナスのどちらでも良い。
さらに、ノズルと基材との距離は、５００［μｍ］以下に保つことにより、溶液の吐出を容易にすることができる。また、図示しないが、ノズル位置検出によるフィードバック制御を行い、ノズルを基材に対し一定に保つようにする。
また、基材を、導電性または絶縁性の基材ホルダーに裁置して保持するようにしても良い。
【００９５】
図７は、本発明の他の基本例の一例としてのインクジェット記録装置の側面断面図を示したものである。ノズル１の側面部には電極１５が設けられており、ノズル内インク３との間に制御された電圧が引加される。この電極１５の目的は、Ｅｌｅｃｔｒｏｗｅｔｔｉｎｇ　効果を制御するための電極である。十分な電場がノズルを構成する絶縁体にかかる場合この電極がなくともＥｌｅｃｔｒｏｗｅｔｔｉｎｇ効果は起こると期待される。しかし、本基本例では、より積極的にこの電極を用いて制御することで、吐出制御の役割も果たすようにしたものである。ノズル１を絶縁体で構成し、その厚さが１μｍ、ノズル内径が２μｍ、印加電圧が３００Ｖの場合、約３０気圧のＥｌｅｃｔｒｏｗｅｔｔｉｎｇ効果になる。この圧力は、吐出のためには、不十分であるが溶液のノズル先端部への供給の点からは意味があり、この制御電極により吐出の制御が可能と考えられる。
【００９６】
前述した図７は、本発明における吐出開始電圧のノズル径依存性を示したものである。インクジェット記録装置として、図１に示すものを用いた。微細ノズルになるに従い吐出開始電圧が低下し、従来より低電圧で吐出可能なことが明らかになった。
【００９７】
上記実施形態において、インク吐出の条件は、ノズル基板間距離（Ｌ）、印加電圧の振幅（Ｖ）、印加電圧振動数（ｆ）のそれぞれの関数になり、それぞれにある一定の条件を満たすことが吐出条件として必要になる。逆にどれか一つの条件を満たさない場合他のパラメーターを変更する必要がある。
【００９８】
この様子を図８を用いて説明する。
まず吐出のためには、それ以上の電界でないと吐出しないというある一定の臨界電界Ｅｃが存在する。この臨界電界は、ノズル径、溶液の表面張力、粘性などによって変わってくる値で、Ｅｃ以下での吐出は困難である。臨界電界Ｅｃ以上すなわち吐出可能電界強度において、ノズル基板間距離（Ｌ）と印加電圧の振幅（Ｖ）の間には、おおむね比例の関係が生じ、ノズル間距離を縮めた場合、臨界印加電圧Ｖを小さくする事が出来る。
【００９９】
逆に、ノズル基板間距離Ｌを極端に離し、印加電圧Ｖを大きくした場合、仮に同じ電界強度を保ったとしても、コロナ放電による作用などによって、流体インク滴の破裂すなわちバーストが生じてしまう。そのため良好な吐出特性を得るためには、ノズル基板間距離は１００μｍ程度以下に抑えることが吐出特性並びに、着弾精度の両面から望ましい。
【０１００】
【発明の効果】
本発明によれば、基材へのインクの滲み、インク自体の乾燥不良、基材に付着したインクによる基材のコックリング等の弊害を抑制でき、ひいては複数の超微細なドットによる高精細な画像を記録することができる。
【図面の簡単な説明】
【図１】インクジェット記録装置の側面図である。
【図２】記録ヘッド及び記録ヘッドに係る部材の断面図である。
【図３】インクの吐出動作とインクに印加される電圧との関係を示す説明図であって、図３（Ａ）は吐出を行わない状態であり、図３（Ｂ）は吐出状態を示す。
【図４】（ａ）〜（ｇ）種々の各記録方式を示す概略図である。
【図５】ノズルの電界強度の計算を説明するために示したものである。
【図６】ノズルのノズル径とメニスカス部で吐出する液滴が飛翔を開始する吐出開始電圧、該初期吐出液滴のレイリー限界での電圧値及び吐出開始電圧とレイリー限界電圧値の比との関係を示す線図である。
【図７】インクジェット記録装置の側面断面図を示したものである。
【図８】インクジェット記録装置における距離−電圧の関係による吐出条件を説明した図である。
【符号の説明】
１００　　　インクジェット記録装置
２０　　　記録ヘッド
２０ａ　　　ラインヘッド
２１　　　ノズル
２２　　　流路
２３　　　対向電極
２４　　　インク室
２５　　　吐出電圧印加手段
２６　　　ノズルプレート
２７　　　インク供給路
２８　　　吐出電極
２９　　　吐出電圧電源
３０　　　バイアス電源
４０　　　ドラム
４１　　　中間転写体
１０２　　　プラテン
１０３ａ，１０３ｂ　　　搬送ローラ
１０４ａ，１０４ｂ　　　圧接ローラ
Ｋ　　　基材[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an ink jet recording apparatus and an ink used for the apparatus.
[0002]
[Prior art]
The conventional ink jet recording method includes a piezo method in which ink droplets are ejected by deforming an ink flow path by vibrating a piezoelectric element.A heating element is provided in the ink flow path, and the heating element generates heat to generate bubbles. There are known a thermal method in which ink droplets are ejected according to a pressure change in an ink flow path due to bubbles, and an electrostatic suction method in which ink in an ink flow path is charged and ink droplets are ejected by an electrostatic suction force of the ink. ing.
[0003]
An ink jet recording apparatus using the above-described ink jet recording method forms a plurality of dots with ink droplets that have landed on a recording medium and records a desired image on the recording medium. (Picoliter) to several tens pl of fine ink droplets can be ejected (for example, see Patent Documents 3 and 4).
[0004]
[Patent Document 1]
JP-A-8-238774
[Patent Document 2]
JP 2000-127410 A
[Patent Document 3]
JP-A-10-291303
[Patent Document 4]
JP-A-11-268266
[0005]
[Problems to be solved by the invention]
However, regarding the recording by the ink jet recording apparatus, even if the amount of each ink droplet is several pl to several tens pl as described above, the dot diameter of each ink droplet that lands on the recording medium is several tens μm (micrometer). ), And the ink adhesion amount per color to the recording medium is also 10 ml / m. ² Reached nearby.
[0006]
More specifically, in a normal inkjet recording apparatus, an ink droplet of about 2 to 80 pl is ejected from a nozzle that ejects ink as a drop, but when the amount of ink ejected from the nozzle is 20 pl, Indicates that the dot diameter of each ink droplet landing on the recording medium has reached 60 μm. The resolution at this time is equivalent to 600 dpi (dot per inch), and the amount of ink adhering to one color on the recording medium is 11.11 ml / m. ² Has been reached.
[0007]
When the amount of ink ejected from the nozzle is 2 pl, the dot diameter of each ink droplet landing on the recording medium has reached 25 μm. The resolution at this time is equivalent to 1440 dpi, and the amount of ink adhered to one color on the recording medium is 6.4 ml / m. ² As a result, the ink adhesion amount can be relatively small. However, in practice, minute errors such as errors in the ejection angle of the ink droplets, errors in the ejection speed of the ink droplets, errors in the conveyance of the recording medium, and landing errors of the ink drops due to the distance between the nozzle and the recording medium occur. It is necessary to completely fill each pixel of the medium with ink, and as a result, the ink adhesion amount per color is 6.4 ml / m ² More than.
[0008]
In such printing, the diameter of each dot and the amount of ink adhering per color are relatively large. For example, in a printing portion (hereinafter, referred to as a “highlight portion”) composed of bright ink droplets, dark ink is used. Each ink droplet is recognized remarkably in comparison with other ink droplets, and even in a recording portion (hereinafter, referred to as a “color mixture portion”) composed of dark ink droplets different from each other. Is done. In other words, it is difficult to express a smooth gradation on an image to be printed only with dark ink droplets. In order to realize a smooth gradation expression, not only dark ink but also coloring material Must use a light-colored ink with reduced density.
[0009]
However, when a light-colored ink is used, the amount of ink droplets per unit area for obtaining a certain density increases, and as a result, as described above, the ink adhesion per color adhered to the recording medium is increased. The amount also increases. In such a situation, the quality of the image deteriorates because the ink bleeds into the recording medium, the ink that has landed on the recording medium cannot be dried sufficiently, or the recording medium cockles due to a so-called “undulation phenomenon”. May be caused. In particular, when the number of nozzles is increased in order to perform high-speed printing by ejecting a large number of ink droplets at a time, this becomes remarkable.
[0010]
It is possible to solve the above-mentioned inconvenience by irradiating ultraviolet rays to the ink that has landed on the recording medium and curing the ink by using an ultraviolet curable ink that is cured by being irradiated with ultraviolet rays. Since the ultraviolet curable ink is immediately cured without being absorbed by the recording medium, each ink droplet that has landed on the recording medium swells greatly on the recording medium, and impairs the texture of the recorded image. That is, unevenness due to each ink droplet appears remarkably on the recording medium.
[0011]
Further, the above-mentioned ink jet recording apparatus has the following problems.
(1) Stability of minute ink droplet formation
Since the nozzle diameter is large, the shape of the ink droplet ejected from the nozzle is not stable. (2) Insufficient landing accuracy of minute ink droplets
The kinetic energy given to the ink droplet ejected from the nozzle becomes smaller in proportion to the cube of the ink droplet radius. For this reason, the minute ink droplet cannot secure sufficient kinetic energy enough to withstand air resistance, and is disturbed by air convection or the like, so that accurate landing cannot be expected. Further, as the ink droplets become finer, the effect of surface tension increases, so that the vapor pressure of the ink droplets increases and the amount of evaporation increases. For this reason, fine ink droplets cause a significant loss of mass during flight, and it is difficult to maintain the shape of the ink droplets even when landing.
[0012]
As described above, miniaturization and high accuracy of ink droplets are conflicting subjects, and it has been difficult to realize both at the same time.
This poor landing position accuracy not only degrades the print quality, but also poses a particularly serious problem, for example, when drawing a circuit wiring pattern using conductive ink by inkjet technology. That is, poor positioning accuracy not only makes it impossible to draw a wiring having a desired thickness, but also may cause disconnection or short-circuit.
(3) High applied voltage
In the principle of the conventional electrostatic suction method, charges are concentrated on the center of the meniscus to cause the meniscus to protrude. The radius of curvature of the tip of the raised tailor cone is determined by the amount of charge concentration, and the separation of ink droplets starts when the concentrated charge amount and the electrostatic force due to the electric field strength exceed the surface tension of the meniscus at that time.
Since the maximum charge amount of the meniscus is determined by the physical properties of the ink and the radius of curvature of the meniscus, the minimum size of the ink droplet is determined by the physical properties of the ink (particularly, surface tension) and the electric field strength formed in the meniscus.
[0013]
In general, the surface tension of ink tends to be lower when a solvent contains a solvent than when a pure solvent is used.Since an actual ink also contains various solvents, it is not possible to increase the surface tension. difficult. For this reason, a method has been adopted in which the surface tension of the ink is assumed to be constant and the size of the ink droplet is reduced by increasing the electric field intensity.
Therefore, in the ink jet recording apparatuses disclosed in Patent Documents 1 and 2 described above, as a principle of ejection, a field having a strong electric field strength is formed in a meniscus region having a much larger area than the projected area of the ejected ink droplet. Since the electric charge is concentrated at the center of the meniscus and the discharge is performed by the electrostatic force including the concentrated electric charge and the formed electric field strength, it is necessary to apply a very high voltage close to 2000 [V]. However, there is a problem in terms of safety in operating the ink jet recording apparatus.
[0014]
(4) Discharge response
In the ink jet recording apparatuses disclosed in the above Patent Documents 1 and 2, both of the ink jet recording devices use a meniscus by forming a field having a strong electric field strength in a meniscus region having an area much larger than the projected area of the ejected ink droplet. Charge is concentrated at the center of the surface, and the discharge is performed by the electrostatic force composed of the concentrated charge and the electric field strength that is formed. Therefore, the movement time of the charge for moving the charge to the center of the meniscus portion affects the ejection responsiveness. However, there has been a problem in improving the printing speed.
[0015]
An object of the present invention is to discharge a super-fine ink droplet, thereby suppressing bleeding of the ink on the recording medium, poor drying of the ink itself, and suppressing adverse effects such as cockling of the recording medium due to the ink, thereby forming a high-definition image. An object of the present invention is to provide an ink jet recording apparatus capable of recording.
[0016]
[Means for Solving the Problems]
The invention according to claim 1 is an inkjet recording apparatus that includes a nozzle that ejects ink as droplets, and that records a desired image on a substrate by ejecting ink droplets from the nozzle to the substrate.
The amount of each ink droplet ejected from the nozzle is 1 to 400 fl (femtoliter), and the amount of ink adhering to a base material per color is 0.2 to 5.6 ml / m. ² It is characterized by being.
[0017]
According to the first aspect of the present invention, an extremely fine ink droplet of 1 to 400 fl, which is much less than several pl, is ejected, and the amount of ink per color adhered to the base material is 0.2 to 5.6 ml / m. ² Therefore, the dot diameter of each ink droplet adhering to the substrate can be greatly reduced. This can suppress adverse effects such as ink bleeding into the base material, poor drying of the ink itself, and cockling of the base material due to the ink attached to the base material, and thus, recording of a high-definition image using a plurality of ultra-fine dots. can do. Further, in this case, since ultra-fine dots are formed on the base material, smooth gradation expression can be performed on the highlight portion and the mixed color portion.
[0018]
In the above configuration, the amount of ink droplets per droplet is less than 1 fl, and the amount of ink adhering to a substrate per color is 0.2 ml / m2. ² If it is less than 3, the landing accuracy of each ink droplet on the base material cannot be sufficiently maintained, and further, an ink density sufficient for recording cannot be maintained. In addition, the ink droplet amount per ink droplet is larger than 400 fl, and the ink adhesion amount per color to the base material is 5.6 ml / m2. ² When it is larger than the above, conventional harmful effects such as bleeding of the ink on the base material, poor drying of the ink itself, and cockling of the base material by the ink attached to the base material cannot be suppressed.
Further, the “substrate” described above and below refers to an object (recording medium) that receives landing of ink droplets ejected from a nozzle. Therefore, for example, when a desired image is recorded on a sheet, the sheet corresponds to the base material.
[0019]
According to a second aspect of the present invention, in the inkjet recording apparatus according to the first aspect,
The amount of ink droplets ejected from the nozzle per droplet is 1 to 100 fl, and the amount of ink adhered to a substrate per color is 0.2 to 2.5 ml / m. ² It is characterized by being.
[0020]
According to the second aspect of the invention, the effect derived from the first aspect can be more reliably achieved. Therefore, in the second aspect of the invention, the ink bleeds into the base material, Defects such as poor drying and cockling of the substrate due to ink attached to the substrate can be more reliably suppressed, and a high-definition image using a plurality of ultra-fine dots can be more reliably recorded. Further, in this case, since ultrafine dots are reliably formed on the base material, smooth gradation expression can be more reliably performed on the highlight portion and the mixed color portion.
[0021]
According to a third aspect of the present invention, in the inkjet recording apparatus according to the first or second aspect,
A cylindrical drum is provided so as to face the nozzle, and a base material is wound around the outer surface,
Each ink droplet ejected from the nozzle is attached to a substrate wound around the outer surface of the drum to record a desired image on the substrate.
[0022]
The invention according to claim 4 is the inkjet recording apparatus according to any one of claims 1 to 3,
An intermediate transfer member arranged to face the nozzle,
Each ink droplet ejected from the nozzle adheres to the intermediate transfer body, and each ink droplet adhered to the intermediate transfer body is transferred to a base material.
[0023]
According to a fifth aspect of the present invention, in the inkjet recording apparatus according to any one of the first to fourth aspects,
A nozzle having an ultra-fine diameter that is disposed with the tip end thereof opposed to a base material having a receiving surface that receives the ejection of the ink droplets of the charged ink and ejects ink droplets from the tip end,
An ink supply unit for supplying ink into the nozzle,
Discharge voltage applying means for applying a discharge voltage to the ink in the nozzle,
It is characterized by having.
[0024]
In the configuration according to the fifth aspect, the nozzle or the base material is arranged such that the ink droplet receiving surface faces the tip of the nozzle. The arrangement work for realizing the mutual positional relationship may be performed by either moving the nozzle or moving the base material.
Then, ink is supplied into the nozzles by the ink supply means. The ink in the nozzle is required to be in a charged state in order to perform ejection. The ink may be charged by applying a voltage within a range in which the ink is not ejected by an ejection voltage applying unit that applies an ejection voltage, or an electrode dedicated to charging may be provided.
[0025]
The structure according to the fifth aspect is characterized in that the electric field is concentrated at the tip of the nozzle by increasing the electric field strength by making the nozzle have an unprecedented ultrafine diameter. The reduction in the diameter of the nozzle will be described in detail later. In such a case, it is possible to discharge ink droplets even if there is no counter electrode facing the tip of the nozzle. For example, in a state where the counter electrode is not present, when the base material is arranged so as to face the nozzle tip portion, and when the base material is a conductor, the surface symmetry of the nozzle tip portion is set on the basis of the receiving surface of the base material. A mirror image charge of opposite polarity is induced at a certain position, and when the base material is an insulator, an image charge of opposite polarity is induced at a symmetric position determined by the dielectric constant of the base material with respect to the receiving surface of the base material. . Then, the ink droplet is caused to fly by the electrostatic force between the charge induced at the nozzle tip and the mirror image charge or the image charge.
[0026]
However, the configuration according to claim 5 makes it possible to eliminate the need for the counter electrode, but the counter electrode may be used in combination. By using the counter electrode together, it is also possible to use the electrostatic force due to the electric field between the nozzle and the counter electrode for guiding the flying electrode, and if the counter electrode is grounded, the charge of the charged ink droplet is reduced. Since it is possible to escape through the counter electrode and obtain an effect of reducing the accumulation of electric charge, it can be said that it is rather desirable to use them together.
[0027]
As a cited invention citing the invention of claim 5, the same configuration as that of the invention of claim 5 is provided, the discharge voltage is fixed, and the operating frequency is
f = σ / 2πε
The frequency control means may be configured to perform an operation control for switching between ejection and stop by switching between a value larger and smaller than the boundary value f represented by. Here, σ: conductivity of the solution, ε: relative permittivity of the solution.
[0028]
In the above configuration, the same operation as the invention described in claim 5 is performed, and a discharge voltage of a potential capable of discharging ink droplets is continuously applied to the ink in the nozzle, and the frequency of the discharge voltage is adjusted. On / off control of the ejection operation is performed based on the fluctuation. That is, the boundary value f = σ / 2πε is a critical frequency determined according to the charging speed based on the conductivity and relative dielectric constant of the ink, and when the ejection voltage is applied at a frequency higher than this, for example, the potential Is not in the proper range, ink droplets are not ejected. Therefore, while continuously applying a discharge voltage of a dischargeable potential to the ink in the nozzle, when not discharging, the frequency is maintained at a state larger than the boundary value f, and when discharging, By switching to a state smaller than the boundary value f, ink droplets can be ejected.
[0029]
In addition, when the on / off of the ink droplet ejection is controlled by switching the frequency of the ejection voltage, the on / off of the ink droplet ejection is switched by the on / off of the application of the ejection voltage, and the on / off of the ink droplet ejection is switched by the level of the potential of the ejection voltage. The time response is superior to that in the case, and the response at the time of re-discharge can be improved.
[0030]
A cited invention citing the invention according to claim 5 has the same configuration as the invention according to claim 5, and at least insulates the inner surface of the flow path of the nozzle and surrounds the ink in the flow path. However, a configuration may be adopted in which the flow supply electrode is provided outside the insulated portion.
[0031]
In the above configuration, "providing the flow supply electrode outside the insulated portion" means that even when the flow supply electrode is provided via an insulating film inside the nozzle, the entire nozzle is formed of an insulating material. This means that a flow supply electrode is provided outside the nozzle.
[0032]
Generally, when a voltage is applied to each electrode by providing an electric potential difference between an electrode that insulates an inner surface of a conduit and an electrode provided through the insulating portion and an electrode that applies a voltage to ink inside the conduit, The effect of the so-called electrowetting phenomenon that the wettability of the ink to the inner surface of the insulated conduit is improved can be obtained.
In the configuration of the above invention, electrowetting is performed by providing a potential difference between an applied voltage by the flow supply electrode provided outside the portion where the inner side surface of the nozzle is insulated and an applied voltage by the discharge voltage applying means. The effect can improve the wettability in the nozzle, and the electrowetting effect can facilitate the supply of ink into the nozzle.
[0033]
The invention according to claim 6 is an ink used for the inkjet recording apparatus according to any one of claims 1 to 5,
The color material has a concentration of 8 to 90 wt% (mass%) and does not contain particles having a diameter of 0.3 μm or more.
[0034]
In the ink according to the sixth aspect, if the concentration of the coloring material is less than 8 wt%, it is not possible to maintain an ink concentration sufficient for recording. When the concentration of the coloring material is higher than 90 wt%, the fluidity of the ink itself deteriorates, and the ejection of the ink from the nozzle becomes unstable, or the viscosity of the ink increases during long-term storage of the ink. Such inconveniences occur. Further, the ink according to the invention of claim 6 does not include particles having a diameter of 0.3 μm or more (preferably particles having a diameter of 0.05 μm or more).
[0035]
In the ink according to the sixth aspect of the invention, both a pigment and a dye can be used as the coloring material. Here, when a pigment is applied as a coloring material, there is no particular problem as long as each particle constituting the pigment is less than 0.3 μm in diameter. In this case, the pigment can be sufficiently dispersed in the ink. Since it is required, it is preferable to apply a dye as a coloring material.
[0036]
According to a seventh aspect of the present invention, in the inkjet recording apparatus according to any one of the first to fifth aspects,
The nozzle diameter of the nozzle is 30 μm or less.
[0037]
According to an eighth aspect of the present invention, in the inkjet recording apparatus according to the seventh aspect,
The nozzle diameter of the nozzle is less than 20 μm.
[0038]
According to a ninth aspect of the present invention, in the inkjet recording apparatus according to the eighth aspect,
The nozzle diameter of the nozzle is 8 μm or less.
[0039]
The invention according to claim 10 is the ink jet recording apparatus according to claim 9,
The nozzle diameter of the nozzle is 4 μm or less.
[0040]
In the present invention, “nozzle diameter” refers to the internal diameter of the tip of the nozzle.
When the nozzle diameter is less than 20 [μm], the electric field intensity distribution becomes narrow. Thereby, the electric field can be concentrated. As a result, the formed ink droplets can be minute and have a stable shape, and the total applied voltage can be reduced. Immediately after being ejected from the nozzle, the ink droplet is accelerated by an electrostatic force acting between the electric field and the electric charge. However, when the ink droplet is separated from the nozzle, the electric field sharply decreases. However, the ink droplet, which is a minute ink droplet and in which the electric field is concentrated, is accelerated by a mirror image force as approaching the counter electrode. By balancing the deceleration due to the air resistance and the acceleration due to the mirror image force, it is possible to stably fly the fine ink droplet and improve the landing accuracy.
[0041]
Further, the internal diameter of the nozzle is preferably 8 [μm] or less. By making the internal diameter of the nozzle 8 [μm] or less, it is possible to further concentrate the electric field, and it is possible to further reduce the size of the ink droplets and that the fluctuation of the distance between the opposing electrodes during flight affects the electric field intensity distribution. Since it can be reduced, it is possible to reduce the influence of the positional accuracy of the counter electrode, the properties and thickness of the base material on the ink droplet shape, and the impact on the landing accuracy.
[0042]
Further, by setting the internal diameter of the nozzle to 4 [μm] or less, a remarkable electric field can be concentrated, the maximum electric field strength can be increased, and the ink droplet having a stable shape can be made ultra-miniaturized. In addition, the initial ejection speed of the ink droplet can be increased. As a result, the flight stability is improved, so that the landing accuracy can be further improved and the ejection responsiveness can be improved.
[0043]
Further, it is desirable that the inner diameter of the nozzle is larger than 0.2 [μm]. By making the inner diameter of the nozzle larger than 0.2 [μm], the charging efficiency of the ink droplets can be improved, so that the ejection stability of the ink droplets can be improved.
[0044]
Furthermore, in the structure of the cited invention citing the invention described in each of the above claims and claim 5,
(1) It is preferable that the nozzle is formed of an electrically insulating material, and an electrode is inserted or plated in the nozzle.
(2) In the configuration of each claim or the configuration of (1), it is preferable that the nozzle is formed of an electrically insulating material, an electrode is inserted in the nozzle or formed by plating, and an electrode is provided outside the nozzle.
According to (1) and (2), in addition to the functions and effects according to the above-described claims, the discharge force can be improved, so that the liquid can be discharged at a low voltage even if the nozzle diameter is further reduced.
(3) In the structure of each of the above-mentioned claims, the structure of the above (1) or (2), it is preferable that the base material is formed of a conductive material or an insulating material.
(4) In the configuration of each of the above claims, the configuration of (1), (2) or (3), the voltage V applied to the nozzle is
(Equation 1)

It is preferable to drive in a basin represented by
Where γ: surface tension of liquid, ε ₀ : A dielectric constant of vacuum, r: nozzle radius, h: distance between nozzle and substrate, k: proportional constant (1.5 <k <8.5) depending on nozzle shape.
(5) In the configuration of each of the above-mentioned claims, the above-mentioned (1), (2), (3) or (4), it is preferable that the applied arbitrary waveform voltage is 1000 V or less.
(6) In the configuration of each of the above-mentioned claims, and in the configuration of (1), (2), (3), (4) or (5), it is preferable that an arbitrary waveform voltage to be applied is 500 V or less.
Preferably, the substrate is placed in a conductive or insulating substrate holder.
(7) In any one of the constitutions described in the above claims and any one of the constitutions (1) to (6), the distance between the nozzle and the substrate is 500 [μm] or less, which is high even when the nozzle diameter is small. This is preferable because landing accuracy can be obtained.
(8) In any one of the above-described claims and any one of the above-mentioned constitutions (1) to (7), it is preferable that pressure is applied to the solution in the nozzle.
(9) In the configuration according to any one of the claims, and in any one of the configurations (1) to (8), when discharging by a single pulse,
(Equation 2)

May be applied to apply a pulse width Δt that is equal to or greater than a time constant τ determined by the following equation. Here, ε: dielectric constant of fluid, σ: electrical conductivity.
[0045]
BEST MODE FOR CARRYING OUT THE INVENTION
The nozzle diameter (inner diameter) of the inkjet recording apparatus described in the following embodiment is preferably 30 [μm] or less, more preferably less than 20 [μm], further preferably 8 [μm] or less. Is preferably 4 [μm] or less. Further, the nozzle diameter is preferably larger than 0.2 [μm].
[0046]
(Overall configuration of inkjet recording apparatus)
Hereinafter, the ink jet recording apparatus will be described with reference to FIG.
FIG. 1 is a schematic configuration diagram of an ink jet recording apparatus. The ink jet recording apparatus 100 includes a platen 102 for supporting a predetermined base material K, a recording head 20 disposed above the platen 102, and a platen 102. Transport rollers 103a and 103b arranged on the upstream and downstream sides in the transport direction of the base material K, and press-contact which presses the base material K between the transport rollers 103a and 103b while being pressed against the transport rollers 103a and 103b. Rollers 104a and 104b.
[0047]
(Configuration of Recording Head and Member Related to Recording Head)
Hereinafter, the recording head 20 and members related to the recording head 20 will be described with reference to FIGS. FIG. 2 is a cross-sectional view of the recording head 20 and members related to the recording head 20 along the nozzle 21, and FIG. 3 is an explanatory diagram showing a relationship between an ink discharging operation and a voltage applied to the ink. 3 (A) shows a state where ejection is not performed, and FIG. 3 (B) shows an ejection state.
[0048]
As shown in FIGS. 1 and 2, the recording head 20 is provided with an ultrafine nozzle 21 for discharging an ink droplet of a chargeable ink from the tip. The nozzle 21 is directed to the platen 102. A counter electrode 23 is provided below the nozzle 21 of the recording head 20 so as to face the nozzle 21. The opposing electrode 23 is formed integrally with the platen 102, has an opposing surface facing the tip of the nozzle 21, and supports the base material K on which the ink droplet lands. In addition, the recording head 20 is connected to an ink supply unit that supplies ink to the flow path 22 in the nozzle 21 and an ejection voltage application unit 25 that applies an ejection voltage to the ink in the nozzle 21.
[0049]
The nozzle 21 and a part of the ink supply means and a part of the discharge voltage applying means 25 are integrally formed by a nozzle plate 26.
[0050]
The recording head 20 is a scanning type recording head which can be freely scanned by a driving mechanism (not shown) in a direction orthogonal to the transport direction of the base material K (left direction in FIG. 1). It has. In the present embodiment, each process color ink including a color material such as yellow (Y), magenta (M), cyan (C), and black (K) is supplied to the recording head 20 by an ink tank described later. The recording head 20 ejects each process color ink from each nozzle 21 as a droplet.
[0051]
(nozzle)
The nozzle 21 is formed integrally with a lower surface layer 26c of the nozzle plate 26, which will be described later, and stands vertically from the flat surface of the nozzle plate 26. Further, the nozzle 21 is formed with an in-nozzle flow path 22 penetrating from the tip end thereof along the center line.
[0052]
The nozzle 21 will be described in more detail. As described above, the nozzle 21 is formed with an ultrafine diameter. Specifically, the inner diameter at the tip of the nozzle 21 is 30 μm or less, but may be less than 20 μm, 8 μm or less, or 4 μm or less. To give an example of specific dimensions of each part, the internal diameter of the nozzle internal flow path 22 is 1 [μm], the external diameter at the tip of the nozzle 21 is 2 [μm], and the root diameter of the nozzle 21 is 5 [μm]. μm], and the height of the nozzle 21 is set to 100 μm, and the shape of the nozzle 21 is formed as a truncated cone almost infinitely conical. The entire nozzle 21 is formed of an insulating resin material together with the lower surface layer 26c of the nozzle plate 26.
[0053]
Note that the dimensions of the nozzle are not limited to the above example. In particular, the inner diameter of the nozzle is a range in which an ejection voltage enabling ejection of ink droplets to be less than 1000 [V] due to the effect of electric field concentration described later is, for example, a nozzle diameter of 70 [μm] or less. Desirably, the lower limit value is a diameter of 20 [μm] or less, which is a range in which it is feasible to form a through-hole through which ink passes by the current nozzle forming technology.
[0054]
(Ink supply means)
The ink supply means is provided inside the nozzle plate 26 at a position which is the root of the nozzle 21 and communicates with the ink passage 22 in the nozzle. The ink supply means supplies ink from an external ink tank (not shown) to the ink chamber 24. An ink supply path 27 for guiding the ink and a supply pump (not shown) for applying a supply pressure of the ink to the ink chamber 24 are provided.
Each of the Y, M, C, and A process color inks is stored in the ink tank. The supply pump supplies each process color ink to the tip of the nozzle 21 and does not spill out from the tip. Each process color ink is supplied to the recording head 20 while maintaining the supply pressure in the range (see FIG. 3A).
[0055]
(Ejection voltage applying means)
The ejection voltage application means 25 includes an ejection electrode 28 for applying an ejection voltage provided inside the nozzle plate 26 and at a boundary between the ink chamber 24 and the flow path 22 in the nozzle, and a direct current There is provided a bias power supply 30 for applying a bias voltage, and an ejection voltage power supply 29 for applying a pulse voltage to the ejection electrode 28 which is superimposed on the bias voltage and has a potential required for ejection.
[0056]
The discharge electrode 28 directly contacts the ink inside the ink chamber 24, charges the ink and applies a discharge voltage.
The bias voltage by the bias power supply 30 is always applied within a range where the ink is not ejected, so that the width of the voltage to be applied at the time of ejection is reduced in advance, thereby improving the reactivity at the time of ejection. I have.
[0057]
The ejection voltage power supply 29 applies a pulse voltage superimposed on a bias voltage only when ejecting ink. At this time, the value of the pulse voltage is set so that the superimposed voltage V satisfies the following condition.
[Equation 3]

Where γ: surface tension of ink, ε ₀ : Dielectric constant of vacuum, r: nozzle radius, k: proportional constant (1.5 <k <8.5) depending on the nozzle shape.
In one example, the bias voltage is applied at 300 [V] DC and the pulse voltage is marked at 100 [V]. Therefore, the superimposed voltage at the time of ejection is 400 [V].
[0058]
(Nozzle plate)
The nozzle plate 26 includes a base layer 26a located at the uppermost layer in FIG. 2, a flow path layer 26b forming an ink supply path positioned therebelow, and a lower surface layer formed further below the flow path layer 26b. 26c, and the above-described ejection electrode 28 is interposed between the flow path layer 26b and the upper surface layer 26c.
[0059]
The base layer 26a is formed of a silicon substrate or a highly insulating resin or ceramic, forms a dissolvable resin layer thereon, and removes only a portion that follows the pattern of the supply path 27 and the ink chamber 24, An insulating resin layer is formed on the removed portion. This insulating resin layer becomes the flow path layer 26b. Then, a discharge electrode 28 is formed on the lower surface of the insulating resin layer by plating with a conductive material (for example, NiP), and an insulating resist resin layer is formed thereunder. Since this resist resin layer becomes the lower surface layer 26c, this resin layer is formed with a thickness in consideration of the height of the nozzle 21. Then, the insulating resist resin layer is exposed by an electron beam method or a femtosecond laser to form a nozzle shape. The nozzle flow path 22 is also formed by laser processing. Then, the dissolvable resin layer according to the pattern of the supply path 27 and the ink chamber 24 is removed, and the supply path 27 and the ink chamber 24 are opened to complete the nozzle plate.
[0060]
(Counter electrode)
The opposing electrode 23 has an opposing surface perpendicular to the nozzle 21 and supports the substrate K along the opposing surface. The distance from the tip of the nozzle 21 to the opposing surface of the opposing electrode 23 is set to, for example, 100 [μm].
Further, since the counter electrode 23 is grounded, it always maintains the ground potential. Therefore, at the time of application of the pulse voltage, the ink droplet ejected by the electrostatic force due to the electric field generated between the tip of the nozzle 21 and the opposing surface is guided to the opposing electrode 23 side.
[0061]
The ink jet recording apparatus 20 ejects ink droplets by increasing the electric field strength by concentrating the electric field at the tip of the nozzle 21 due to the ultra-miniaturization of the nozzle 21. Although it is possible to discharge droplets, it is desirable that guidance by electrostatic force be performed between the nozzle 21 and the counter electrode 23. It is also possible to release the charge of the charged ink droplet by grounding the counter electrode 23.
[0062]
(ink)
As the ink, an ink of a type used in an ordinary inkjet recording apparatus (specifically, an aqueous ink, an oil-based ink, a solvent ink, an ultraviolet curable ink, a solid ink, or the like) can be applied. The ink preferably has a colorant concentration of 8 to 90 wt% and does not include particles having a diameter of 0.3 μm or more.
[0063]
More specifically, the ink has a viscosity of 0.1 to 1000 mPa · s (preferably 1 to 100 mPa · s) and a surface tension of 20 to 70 mN / m (preferably 25 to 50 mN / m). Certain inks are applicable. When the viscosity of the ink is less than 0.1 mPa · s or greater than 1000 mPa · s, the ejection of the ink from the nozzle 21 becomes unstable. Further, when the surface tension of the ink is less than 20 mN / m, the ink droplets ejected from the nozzle 21 tend to bleed into the base material K. When the surface tension of the ink is larger than 70 mN / m, each pixel of the base material K cannot be completely filled with the ink droplets discharged from the nozzle 21, and the amount of ink adhering to the base material K per one color is reduced. Must do more. After all, in this case, it is not possible to suppress the conventional adverse effects such as bleeding of the ink on the base material, poor drying of the ink itself, and cockling of the base material due to the ink attached to the base material.
[0064]
When an ultraviolet curable ink is used as the ink, an ultraviolet irradiation unit is provided at a position (for example, in the vicinity of the nozzle 21) at which ink droplets landed on the base material K can be irradiated with ultraviolet light, and each of the ink droplets landed on the base material K is provided. It is necessary to cure each of these ink droplets by irradiating the ink droplets with ultraviolet rays. Examples of the "light source" of the ultraviolet irradiation means include a low-pressure mercury lamp, an ultraviolet laser, a xenon flash lamp, an insect trap, a black light, a germicidal lamp, a cold cathode tube, an LED (Light Emitting Diode), an LED high-pressure mercury lamp, a metal halide lamp, and an excimer. Lamps, electrodeless ultraviolet lamps, and the like are applicable, and light sources other than these may be applied.
[0065]
(Base material)
As the base material K, various papers such as art paper, coated paper, high-quality paper, various fabrics, and various nonwoven fabrics can be applied, and those made of materials such as resin, metal, and glass can also be applied. As the form of the base material K, the recording surface may have a two-dimensional spread such as a roll shape, a cut sheet shape, a plate shape, or the like, or may have a cylindrical shape, a conical shape, a polygonal shape, or the like. The recording surface may have a three-dimensional spread.
[0066]
(Image recording operation by inkjet recording device)
Next, the operation of the inkjet recording apparatus 100 will be described with reference to FIG.
First, by rotating the transport roller 103a, a predetermined base material K is transported to the platen 102 while being sandwiched between the transport roller 103a and the pressing roller 104a.
Then, while the substrate K is supported by the platen 102, the substrate K is intermittently transported by the transport roller 103a and the pressing roller 104a, and the scanning recording head 20 is moved in a direction orthogonal to the transport direction of the substrate K. Each process color ink is ejected from the recording head 20 while scanning is performed, and a desired image is recorded on the base material K. The base material K after image recording is nipped by the transport roller 103b and the pressure contact roller 104b and removed from the platen 102.
[0067]
(Discharge operation of minute ink droplets by recording head)
The operation of the recording head 20 will be described with reference to FIGS.
Ink is supplied to the nozzle flow path 22 by the supply pump of the ink supply means. In this state, a bias voltage is applied to the ink from the bias power supply 30 via the ejection electrode 28. In this state, the ink is charged and a meniscus that is concavely formed by the ink is formed at the tip of the nozzle 21 (FIG. 3A).
When a pulse voltage is applied by the ejection voltage power supply 29, ink is guided to the tip end side of the nozzle 21 by electrostatic force due to the electric field strength of the concentrated electric field at the tip end of the nozzle 21, and the convex meniscus projecting to the outside Is formed, and the electric field is concentrated by the apex of the convex meniscus, and finally, a minute ink droplet is ejected to the counter electrode 23 side against the surface tension of the ink (FIG. 3B).
[0068]
In this case, the ink droplets of 1 to 400 fl per droplet are ejected from the nozzle 21, and the ink adhering amount per color of the ink droplet ejected from the nozzle 21 and landed on the base material K is 0.1 μm. 2 to 5.6 ml / m ² It has become.
[0069]
In the ink jet recording apparatus 100 as described above, since the recording head 20 discharges ink droplets from the nozzle 21 having a small diameter, the electric field is concentrated by the charged ink in the nozzle flow path 22, and the electric field strength is reduced. Enhanced. For this reason, in a nozzle having a structure in which the electric field is not concentrated (for example, an inner diameter of 100 [μm]), the voltage required for the ejection becomes too high, and the ink is ejected by the nozzle with a small diameter, which was virtually impossible to eject. Can be performed at a lower voltage.
[0070]
Further, in the ink jet recording apparatus 100, since the diameter is small, it is possible to easily perform the control for reducing the discharge flow rate per unit time due to the low nozzle conductance, and sufficiently reduce the pulse width without narrowing the pulse width. Ink is ejected by small ink droplets (ink droplets having a droplet amount of 1 to 400 fl per droplet).
[0071]
In addition, since the ejected ink droplets are charged, the vapor pressure is reduced even for minute ink droplets, and by suppressing evaporation, loss of ink droplet mass is reduced and flight is stabilized. This prevents a drop in ink droplet landing accuracy.
[0072]
From these facts, as described above, the amount of ink droplets discharged from the nozzles 21 per droplet can be set to 1 to 400 fl, and the ink droplets discharged from the nozzles 21 land on the base material K. 0.2 to 5.6 ml / m ² Therefore, the diameter of each dot of the ink droplet attached to the base material K can be significantly reduced. Therefore, adverse effects such as bleeding of the ink on the base material K, poor drying of the ink itself, and cockling of the base material K by the ink attached to the base material K, which have been problems in the related art, can be suppressed. A high-definition image with fine dots can be recorded on the base material K. Further, in this case, since ultra-fine dots are formed on the base material K, smooth gradation expression can be performed on the highlight portion and the mixed color portion.
[0073]
In the above embodiment, in order to obtain an electrowetting effect on the nozzle 21, an electrode is provided on the outer periphery of the nozzle 21, or an electrode is provided on the inner surface of the nozzle channel 22, and an insulating film is formed thereon. It may be coated. By applying a voltage to this electrode, the wettability of the inner surface of the nozzle channel 22 can be improved by an electrowetting effect on the ink to which the voltage is applied by the ejection electrode 28, Ink can be smoothly supplied to the flow channel 22, and excellent ejection can be performed, and the response of ejection can be improved.
[0074]
In the above-described embodiment, the ejection voltage application unit 25 always applies the bias voltage and performs the ejection of the ink droplets by using the pulse voltage as a trigger. However, the AC voltage or the continuous rectangular wave is always applied with the amplitude required for the ejection. At the same time, the discharge may be performed by switching the level of the frequency. In order to discharge ink droplets, it is necessary to charge the ink.If the discharge voltage is applied at a frequency higher than the speed at which the ink is charged, the discharge is not performed, and if the frequency is changed to a frequency at which the ink can be charged sufficiently. Discharge is performed. Therefore, when the ejection is not performed, the ejection voltage is applied at a frequency higher than the frequency at which the ink can be ejected, and the ejection of the ink is controlled by performing control to reduce the frequency to a frequency band at which the ejection can be performed only when the ejection is performed. Becomes possible. In such a case, there is no change in the potential itself applied to the ink, so that it is possible to further improve the time responsiveness and thereby to improve the landing accuracy of the ink droplet.
[0075]
Further, in the above-described embodiment, as shown in FIG. 4A, an example of a recording method in which recording is performed while scanning the recording head 20 in a direction orthogonal to the transport direction of the base material K has been described. In the apparatus 100, the recording method shown in FIG. 4A may be changed to each of the following recording methods. FIGS. 4A to 4G schematically show various recording methods.
[0076]
That is, as shown in FIG. 4B, a recording method in which the recording head 20 is intermittently moved in a predetermined direction and the substrate K is reciprocated in a direction orthogonal to the moving direction of the recording head 20 may be used. Further, as shown in FIG. 4C, a line type line head 20a having a large number of nozzles 21 arranged in a direction orthogonal to the direction of transport of the base material K is used instead of the scanning type recording head 20. The recording method to be applied may be used.
[0077]
Further, as shown in FIG. 4D, a cylindrical drum 40 having a base material K wound around the outer surface is disposed so as to face the recording head 20, and the recording head 20 is rotated while rotating the drum 40 in a predetermined direction. A scanning recording method may be used. Further, as shown in FIG. 4E, a recording method in which the recording head 20 in the recording method shown in FIG. 4D is replaced with a line type line head 20a as shown in FIG.
[0078]
Further, as shown in FIG. 4F, a drum-shaped intermediate transfer member 41 rotating in a predetermined direction is arranged between the recording head 20 and the base material K, and the recording medium is directed from the recording head 20 to the intermediate transfer member 41. A recording method in which ink is ejected and the ink attached to the intermediate transfer body 41 is transferred to the base material K may be used. In this case, the intermediate transfer body 41 has a function of transporting each ink droplet ejected from the recording head 20 while holding the ink droplet on its own surface layer, and a function of pressing and transferring each transported ink droplet to the substrate K. . Also, as shown in FIG. 4G, the recording head 20 in the recording method shown in FIG. 4F is replaced with a line type line head 20a as shown in FIGS. 4C and 4E. A recording method may be used.
[0079]
【Example】
[Example 1]
The following aqueous ink A was prepared.
(Aqueous ink A)
Concentration of each color material: 20 wt%
Coloring material (Y): Water Yellow 6 (manufactured by Orient Chemical Industry Co., Ltd.)
Coloring material (M): Chuganol-Fast Red 3B (manufactured by Chugai Kasei Co., Ltd.)
Coloring material (C): Kayara Star Cois-Blue GL (Nippon Kayaku Co., Ltd.)
Coloring material (K): Special black 7984 (manufactured by Bayer)
Ethylene glycol concentration: 20 wt%
Water concentration: 60 wt%
[0080]
Using the aqueous ink A, a predetermined image (drop volume per ink droplet: 100 fl, resolution: 5000 dpi) was recorded on art paper by the inkjet recording device 20. In this case, in the recording area of the art paper, the ink adhesion amount per color in a predetermined area where each pixel is completely filled with ink droplets is 3.9 ml / m. ² Met. In this case, the bleeding of the ink into the art paper and the cockling of the art paper were very slight, and the ink droplets of the ink adhering to the art paper were quickly fixed on the art paper by drying with a dryer.
[0081]
[Comparative Example to Example 1]
The following aqueous ink B was prepared.
(Water-based ink B)
Concentration of each color material: 3.3 wt%
Each color material: same as in Example 1
Ethylene glycol concentration: 20 wt%
Water concentration: 76.7 wt%
[0082]
Then, a predetermined image (drop volume per ink droplet: 2 pl, resolution: 1800 dpi) was recorded on art paper using the aqueous ink B by an ordinary piezo element type ink jet recording device. In this case, in the recording area of the art paper, the ink adhesion amount per color in a predetermined area in which each pixel is completely filled with ink droplets is 10 ml / m2. ² Met. Further, in this case, in the mixed color portion, each ink droplet bleeds into the art paper, cockling of the art paper was confirmed, and a predetermined time was required for drying each ink droplet attached to the art paper.
[0083]
[Example 2]
The following water-based ink C was prepared.
(Aqueous ink C)
Concentration of each color material: 50 wt%
Each color material: same as in Example 1
Water concentration: 50 wt%
[0084]
Then, a predetermined image (drop volume per ink droplet: 15 fl, resolution: 7000 dpi) was recorded on art paper by the inkjet recording device 20 using the aqueous ink C. In this case, in the recording area of the art paper, the ink adhesion amount per color in a predetermined area in which each pixel is completely filled with ink droplets is 1.1 ml / m. ² Met. In this case, neither ink bleeding on the art paper nor cockling of the art paper occurred, and each ink droplet of the ink adhering to the art paper dried immediately.
[0085]
[Example 3]
The inks of the respective colors having the composition ratios shown in Table 1 below were dispersed for 8 hours by a sand grinder, and then the inks of the respective colors were passed through a 0.3 μm membrane filter to prepare an ultraviolet curable ink D.
[0086]
[Table 1]

In Table 1, dispersants: Solsperse 24000, OXT221 manufactured by Avecia, Aronoxetane OXT221, 2021P manufactured by Toagosei Co., Ltd .: Celloxide 2021P manufactured by Daicel, UVI6990: Photoacid generator manufactured by Dow Chemical.
[0087]
Using the ink D, a predetermined image (a droplet amount of each ink droplet: 300 fl, resolution: 3000 dpi) was recorded on a PET (PolyEthylene Terephthalate) film by the inkjet recording device 20. Thereafter, the image is simultaneously irradiated with short-wavelength ultraviolet light (ultraviolet light having a wavelength of about 280 nm or less) and long-wavelength ultraviolet light (ultraviolet light having a wavelength of about 320 to about 400 nm) to cure each ink droplet constituting the image. The film was fixed on a PET film. In this case, in the recording area of the PET film, the ink adhesion amount per color in a predetermined area in which each pixel is completely filled with ink droplets is 5.6 ml / m. ² Met. In this case, there was no bleeding of the ink into the PET film, and further, almost no irregularities on the substrate, which were peculiar to the ordinary ultraviolet curable ink, were observed, and the texture of the image was good.
[0088]
(Measures for realizing stable ejection of low applied voltage and minute ink droplets)
In the present invention, reconsidering the role of the nozzle that plays in the electrostatic suction type inkjet system,
(Equation 4)

That is,
(Equation 5)

Or
(Equation 6)

By using Maxwell force or the like in a region which has not been conventionally attempted to be undischargeable, a fine ink droplet can be formed.
[0089]
An expression that approximately derives ejection conditions and the like for such a drive voltage reduction and a technique for realizing minute ejection is described below.
The following description is applicable to the ink jet recording apparatuses described in the above embodiments of the present invention.
Now, it is assumed that a conductive solution is injected into a nozzle having a radius r, and the nozzle is positioned vertically at a height h from an infinite plate conductor as a base material. This is shown in FIG. At this time, it is assumed that the electric charge induced at the nozzle tip concentrates on the hemisphere at the nozzle tip, and is approximately represented by the following equation.
(Equation 7)

Here, Q: electric charge induced at the tip of the nozzle, ε ₀ : Dielectric constant of vacuum, ε: dielectric constant of substrate, h: distance between nozzle and substrate, r: radius of nozzle inner diameter, V: voltage applied to nozzle. α is a proportionality constant depending on the nozzle shape and the like, and takes a value of about 1 to 1.5, and particularly becomes about 1 when r << h.
[0090]
When the substrate as the base material is a conductive substrate, it is considered that a mirror image charge Q ′ having an opposite sign is induced at a symmetric position in the substrate. When the substrate is an insulator, the image charge Q ′ having the opposite sign is similarly induced at a symmetric position determined by the dielectric constant.
By the way, the electric field strength E at the nozzle tip _loc. Assuming that the radius of curvature at the tip is R,
(Equation 8)

Given by Here, k is a proportional constant, which varies depending on the nozzle shape and the like, but takes a value of about 1.5 to 8.5, and is considered to be about 5 in many cases. (PJ Birdsey and DA Smith, Surface Science, 23 (1970) 198-210). For the sake of simplicity, it is assumed that r = R. This corresponds to a state in which the conductive solution swells in a hemispherical shape having the same radius as the nozzle diameter r at the nozzle tip due to surface tension.
Consider the balance of the pressure acting on the liquid at the nozzle tip. First, assuming that the liquid pressure at the tip of the nozzle is S, the electrostatic pressure is
(Equation 9)

From equations (8), (9) and (10), setting α = 1,
(Equation 10)

It is expressed as
[0091]
On the other hand, if the surface tension of the liquid at the nozzle tip is Ps,
[Equation 11]

Here, γ: surface tension.
The condition under which the ejection of fluid occurs due to the electrostatic force is a condition where the electrostatic force exceeds the surface tension.
(Equation 12)

It becomes. By using a sufficiently small nozzle diameter r, the electrostatic pressure can exceed the surface tension.
When the relationship between V and r is obtained from this relational expression,
(Equation 13)

Gives the lowest voltage for ejection. That is, from Equations (7) and (14),
[Equation 14]

Is the operating voltage of the present invention.
[0092]
FIG. 6 shows the dependence of the discharge limit voltage Vc on the nozzle having a certain radius r. From this figure, it was clarified that, considering the concentration effect of the electric field by the fine nozzle, the discharge start voltage decreases as the nozzle diameter decreases. That is, when the nozzle diameter is less than 20 μm, the electric field can be concentrated, fine droplets can be stably ejected, and the ejection start voltage can be reduced. FIG. 6 shows that when the nozzle diameter is 8 μm or less, the electric field can be further concentrated, fine droplets can be discharged more stably, and the discharge start voltage can be further reduced. Further, the nozzle diameter is preferably 4 μm or less.
In the conventional concept of an electric field, that is, when only the electric field defined by the voltage applied to the nozzle and the distance between the opposing electrodes is considered, the voltage required for ejection increases as the size of the nozzle becomes smaller. On the other hand, if attention is paid to the local electric field strength, the discharge voltage can be reduced by making the nozzle fine.
[0093]
Discharge by electrostatic suction is basically based on charging of fluid at the nozzle end. It is considered that the charging speed is about a time constant determined by dielectric relaxation.
[Equation 15]

Here, ε: relative permittivity of the fluid, σ: conductivity of the fluid. The fluid has a dielectric constant of 10 and a conductivity of 10 ^-6 Assuming S / m, τ = 1.854 × 10 ^-5 sec. Alternatively, if the critical frequency is fc,
(Equation 16)

It becomes. It is considered that no response can be made to a change in the electric field having a frequency earlier than fc and ejection becomes impossible. Estimating the above example results in a frequency of about 10 kHz. At this time, if the nozzle radius is 2 μm and the voltage is slightly less than 500 V, G is 10 ^-13 m ³ / S, but the liquid of the above example can be discharged at 10 kHz, so the minimum discharge amount in one cycle is 10 fl (femtoliter, 1 fl: 10 ^-15 l) degree can be achieved.
[0094]
Each of the above embodiments is characterized by the effect of concentrating the electric field at the tip of the nozzle and the effect of the image force induced on the opposing substrate, as shown in FIG. Thus, there is no need to make the substrate or substrate support conductive or to apply a voltage to these substrates or substrate support as in the prior art. That is, an insulating glass substrate, a plastic substrate such as polyimide, a ceramic substrate, a semiconductor substrate, or the like can be used as the substrate.
In each of the above embodiments, the voltage applied to the electrode may be either positive or negative.
Further, by keeping the distance between the nozzle and the substrate at 500 [μm] or less, the discharge of the solution can be facilitated. Although not shown, feedback control based on nozzle position detection is performed to keep the nozzle constant with respect to the base material.
Further, the base material may be placed and held in a conductive or insulating base material holder.
[0095]
FIG. 7 is a side sectional view of an ink jet recording apparatus as another example of the basic example of the present invention. An electrode 15 is provided on the side surface of the nozzle 1, and a controlled voltage is applied between the nozzle 15 and the ink 3 in the nozzle. The purpose of this electrode 15 is an electrode for controlling the ElectroWetting effect. If a sufficient electric field is applied to the insulator forming the nozzle, the Electrowetting effect is expected to occur without this electrode. However, in this basic example, the role of the ejection control is also achieved by more positively controlling using this electrode. When the nozzle 1 is made of an insulator, has a thickness of 1 μm, an inner diameter of the nozzle of 2 μm, and an applied voltage of 300 V, an Electrowetting effect of about 30 atm is obtained. Although this pressure is insufficient for discharge, it is significant from the viewpoint of supplying the solution to the nozzle tip, and it is considered that discharge can be controlled by this control electrode.
[0096]
FIG. 7 described above shows the nozzle diameter dependence of the ejection start voltage in the present invention. The ink jet recording apparatus shown in FIG. 1 was used. It has been clarified that the discharge start voltage decreases as the nozzle becomes finer, and that discharge can be performed at a lower voltage than in the past.
[0097]
In the above embodiment, the conditions for ink ejection are functions of the distance between nozzle substrates (L), the amplitude of applied voltage (V), and the frequency of applied voltage (f). Is required as a discharge condition. Conversely, if any one of the conditions is not satisfied, it is necessary to change other parameters.
[0098]
This will be described with reference to FIG.
First, for discharge, there is a certain critical electric field Ec in which the discharge is performed only when the electric field is larger than that. The critical electric field varies depending on the diameter of the nozzle, the surface tension of the solution, the viscosity, and the like, and it is difficult to discharge below Ec. Above the critical electric field Ec, that is, at the dischargeable electric field strength, there is a roughly proportional relationship between the distance (L) between the nozzle substrates and the amplitude (V) of the applied voltage, and when the distance between the nozzles is reduced, the critical applied voltage V Can be reduced.
[0099]
Conversely, when the distance L between the nozzle substrates is extremely large and the applied voltage V is increased, even if the same electric field strength is maintained, the burst or burst of the fluid ink droplet is generated by the action of corona discharge or the like. Therefore, in order to obtain good ejection characteristics, it is desirable to keep the distance between the nozzle substrates to about 100 μm or less in terms of both ejection characteristics and landing accuracy.
[0100]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the bleeding of the ink to a base material, the bad drying of the ink itself, the bad effects, such as cockling of the base material by the ink which adhered to the base material, can be suppressed, and the high-definition by a plurality of ultra-fine dots can be suppressed. Images can be recorded.
[Brief description of the drawings]
FIG. 1 is a side view of an ink jet recording apparatus.
FIG. 2 is a sectional view of a recording head and members related to the recording head.
3A and 3B are explanatory diagrams illustrating a relationship between an ink ejection operation and a voltage applied to the ink. FIG. 3A illustrates a state where ejection is not performed, and FIG. 3B illustrates an ejection state. .
FIGS. 4A to 4G are schematic diagrams showing various recording methods.
FIG. 5 is a view for explaining calculation of an electric field intensity of a nozzle.
FIG. 6 shows a relationship between a nozzle diameter of a nozzle, a discharge start voltage at which a droplet discharged from a meniscus portion starts to fly, a voltage value at the Rayleigh limit of the initial discharge droplet, and a ratio of a discharge start voltage to a Rayleigh limit voltage value. FIG. 4 is a diagram showing the relationship.
FIG. 7 is a side sectional view of the ink jet recording apparatus.
FIG. 8 is a diagram illustrating ejection conditions based on a distance-voltage relationship in the inkjet recording apparatus.
[Explanation of symbols]
100 inkjet recording device
20 Recording head
20a line head
21 nozzle
22 Channel
23 Counter electrode
24 ink chamber
25 ejection voltage applying means
26 Nozzle plate
27 Ink supply path
28 ejection electrode
29 Ejection voltage power supply
30 bias power supply
40 drums
41 Intermediate transfer member
102 Platen
103a, 103b transport roller
104a, 104b Press roller
K base material

Claims (10)

An ink jet recording apparatus comprising a nozzle for discharging ink as droplets, and recording a desired image on the substrate by discharging ink droplets from the nozzle to the substrate.
The amount of each ink droplet ejected from the nozzle is 1 to 400 fl, and the amount of ink adhered to a substrate per color is 0.2 to 5.6 ml / m ^2. Ink jet recording device. The inkjet recording apparatus according to claim 1,
The amount of each ink droplet ejected from the nozzle is 1 to 100 fl, and the amount of ink adhered to a base material per color is 0.2 to 2.5 ml / m ^2. Ink jet recording device. The ink jet recording apparatus according to claim 1, wherein
A cylindrical drum is provided so as to face the nozzle, and a base material is wound around the outer surface,
An ink jet recording apparatus for recording a desired image on a substrate by attaching each ink droplet discharged from the nozzle to a substrate wound around the outer surface of the drum. The inkjet recording apparatus according to any one of claims 1 to 3,
An intermediate transfer member arranged to face the nozzle,
An ink jet recording apparatus, wherein each ink droplet discharged from the nozzle adheres to the intermediate transfer member, and each ink droplet adhered to the intermediate transfer member is transferred to a base material. The inkjet recording apparatus according to any one of claims 1 to 4,
A nozzle having an ultra-fine diameter that is disposed with the tip end thereof opposed to a base material having a receiving surface that receives the ejection of the ink droplets of the charged ink and ejects ink droplets from the tip end,
An ink supply unit for supplying ink into the nozzle,
Discharge voltage applying means for applying a discharge voltage to the ink in the nozzle,
An ink jet recording apparatus comprising: An ink used in the inkjet recording apparatus according to any one of claims 1 to 5,
An ink characterized in that the color material has a concentration of 8 to 90 wt% and does not contain particles having a diameter of 0.3 μm or more. The inkjet recording apparatus according to any one of claims 1 to 5,
An ink jet recording apparatus, wherein the nozzle diameter of the nozzle is 30 μm or less. The inkjet recording apparatus according to claim 7,
An ink jet recording apparatus, wherein the nozzle diameter of the nozzle is less than 20 μm. The ink jet recording apparatus according to claim 8,
An ink jet recording apparatus, wherein the nozzle diameter of the nozzle is 8 μm or less. The inkjet recording apparatus according to claim 9,
An ink jet recording apparatus, wherein the nozzle diameter of the nozzle is 4 μm or less.

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