JP2004114370A - Electrostatic attraction fluid jet device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrostatic attraction fluid jet device capable of putting a versatile recorder to practical use while satisfying both high resolution and safety. <P>SOLUTION: A process control section 10 controls a voltage being applied to ink 2 in a nozzle 4 through an electrostatic field applying electrode 9. The nozzle 4 has a shape corresponding to a meniscus equivalent to the forward end part of a Taylor cone being formed during electrostatic attraction process of the ink 2 as a conventional fluid, and the diameter of the ink ejection hole 4b of the nozzle 4 is set substantially equal to the diameter at the forward end part of the meniscus 14 immediately before ink ejection and not larger than ϕ8μm. The process control section 10 controls the voltage being applied to the ink 2 such that the quantity of charges being induced in a liquid drop 3 immediately after ejection from the ink ejection hole 4b does not exceed 90% of the quantity of charges corresponding to Rayleigh limit of the liquid drop 3. <P>COPYRIGHT: (C)2004,JPO

Description

【０１１２】
表１中の記号は、各ノズルの吐出安定性を示しており、×：目詰まり等での不吐出あり、△：連続吐出にて吐出不安定、○：安定吐出である。
【０１１３】
表１から、微粒子径としてはφ３０ｎｍ以下が好ましいことが分かった。特に微粒子径φ１０ｎｍ以下になるとインク中の微粒子１個の帯電量はインク吐出における電荷としての影響がほぼ無視できるとともに、電荷による移動速度も非常に遅くなり微粒子のメニスカス中心への集中も発生しない。また、ノズル径がφ３μｍ以下では、メニスカス部の電界集中により極端に最大電界強度が高くなり、微粒子１個毎の静電力も大きくなるためφ１０ｎｍ以下の微粒子を含んだインクを用いることが好ましい。但し、微粒子径がφ１ｎｍ以下になると、微粒子の凝集及び濃度の不均一の発生が大きくなるため、微粒子径は、φ１ｎｍからφ１０ｎｍの範囲が好ましい。
【０１１４】
本実施の形態においては、平均粒径がφ３ｎｍからφ７ｎｍの間の銀の微粒子を含んだペーストを使用しており、該微粒子には凝集防止のコーティングを施している。
【０１１５】
ここで、ノズル４のノズル径と電界強度との関係について、図３（ａ）（ｂ）〜図８（ａ）（ｂ）を参照しながら以下に説明する。図３（ａ）（ｂ）から図８（ａ）（ｂ）に対応して、ノズル径をφ０．２、０．４、１、８、２０μｍ及び参考として従来にて使用されているノズル径φ５０μｍの場合の電界強度分布を示す。
【０１１６】
ここで、各図において、ノズル中心位置とは、ノズル４のインク吐出孔４ｂのインク吐出面の中心位置を示す。また、各々の図の（ａ）は、ノズルと対向電極との距離が２０００μｍに設定したときの電界強度分布を示し、（ｂ）は、ノズルと対向電極との距離が１００μｍに設定されたときの電界強度分布を示す。なお、印加電圧は、各条件とも２００Ｖと一定にした。図中の分布線は、電界強度が１×１０^６Ｖ／ｍから１×１０^７Ｖ／ｍまでの範囲を示している。
【０１１７】
以下の表２に、各条件下での最大電界強度を示す。
【０１１８】
【表２】

【０１１９】
図３（ａ）（ｂ）〜図８（ａ）（ｂ）から、ノズル径がφ２０μｍ（図７（ａ）（ｂ））以上だと電界強度分布は広い面積に広がっていることが分かった。また、表２から、ノズルと対向電極の距離が電界強度に影響していることも分かった。
【０１２０】
これらのことから、ノズル径がφ８μｍ（図６（ａ）（ｂ））以下であると電界強度は集中するとともに、対向電極の距離の変動が電界強度分布にほとんど影響することがなくなる。したがって、ノズル径がφ８μｍ以下であれば、対向電極の位置精度及び被記録媒体の材料特性のバラツキや厚さバラツキの影響を受けずに安定した吐出が可能となる。ここで、１ｐｌの液滴量のインク２を吐出するには、ノズル径をφ１０μｍにする必要があるので、上記のように、ノズル径が８μｍ以下であれば、液滴量が１ｐｌ以下にすることは可能である。
【０１２１】
次に、上記ノズル４のノズル径とメニスカス部１４の最大電界強度と強電界領域の関係を図９に示す。
【０１２２】
図９に示すグラフから、ノズル径がφ４μｍ以下になると、電界集中が極端に大きくなり最大電界強度を高くすることができるのが分かった。これによって、インクの初期吐出速度を大きくすることができるので、インク（液滴）の飛翔安定性が増すとともに、メニスカス部での電荷の移動速度が増すため吐出応答性が向上する。
【０１２３】
続いて、吐出したインク２の液滴３における帯電可能な最大電荷量について、いかに説明する。液滴３に帯電可能な電荷量は、液滴３のレイリー分裂（レイリー限界）を考慮した以下の（５）式で示される。
【０１２４】
ｑ＝８×π×（ε０×γ×ｒ^３）^２　　　（５）
ここで、ｑはレイリー限界を与える電荷量、ε０は真空の誘電率、γはインクの表面張力、ｒはインク液滴の半径である。
【０１２５】
上記（５）式で求められる電荷量ｑがレイリー限界値に近い程、同じ電界強度でも静電力が強く、吐出の安定性が向上するが、レイリー限界値に近すぎると、逆にノズル４のインク吐出孔４ｂでインク２の霧散が発生してしまい、吐出安定性に欠けてしまう。
【０１２６】
ここで、ノズルのノズル径とメニスカス部で吐出する該ノズル径の約２倍の径の初期吐出液滴が飛翔を開始する吐出開始電圧、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グラフを図１０に示す。
【０１２７】
図１０に示すグラフから、ノズル径がφ０．２μｍからφ４μｍの範囲において、吐出開始電圧とレイリー限界電圧値の比が０．６を超え、液滴の帯電効率が良い結果となっており、該範囲において安定した吐出が行えることが分かった。
【０１２８】
例えば、図１１に示すノズル径とメニスカス部の強電界（１×１０^６Ｖ／ｍ以上）の領域の関係で表されるグラフでは、ノズル径がφ０．２μｍ以下になると電界集中の領域が極端に狭くなることが示されている。このことから、吐出する液滴は、加速するためのエネルギーを十分に受ける事ができず飛翔安定性が悪くなることを示す。よって、ノズル径はφ０．２μｍより大きいく設定する必要がある。
【０１２９】
次に、上記構成のインクジェット装置を実際に駆動する場合の印加電圧、すなわち液滴の吐出開始電圧以上の電圧で最適な電圧値を変動した場合の最大電界強度から誘導されるメニスカス部の初期吐出液滴を一定とした場合の該液滴の電荷量と、液滴の表面張力からくるレイリー限界値との関係を図１２のグラフに示す。
【０１３０】
図１２に示すグラフにおいて、Ａ点は上記液滴の電荷量と液滴の表面張力からくるレイリー限界値との交点であり、インクへの印加電圧が、Ａ点より高い電圧であれば、初期吐出液滴にはほぼレイリー限界に近い最大電荷量が形成されており、Ａ点より低い電圧であればレイリー限界以下でかつ吐出に必要な電荷量が形成されていることを示している。
【０１３１】
ここで、吐出液滴の運動方程式にのみ着目すると、強電界かつ最大電荷量の吐出エネルギーとして最適な条件での飛翔が行われるため、印加電圧としてはＡ点より高い電圧が好ましい。
【０１３２】
ところで、図１３に、環境湿度を５０％とした場合のインク（ここでは純水）の初期吐出液滴径と乾燥時間（液滴の溶剤が全て蒸発してしまう時間）との関係を示すグラフを示す。このグラフから、初期吐出液滴径が小さい場合には、蒸発によるインクの液滴径の変化が非常に早く、飛翔中の短い時間においても乾燥が進んでしまうことが分かる。
【０１３３】
このため、初期吐出時に最大電荷量が液滴に形成されていると乾燥による液滴径の減少すなわち電荷が形成されている液滴の表面積が減少することにより、インクの飛翔中にレイリー分裂が発生し、過分の電荷を放出する際に電荷は液滴の一部を引き連れて放出されるため、蒸発以上の飛翔液滴の減少が発生することなる。
【０１３４】
従って、着弾時の液滴径のバラツキ及び着弾精度が悪化するとともに、ノズルと被記録媒体中に分裂したミストが浮遊することになり、被記録媒体を汚染することになる。このため、安定した吐出ドットの形成を考慮すると、初期吐出液滴に誘導される電荷量をレイリー限界に相当する電荷量よりもある程度小さくする必要がある。この場合、該電荷量をレイリー限界値に相当する電荷量の９５％程度では、着弾ドット径のバラツキの精度が向上できず、結果的として９０％以下にすることが好ましい。
【０１３５】
具体的な数値としては、ノズル孔径を針電極の先端形状と見なした場合のメニスカスの最大電界強度による初期吐出液滴径のレイリー限界を算出し、該算出値以下の範囲とすることにより着弾時の液滴のバラツキを抑えることができた。これは、吐出液滴が分離する直前の表面積が吐出直後の液滴に比べ小さく、かつ電荷の移動時間のタイムラグにより、実際の初期吐出液滴に誘導される電荷量は、上記計算により求められる電荷量より小さくなっているためと考えられる。
【０１３６】
このような条件であれば、飛翔時のレイリー分裂を防げると共にメニスカス部での吐出液滴の分離時に電荷量が多いことによるミスト化等の安定吐出を軽減する事ができる。
【０１３７】
なお、帯電した液滴は、蒸気圧が減少して蒸発しにくくなる。これは、以下の（６）式から分かる。
【０１３８】
ＲＴρ／Ｍ×ｌｏｇ（Ｐ／Ｐ０）＝２γ／ｄ−ｑ^２／（８πｄ^４）　　（６）
ここで、Ｒは気体定数、Ｍは気体の分子量、Ｔは気体の温度、ρは気体の密度、Ｐは微小液滴での蒸気圧、Ｐ０は平面での蒸気圧、γはインクの表面張力、ｄはインク液滴の半径である。
【０１３９】
上記の（６）式に示されるように、帯電した液滴は、該液滴の帯電量により蒸気圧が減少するもので、帯電量が少なすぎると蒸発の緩和に影響が少ないため、レイリー限界に相当する電界強度及び電圧値の６０％以上が好ましい結果となった。この結果は、上記と同様にノズル孔径を針電極の先端形状と見なした場合のメニスカスの最大電界強度による初期吐出液滴径のレイリー限界を算出し、該算出値の０．８倍以上の範囲を示すことと同じである。
【０１４０】
特に、図１３に示すように、初期吐出液滴径がφ５μｍ以下になると乾燥時間は極端に短くなり蒸発の影響を受けやすくなるため、初期吐出液滴の電荷量を低く抑えることは蒸発を抑える観点からより効果があることが分かる。なお、図１３に示す乾燥時間と初期吐出液滴径との関係を求める場合の周囲湿度は５０％とした。
【０１４１】
また、吐出液滴の乾燥を考慮すると、被記録媒体までの液体の吐出時間を短くする必要がある。
【０１４２】
ここで、吐出液滴がメニスカス部より分離してノズルより被記録媒体に着弾するまでの平均飛翔速度を５ｍ／ｓ、１０ｍ／ｓ、２０ｍ／ｓ、３０ｍ／ｓ、４０ｍ／ｓ、５０ｍ／ｓとして、吐出の安定性と着弾ドットの位置精度を比較し、以下の表３に示す。
【０１４３】
【表３】

【０１４４】
表３中の吐出安定性の記号においては、×：ほとんど吐出せず、△：連続吐出にて不吐出あり、○：不吐出なしを示しており、着弾精度の記号においては、×：着弾ズレ＞着弾ドット径、△：着弾ズレ＞着弾ドット径×０．５、○：着弾ズレ＜着弾ドット径×０．５、◎：着弾ズレ＜着弾ドット径×０．２を示している。
【０１４５】
上記の表３から分かるように、平均飛翔速度５ｍ／ｓでは、着弾精度が悪く、吐出安定性も悪くなる。特に、ノズル径がφ１μｍ以下では、吐出速度が遅いと液滴にかかる空気抵抗の要因が大きくかつ蒸発によるドット径の更なる微少化により、着弾できない場合があった。逆に、平均飛翔速度５０ｍ／ｓでは、印加電圧を高くする必要があるため、メニスカス部での電界強度が非常に強くなり、吐出液滴のミスト化が頻繁に発生してしまい、安定した吐出が難しいことが分かった。
【０１４６】
以上のことから、吐出液滴がメニスカス部より分離して被記録媒体に着弾するまでの平均飛翔速度は１０ｍ／ｓから４０ｍ／ｓの間が好ましいことが分かった。
【０１４７】
ところで、図１３では、周囲湿度として５０％とした場合の、初期吐出液滴径と乾燥時間との関係を示したが、図１４では、初期吐出液滴径がφ０．５μｍでノズルと被記録媒体の距離を０．２ｍｍとした場合の周囲湿度と乾燥時間の関係を示す。
【０１４８】
図１４に示すグラフから、周囲湿度が６０％以下では該乾燥速度の数値は大きく変動しないことが分かった。しかしながら、周囲湿度が７０％を超えるとインクの蒸発を極端に抑える事が可能であり、周囲湿度を７０％以上とする場合には、上記条件等の影響は低いものとなり、特に周囲湿度を９５％以上に設定すると乾燥の影響をほぼ無視する事ができ、本発明のインクジェット装置の設計条件の自由度を広くかつ適用範囲を広げる事が可能であることが分かった。
【０１４９】
ここで、ノズル径をφ１及びφ３μｍとして、初期吐出液滴径を変動した場合の吐出安定性及び吐出ドット径バラツキ（着弾バラツキ）を以下の表４に示す。なお、ノズルによる初期吐出径は、印加電圧値を変動することにより制御可能であり、又印加する電圧パルスのパルス幅を調整する事によっても制御可能であり、ここでは、同一ノズル径での電界強度の影響を排除するため、前記パルス幅を変動させて初期吐出径を調整している。
【０１５０】
【表４】

【０１５１】
表４中の吐出安定性の記号において、×：ほとんど吐出せず、△：１０分間連続吐出にて不吐出あり、○：１０分間連続吐出にて不吐出なし、◎：３０分間連続吐出にて不吐出なしを示しており、バラツキの記号においては、△：着弾ドットのバラツキ＞着弾ドット径×０．２、○：着弾ドットのバラツキ≦着弾ドット径×０．２、◎：着弾ドットのバラツキ≦着弾ドット径×０．１を示している。
【０１５２】
表４から、ノズル径に対し１．５倍〜３倍程度において吐出の安定性が良く、特に１．５倍〜２倍において着弾ドット径のバラツキが極端に抑えられることが分かった。これは、メニスカス部から引き出されるインク形状を液柱と見なした場合、該液柱の表面積が該液柱の体積分の球の表面積より大きくなる条件での液滴分離が最も安定するためと考えられる。
【０１５３】
上記の構成によれば、インクの吐出直後の液滴量が１ｐｌ以下の微少なインク液滴を吐出する静電吸引型インクジェット装置において、ノズル４のインク吐出孔４ｂの直径を、インクの吐出直後の液滴直径と同等以下とすることによりノズル４のメニスカス部１４に吐出のための電界を集中させることができるので、インクを吐出するのに必要な印加電圧を大幅に下げることができ、個々に分離、吐出する液滴の径のバラツキを小さく安定した吐出を実現可能とした。
【０１５４】
また、従来必要とされていたバイアス電圧の印加が不要となり、駆動電圧を正負交互に印加する事が可能となり、被記録媒体の表面電位の増加による着弾精度への影響を軽減する事ができた。
【０１５５】
また、ノズルの孔の直径をφ８μｍ以下の範囲とすることによりノズルのメニスカス部に電界を集中させることができると共に、対向電極の位置精度及び被記録媒体の材料特性のバラツキや厚さバラツキの影響を受けずに安定した吐出が可能となった。
【０１５６】
特に、ノズル４のインク吐出孔４ｂの直径をφ０．２μｍ以上φ４μｍ以下の範囲とすることにより、電界集中が極端に大きくなる。このように、最大電界強度を高くすることが、インクの初期吐出速度を大きくすることになるので、飛翔安定性が増すとともに、メニスカス部での電荷の移動速度が増すため吐出応答性が向上すると共に、レイリー分裂の影響による着弾ドット径のバラツキを抑える事ができる。
【０１５７】
更に、ノズル４からのインクの吐出直後の液滴直径を、ノズル４のインク吐出孔４ｂの直径の１．５倍から３倍以下の範囲とすることにより、吐出の安定性が向上でき、特にインクの吐出直後の液滴直径を該ノズル径の１．５倍から２倍以下の範囲とすることにより吐出ドット径のバラツキを極端に抑えることができる。
【０１５８】
本実施の形態では、上述のように、インク室１内のインクに負圧が印加された例について説明したが、インクに正圧が印加された場合でも構わない。インク室１内のインクに正圧を印加するには、例えば、図１５に示すように、インク供給路６の図示しないインクタンク側にポンプ１２を設け、該ポンプ１２を用いてインク室１内のインクに正圧を印加することが考えられる。この場合、インク室１からのインク吐出のタイミングに合わせて駆動させるようにプロセス制御部１３を用いて上記ポンプ１２を駆動制御すればよい。このように、インク室１内のインクに正圧を印加するようにすれば、メニスカス部の凸形状を静電力で形成する手間が省け、印加電圧の低減及び応答速度の向上が図ることができる。
【０１５９】
なお、本実施の形態では、説明の簡単化のため単一ノズルを備えたインクジェット装置について説明を行ったが、これに限定されるものではなく、隣接ノズルでの電界強度の影響を考慮した設計を行えば、複数のノズルを有するマルチヘッドを備えたインクジェット装置にも適用可能である。
【０１６０】
更に、本実施の形態では、図１および図１５に示すように、対向電極７を常に設けたインクジェット装置について説明したが、表２から分かるように、対向電極７とノズル４のインク吐出孔４ｂとの間の距離（ギャップ）は、被記録媒体とノズル間の電界強度にほとんど影響せず、該被記録媒体とノズル間の距離が近く、被記録媒体の表面電位が安定しているならば対向電極は不要となる。
【０１６１】
【発明の効果】
本発明の静電吸引型流体ジェット装置は、以上のように、電圧印加により帯電された流体を、絶縁材料からなるノズルの流体吐出孔から静電吸引により液滴の状態で吐出させる静電吸引型流体ジェット装置において、上記ノズル内の流体に印加される電圧を制御する印加電圧制御手段を備え、該ノズルの流体吐出孔の直径が、φ８μｍ以下に設定され、上記印加電圧制御手段は、上記流体吐出孔から吐出した直後の流体の液滴に誘導される電荷量が、該液滴のレイリー限界に相当する電荷量の９０％以下になるように、上記流体に印加する電圧を制御する構成である。
【０１６２】
それゆえ、従来の流体の静電吸引の過程において、従来のノズルの流体吐出孔の直径よりも小さな液滴直径の流体を吐出するために形成されるテーラーコーン形状の電荷が集中した先端部の直径と略同一に本発明ではノズル径を設定することで、広範囲に必要であった電場の形成を狭くすることができる。
【０１６３】
以上のことから、電荷の移動に必要な電圧、すなわち流体を静電吸引させるのに必要な帯電量を該流体に付与するために必要な電圧を大幅に低減させることが可能となる。これにより、従来のように、２０００Ｖといった高電圧を必要としないので、流体ジェット装置を使用する際の安全性の向上を図ることができる。
【０１６４】
しかも、ノズルの流体吐出孔の直径が、φ８μｍ以下に設定されていることで、電界強度分布が該流体吐出孔の吐出面近傍に集中すると共に、対向電極からノズルの流体突出孔までの距離の変動が電界強度分布に影響することがなくなる。
【０１６５】
これにより、対向電極の位置精度、被記録媒体の材料特性のバラツキや厚さバラツキの影響を受けずに安定した流体の吐出を行うことができる。
【０１６６】
また、上記のように、電場を狭くできることにより、狭い領域に強い電場を形成することが可能となり、この結果、形成できる液滴を微小なものにすることが可能となる。これにより、液滴をインクとした場合に、印字画像を高解像度にすることが可能となる。
【０１６７】
さらに、上述のように、電荷の集中領域と流体のメニスカス領域とがほぼ同等のサイズになることから、メニスカス領域内での電荷の移動時間が吐出応答性に影響を与えることがなくなり、液滴の吐出速度（液滴がインクである場合の印字速度）の向上を図ることが可能となる。
【０１６８】
また、電荷の集中領域と流体のメニスカス領域とがほぼ同等のサイズになることから、広い範囲のメニスカス領域に強い電場を形成する必要がなくなる。これにより、従来のように、広い範囲のメニスカス領域に強い電場を形成するために対向電極の配置を精度良く行う必要がなくなり、且つ、被記録媒体の誘電率及び厚さが対向電極の配置に影響しなくなる。
【０１６９】
したがって、静電吸引型流体ジェット装置において、対向電極の配置に対する自由度が増大する。つまり、静電吸引型流体ジェット装置の設計の自由度が増大する。この結果、誘電率や厚みに影響されず、従来、使用が困難であった被記録媒体に対して印字することが可能となり、汎用性の高い流体ジェット装置を実現することができる。
【０１７０】
よって、上記の構成の静電吸引型流体ジェット装置によれば、高解像度と安全性の両方を満足させ、汎用性の高い装置を実現することができるという効果を奏する。
【０１７１】
しかも、上記印加電圧制御手段は、上記流体吐出孔から吐出した直後の流体の液滴に誘導される電荷量が、該液滴のレイリー限界に相当する電荷量の９０％以下になるように上記流体に印加する電圧を制御することで、吐出した液滴の乾燥での液滴表面積の現象による放電を防ぐと共に、液滴の帯電による蒸気圧の減少を防ぐことができる。
【０１７２】
これにより、吐出した液滴の乾燥時間（液滴の溶剤が全て蒸発するまでの時間）の減少を低減させることができるので、着弾した液滴のドット径のサイズのバラツキを無くすことができる。
【０１７３】
また、吐出した液滴の乾燥時間が長くなるので、液滴が着弾するまでの間に液滴の直径、すなわち液滴量の変化を少なくすることができる。これにより、飛翔中の液滴が受ける空気抵抗や周囲湿度等の環境条件は各液滴で均一になるので、液滴の着弾精度の向上を図ること、すなわち着弾時の液滴のバラツキを抑えることができる。
【０１７４】
さらに、吐出した液滴の乾燥時間が長くなるので、吐出液滴の直径がφ５μｍ程度の微小な液滴であってもであっても、液滴を乾燥せずに着弾させることが可能となる。
【０１７５】
よって、上記構成の静電吸引型流体ジェット装置を使用すれば、微小な液滴を安定して吐出することができると共に、高精度で着弾させることができるという効果を奏する。
【０１７６】
上記印加電圧制御手段は、上記流体吐出孔から吐出した直後の流体の液滴に誘導される電荷量が、該液滴のレイリー限界に相当する電荷量の６０％以上になるように、上記流体に印加する電圧を制御する。
【０１７７】
一般に、帯電した液滴は、該液滴の表面に帯電した電荷量（帯電量）により蒸気圧が減少するので、帯電量が少なすぎると蒸発の緩和に影響を及ぼさなくなる。具体的には、液滴のレイリー限界に相当する電荷量の６０％よりも少ない電荷量の場合に、液滴の蒸発の緩和に影響を及ぼさなくなるという効果を奏する。
【０１７８】
上記ノズルの流体吐出孔の直径は、φ５μｍ以下に設定することが好ましく、さらに、上記ノズルの流体吐出孔の直径は、φ０．２μｍ以上φ４μｍ以下に設定することが好ましい。
【０１７９】
この場合、ノズルの流体吐出孔の直径を、φ５μｍ以下に設定することで、電界強度が集中し、電界集中が極端に大きくなり最大電界強度を高くすることができ、この結果、液滴の帯電効率を良くすることができる。さらに、液滴の帯電効率を良くするには、ノズルの流体吐出孔の直径を、φ０．２μｍ以上φ４μｍ以下に設定すればよい。この場合、電界集中が極端に大きくなり最大電界強度を高くすることができ、この結果、直径の小さな微小な液滴を安定して吐出することが可能となるという効果を奏する。
【図面の簡単な説明】
【図１】本発明の一実施の形態にかかるインクジェット装置の概略構成断面図である。
【図２】（ａ）〜（ｃ）は、図１に示すインクジェット装置におけるインクのメニスカスの挙動を説明する図である。
【図３】（ａ）は、ノズルと対向電極との距離が２０００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフであり、（ｂ）は、ノズルと対向電極との距離が１００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフである。
【図４】（ａ）は、ノズルと対向電極との距離が２０００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフであり、（ｂ）は、ノズルと対向電極との距離が１００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフである。
【図５】（ａ）は、ノズルと対向電極との距離が２０００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフであり、（ｂ）は、ノズルと対向電極との距離が１００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフである。
【図６】（ａ）は、ノズルと対向電極との距離が２０００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフであり、（ｂ）は、ノズルと対向電極との距離が１００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフである。
【図７】（ａ）は、ノズルと対向電極との距離が２０００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフであり、（ｂ）は、ノズルと対向電極との距離が１００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフである。
【図８】（ａ）は、ノズルと対向電極との距離が２０００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフであり、（ｂ）は、ノズルと対向電極との距離が１００μｍのときのノズル中心からの距離と対向電極からの距離との関係を示すグラフである。
【図９】ノズル径と最大電界強度との関係を示すグラフである。
【図１０】ノズル径と各種電圧との関係示すグラフである。
【図１１】ノズル径と強電界領域との関係を示すグラフである。
【図１２】印加電圧と帯電電荷量との関係を示すグラフである。
【図１３】初期吐出液滴径と乾燥時間との関係を示すグラフである。
【図１４】周囲湿度と乾燥時間との関係を示すグラフである。
【図１５】本発明の他の実施の形態にかかるインクジェット装置の概略構成断面図である。
【図１６】本発明の原理を説明する図である。
【図１７】従来の静電吸引型インクジェット装置の概略構成断面図である。
【図１８】（ａ）〜（ｃ）は、図１７に示すインクジェット装置におけるインクのメニスカスの挙動を説明する図である。
【図１９】従来の他の静電吸引型インクジェット装置の概略構成図である。
【図２０】図１９に示すインクジェット装置のノズル部分の概略断面斜視図である。
【図２１】図１９に示すインクジェット装置のインク吐出原理を説明する図である。
【図２２】図１９に示すインクジェット装置のノズル部分での電圧印加時における微粒子の状態を説明する図である。
【図２３】図１９に示すインクジェット装置のノズル部分における微粒子体形成の原理を説明する図である。
【図２４】（ａ）〜（ｃ）は、図１９に示すインクジェット装置におけるインクのメニスカスの挙動を説明する図である。
【符号の説明】
１　　インク室
２　　インク（流体）
３　　液滴
４　　ノズル
４ａ　先端部
４ｂ　インク吐出孔（流体吐出孔）
５　　パッキン
６　　インク供給路
７　　対向電極
８　　被記録媒体
９　　静電界印加用電極
１０　プロセス制御部（印加電圧制御手段）
１２　ポンプ
１３　プロセス制御部
１４　メニスカス部
１４ａ　メニスカス
１４ｂ　メニスカス
１４ｃ　メニスカス[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electrostatic suction type fluid jet device that discharges a fluid onto an object by charging and electrostatically suctioning a fluid such as ink.
[0002]
[Prior art]
In general, there are various types of fluid jet systems for discharging a fluid such as ink onto an object (recording medium). Here, an ink jet method using ink as a fluid will be described.
[0003]
As the on-demand type ink jet system, a piezo system using a piezoelectric phenomenon, a thermal system using a film boiling phenomenon of an ink, an electrostatic suction system using an electrostatic phenomenon, and the like have been developed. The demand for the ink jet system is increasing. In order to realize high-resolution inkjet recording, it is essential to miniaturize the ejected ink droplets.
[0004]
Here, the behavior until the ink droplet discharged from the nozzle lands on the recording medium is as follows.
ρink · (4/3 · π · d ³ ) · Dv / dt
= − Cd · (1/2 · ρair · v ² ) ・ (Π ・ d ^2/4 ) ・ ・ ・ ・ ・ (1)
Can be represented by the equation of motion (Equation (1)).
[0005]
Ρink is the volume density of the ink, V is the droplet volume, v is the droplet velocity, Cd is the drag coefficient, ρair is the air density, d is the ink droplet radius, and Cd is
Cd = 24 / Re. (1 + 3 / 16.Re ^0.62 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (2)
It can be represented by the following equation (2).
[0006]
Re is the Reynolds number, η is the viscosity of air,
Re = 2 · d · ρink · v / η (3)
It can be represented by the following equation (3).
[0007]
The effect of the droplet radius on the kinetic energy of the ink droplet on the left side of the above equation (1) is greater than the effect of the droplet radius on the viscous resistance of air. For this reason, in the case of the same speed, the smaller the droplet, the faster the droplet speed is decelerated, and the droplet cannot reach the recording medium separated by a predetermined distance, or the landing accuracy is poor even if the recording medium is reached.
[0008]
To prevent this, it is necessary to increase the initial discharge speed of the droplets, that is, to increase the discharge energy per unit volume.
[0009]
However, the conventional piezo-type and thermal-type ink jet heads have the following problems when the discharge droplets are miniaturized, that is, when the discharge energy per unit volume of the discharge droplets is increased. It was particularly difficult to make the following, that is, the diameter of the droplet (hereinafter, referred to as the droplet diameter) 10 mm or less.
[0010]
Problem (A): The ejection energy of the piezo-type inkjet head is related to the displacement amount and the generated pressure of the driven piezoelectric element. The amount of displacement of the piezoelectric element is closely related to the amount of ink ejected, that is, the size of the ink droplet. In order to reduce the size of the droplet, it is necessary to reduce the amount of displacement. Is difficult to improve.
[0011]
Problem (B): In the thermal type ink jet head, since the film boiling phenomenon of the ink is used, the pressure at the time of bubble formation has a physical limit, and the discharge energy is almost determined by the area of the heating element. The area of the heating element is substantially proportional to the volume of the generated bubble, that is, the ink ejection amount. For this reason, if the size of the ink droplet is reduced, the volume of the generated bubble is reduced, and the discharge energy is reduced. Therefore, it is difficult to improve the discharge energy per unit volume of the ink discharge droplet.
[0012]
Problem (C): In both the piezo method and the thermal method, since the driving amount of the driving (heating) element is closely related to the ejection amount, it is very difficult to suppress the variation particularly when a small droplet size is ejected. .
[0013]
Therefore, as a method for solving each of the above problems, a method of discharging micro droplets by an electrostatic suction method has been developed.
[0014]
In the electrostatic suction method, the equation of motion of the ink droplet ejected from the nozzle is expressed by the following equation (4).
[0015]
ρink · (4/3 · π · d ³ ) · Dv / dt
= Q · E-Cd · (1/2 · pair · v ² ) ・ (Π ・ d ^2/4 ) ・ ・ ・ ・ ・ (4)
Here, q is the charge amount of the droplet, and E is the intensity of the surrounding electric field.
[0016]
According to the above formula (4), in the electrostatic suction method, the ejected droplet receives an electrostatic force during flight separately from the ejection energy, so that the ejection energy per unit volume can be reduced, and Application to ejection becomes possible.
[0017]
For example, Japanese Patent Application Laid-Open No. 8-238774 (Patent Document 1) discloses an electrostatic suction type inkjet device (hereinafter referred to as an electrostatic suction type inkjet device) in which an electrode for applying a voltage is provided inside a nozzle. An inkjet device provided is disclosed. Further, Japanese Patent Application Laid-Open No. 2000-127410 (Patent Document 2) discloses an ink jet apparatus that uses a nozzle as a slit, has a needle electrode protruding from the nozzle, and discharges ink containing fine particles.
[0018]
The ink jet device disclosed in Patent Document 1 will be described below with reference to FIG. FIG. 17 is a schematic cross-sectional view of the inkjet apparatus.
[0019]
17, 101 is an ink ejection chamber, 102 is ink, 103 is an ink chamber, 104 is a nozzle hole, 105 is an ink tank, 106 is an ink supply path, 107 is a rotating roller, 108 is a recording medium, and 110 is a control element. Reference numeral 111 denotes a process control unit.
[0020]
Further, reference numeral 114 denotes an electrode portion for applying an electrostatic field disposed on the ink chamber 103 side of the ink ejection chamber 101; 115, a counter electrode portion which is a metal drum provided on the rotating roller 107; A bias power supply for applying a negative voltage of 1,000 V. Reference numeral 117 denotes a high-voltage power supply for supplying a high voltage of several hundred volts to the electrostatic field applying electrode 114, and reference numeral 118 denotes a ground.
[0021]
Here, between the electrostatic field applying electrode section 114 and the counter electrode section 115, the bias power supply section 116 having a negative voltage of several thousand V applied to the counter electrode section 115 and the high voltage power supply section 117 having a voltage of several hundred V are applied. The high voltage is superimposed to form a superimposed electric field, and the superimposed electric field controls the ejection of the ink 102 from the nozzle holes 104.
[0022]
Reference numeral 119 denotes a convex meniscus formed in the nozzle hole 104 by a bias voltage of several thousand volts applied to the counter electrode 115.
[0023]
The operation of the electrostatic suction type inkjet apparatus configured as described above will be described below.
[0024]
First, the ink 102 is transferred to the nozzle hole 104 for discharging the ink 102 along the ink supply path 106 by a capillary phenomenon. At this time, the opposing electrode unit 115 on which the recording medium 108 is mounted is arranged to face the nozzle hole 104.
[0025]
The ink 102 reaching the nozzle hole 104 forms a convex ink meniscus 119 by a bias voltage of several thousand volts applied to the counter electrode 115. By applying a signal voltage from the high-voltage power supply unit 117 of several hundred volts to the electrostatic field application electrode unit 114 disposed in the ink chamber 103, the voltage from the bias power supply unit 116 applied to the counter electrode unit 115 is changed. The ink 102 is superimposed and discharged by the superposed electric field onto the recording medium 108 to form a print image.
[0026]
The behavior of the meniscus until the droplet flies in the ink jet device disclosed in Patent Document 1 will be described below with reference to FIGS. 18 (a) to 18 (c).
[0027]
Before the drive voltage is applied, as shown in FIG. 18A, a state in which a raised meniscus 119a is formed on the ink surface due to the balance between the electrostatic force due to the bias voltage applied to the ink and the surface tension of the ink. It has become.
[0028]
When the driving voltage is applied in the above state, as shown in FIG. 18B, the meniscus 119b causes the charge generated on the liquid surface to start to approach the center of the rising of the liquid surface, whereby the center of the rising of the liquid surface becomes A raised meniscus 119b is formed.
[0029]
Thereafter, when the driving voltage is continuously applied, as shown in FIG. 18 (c), the charges generated on the liquid surface are further concentrated at the center, so that a meniscus 119c having a semilunar shape called a Taylor cone is formed. Droplets are separated and ejected when the electrostatic force due to the amount of charge concentrated on the top of the cone exceeds the surface tension of the ink.
[0030]
Next, the ink jet device disclosed in Patent Document 2 will be described below with reference to FIG. FIG. 19 is a schematic configuration diagram of the ink jet device.
[0031]
As shown in FIG. 19, a line type recording head 211 made of a low dielectric material (acrylic resin, ceramics, etc.) as an inkjet head, and ink ejection ports of the recording head 211 are provided inside the housing of the inkjet apparatus. A counter electrode 210 made of metal or high dielectric material, an ink tank 212 for storing ink in which charged pigment particles are dispersed in a non-conductive ink medium, an ink tank 212, and a recording head An ink circulating system (pumps 214a and 214b, pipes 215a and 215b) for circulating ink between the discharge electrodes 211 and 211, and a pulse voltage for drawing ink droplets forming one pixel of a recording image is applied to each ejection electrode 211a. A driving circuit for controlling the pulse voltage generator 213 in accordance with the image data; A recording medium transport mechanism (not shown) for passing the recording medium A through a gap provided between the recording head 211 and the counter electrode 210, a controller (not shown) for controlling the entire apparatus, and the like. Have been.
[0032]
The ink circulation system includes two pipes 215a and 215b connecting the recording head 211 and the ink tank 212, and two pumps 214a and 214b driven by control of a controller.
[0033]
The ink circulation system is divided into an ink supply system for supplying ink to the recording head 211 and an ink collection system for collecting ink from the recording head 211.
[0034]
In the ink supply system, ink is sucked up from the ink tank 212 by the pump 214a, and is pumped to the ink supply unit of the recording head 211 via the pipe 215a. On the other hand, in the ink recovery system, ink is sucked from the ink recovery section of the recording head 211 by the pump 215b, and is forcibly recovered to the ink tank 212 via the pipe 215b.
[0035]
As shown in FIG. 20, the recording head 211 has an ink supply section 220a for expanding the ink fed from the pipe 215a of the ink supply system to a line width, and an ink flow for guiding the ink from the ink supply section 220a in a mountain shape. A passage 221, an ink recovery section 220 b connecting the ink flow path 221 to the pipe 215 b of the ink recovery system, and a slit-shaped ink having an appropriate width (about 0.2 mm) for opening the top of the ink flow path 221 to the counter electrode 210 side. The ejection port 222, a plurality of ejection electrodes 211a arranged in the ink ejection port 222 at a predetermined pitch (about 0.2 mm), and low-dielectric (for example, ceramic) disposed on both sides and the upper surface of each ejection electrode 211a, respectively. ) Partition wall 223 is provided.
[0036]
Each of the ejection electrodes 211a is formed of a metal such as copper or nickel, and a low dielectric film (for example, a polyimide film) having good wettability for preventing pigment adhesion is formed on the surface thereof. The tip of each discharge electrode 211a is formed in a triangular pyramid shape, and each protrudes from the ink discharge port 222 toward the counter electrode 210 by an appropriate length (70 μm to 80 μm).
[0037]
When the above-described drive circuit (not shown) supplies a control signal to the pulse voltage generator 213 for a time corresponding to the grayscale data included in the image data in accordance with the control of the controller, the pulse voltage generator 213 A high voltage signal obtained by applying a pulse top pulse Vp according to the type of the control signal to the bias voltage Vb is superimposed on the bias voltage Vb and output.
[0038]
When the image data is transferred, the controller drives the two pumps 214a and 214b of the ink circulation system. As a result, the ink is fed from the ink supply unit 220a under pressure, and the ink recovery unit 220b becomes negative pressure, so that the ink flowing through the ink flow path 221 climbs up the gap between the partition walls 223 by capillary action, and It spreads even to the tip of 211a. At this time, since a negative pressure is applied to the ink liquid surface near the tip of each ejection electrode 211a, an ink meniscus is formed at the tip of each ejection electrode 211a.
[0039]
Further, by controlling the recording medium transport mechanism by the controller, the recording medium A is sent in a predetermined direction, and by controlling the driving circuit, the above-described high voltage signal is applied to the ejection electrode 211a. Is done.
[0040]
The behavior of the meniscus until the droplet flies in the ink jet device disclosed in Patent Document 2 will be described below with reference to FIGS.
[0041]
As shown in FIG. 21, when a pulse voltage from the pulse voltage generator 213 is applied to the ejection electrode 211a in the recording head 211, an electric field is generated from the ejection electrode 211a toward the counter electrode 210. Here, the sharpest discharge electrode 211a is used, so that the strongest electric field is generated near the front end.
[0042]
When such an electric field is generated, as shown in FIG. 22, each of the charged pigment particles 201a in the ink solvent moves toward the ink liquid surface by the force fE (FIG. 23) exerted from the electric field. Thus, the pigment concentration near the ink liquid level is concentrated.
[0043]
When the pigment concentration is concentrated in this way, as shown in FIG. 23, a plurality of charged pigment particles 201a starts to aggregate near the ink liquid surface by being pushed by the opposite side of the electrode. Then, when the pigment aggregate 201 starts to grow spherically near the ink liquid surface, the electrostatic repulsion fcon from the pigment aggregate 201 starts to act on each of the charged pigment particles 201a. That is, a resultant force ftotal of the electrostatic repulsion force fcon from the pigment aggregate 201 and the force fE from the electric field E by the pulse voltage acts on each of the charged pigment particles 201a.
[0044]
Therefore, as long as the electrostatic repulsion between the charged pigment particles does not exceed the mutual aggregating force, the charged pigment particles 201a (the tip of the discharge electrode 211a and the pigment agglomerate) on which the resultant force ftotal acts toward the pigment agglomerate 201 act. If the force fE exerted by the electric field on the charged pigment particles 201a) on a straight line connecting to the center of the particle 201 exceeds the electrostatic repulsion fcon from the pigment aggregate 201 (fE ≧ fcon), the charged pigment particles 201a It grows into an aggregate 201.
[0045]
The pigment aggregate 201 formed from the n charged pigment particles 201a receives the electrostatic repulsion force FE from the electric field E due to the pulse voltage, while receiving the binding force Fesc from the ink solvent. When the electrostatic repulsion force FE and the restraining force Fesc are balanced, the pigment aggregate 201 stabilizes while slightly protruding from the ink liquid surface.
[0046]
Further, when the pigment aggregate 201 grows and the electrostatic repulsion force FE exceeds the binding force Fesc, as shown in FIGS. 24A to 24C, the pigment aggregate 201 moves from the ink liquid level 200a. Escape.
[0047]
[Patent Document 1] JP-A-8-238774
[0048]
[Patent Document 2] JP-A-2000-127410
[0049]
[Problems to be solved by the invention]
By the way, according to the principle of the conventional electrostatic attraction method, the electric charge is concentrated at the center of the meniscus, and the meniscus is raised. The radius of curvature at the tip of the raised tailor cone is determined by the amount of charge concentration, and when the electrostatic force due to the concentrated charge amount and the electric field strength exceeds the surface tension of the meniscus at that time, separation of droplets starts.
[0050]
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 droplet size is determined by the physical properties of the ink (particularly, surface tension) and the intensity of the electric field formed in the meniscus.
[0051]
In general, the surface tension of a liquid tends to be lower when it contains a solvent than when it is a pure solvent, and since actual inks contain 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 ink is assumed to be constant and the droplet size is reduced by increasing the electric field strength.
[0052]
Therefore, in the ink jet devices disclosed in Patent Documents 1 and 2 described above, as a principle of ejection, the meniscus is formed by forming a field with a strong electric field strength in a meniscus region having a much larger area than the projected area of the ejected droplet. In order to perform discharge by electrostatic force consisting of the concentrated charges and the electric field strength being formed, it is necessary to apply a very high voltage close to 2000 V, so that drive control is difficult and an inkjet apparatus There was also a problem in terms of safety in operating the.
[0053]
In particular, when a strong electric field strength is formed in a wide area, the discharge breakdown strength (for example, the discharge breakdown strength of air between parallel plates is 3 × 10 ⁶ V / m) or less, and the size of the microdroplets that can be formed was limited in principle.
[0054]
In addition, since the electric charge moves to the center of the meniscus portion, the electric charge moving time affects the ejection responsiveness, which has been a problem in improving the printing speed.
[0055]
As a method for solving these problems, which is also used in Patent Documents 1 and 2 described above, a method in which a driving voltage is reduced by applying a bias voltage lower than an ejection voltage in advance, or a method as disclosed in Patent Document 2 In addition, a configuration has been adopted in which the electrodes are projected from the nozzle portion to promote the concentration of electric charges. Further, as in Patent Literature 1, a method of applying a positive pressure to the ink to cause the meniscus to rise in advance has been proposed.
[0056]
However, none of the techniques disclosed in Patent Literatures 1 and 2 has been a fundamental solution. In particular, when a bias voltage is applied, only one of the positive and negative drive voltages can be applied at all times, and when the recording medium is an insulating material, the surface potential due to the adhesion of the charged discharge droplets accumulates. For this reason, the landing accuracy is deteriorated, and it is necessary to take measures such as removing the charge on the surface of the recording medium during printing.
[0057]
Further, in order to form a field having a strong electric field strength in a wide range of meniscus regions, it is necessary to precisely arrange the counter electrode, and the dielectric constant and thickness of the recording medium affect the arrangement of the counter electrode. The degree of freedom in use was narrow. In particular, when the recording medium is thick, the distance between the opposing electrodes is further away from the electrode of the nozzle portion, and a higher voltage has to be applied, and there are many recording media that are difficult to use in practice.
[0058]
Therefore, the conventional electrostatic suction type ink jet device (electrostatic suction type fluid jet device) has a problem that a high versatility device which satisfies both high resolution and safety and has high versatility has not been put to practical use. Was.
[0059]
SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and has as its object to achieve both high resolution and safety, and to provide a versatile recording device that can be used practically. It is to provide a suction type fluid jet device.
[0060]
[Means for Solving the Problems]
As shown in FIG. 16, the inventors of the present application have assumed that, in the conventional method, the curvature 24 of the tip portion of the meniscus 22 of the Taylor-cone-shaped fluid of the nozzle portion 21 formed in the process of electrostatic suction is almost equal to the curvature 24 just before the droplet discharge. By using the nozzle 23 having a shape in which the fluid discharge hole side is narrowed so as to have a nozzle diameter of the same size, it is possible to narrow the formation of an electric field, which is required over a wide range, and to move the electric charge in the meniscus 22 of the fluid. We found that the amount could be reduced.
[0061]
Utilizing the above principle, the present inventors further set the diameter of the fluid discharge hole at the tip of the nozzle to be equal to or less than the droplet diameter of the fluid immediately after the discharge, so that the charge concentration area And the meniscus area can be made almost the same.
[0062]
Therefore, in order to solve the above-described problems, the electrostatic suction type fluid jet device of the present invention uses a fluid ejected from a nozzle made of an insulating material to form a droplet by electrostatic suction from a fluid charged by voltage application. An electrostatic suction type fluid jet device for discharging in a state, comprising an applied voltage control means for controlling a voltage applied to the fluid in the nozzle, wherein a diameter of a fluid discharge hole of the nozzle is set to φ8 μm or less; The applied voltage control means controls the fluid so that the amount of charge induced in the droplet of the fluid immediately after being discharged from the fluid discharge hole is 90% or less of the amount of charge corresponding to the Rayleigh limit of the droplet. It is characterized in that the applied voltage is controlled.
[0063]
According to the above configuration, in the process of the conventional electrostatic attraction of the fluid, the Taylor cone-shaped charges formed to discharge the fluid having a droplet diameter smaller than the diameter of the fluid discharge hole of the conventional nozzle are concentrated. In the present invention, by setting the nozzle diameter to be substantially the same as the diameter of the tip portion, it is possible to narrow the formation of an electric field that is required over a wide range.
[0064]
From the above, it is possible to greatly reduce the voltage required for moving the electric charge, that is, the voltage required for applying the charge amount required for electrostatically attracting the fluid to the fluid. This eliminates the need for a high voltage of 2000 V as in the related art, so that safety when using the fluid jet device can be improved.
[0065]
In addition, since the diameter of the fluid discharge hole of the nozzle is set to φ8 μm or less, the electric field intensity distribution is concentrated near the discharge surface of the fluid discharge hole, and the distance from the counter electrode to the fluid protrusion hole of the nozzle is reduced. The fluctuation does not affect the electric field intensity distribution.
[0066]
This makes it possible to discharge the fluid stably without being affected by the positional accuracy of the opposing electrode and the variations in the material properties and thickness of the recording medium.
[0067]
Further, as described above, since the electric field can be narrowed, a strong electric field can be formed in a narrow region, and as a result, a droplet that can be formed can be minute. This makes it possible to increase the resolution of a printed image when ink droplets are used.
[0068]
Further, as described above, since the charge concentration region and the meniscus region of the fluid have substantially the same size, the movement time of the charge in the meniscus region does not affect the ejection response, and the droplet Can be improved (the printing speed when the droplets are ink).
[0069]
Further, since the charge concentration region and the fluid meniscus region have substantially the same size, it is not necessary to form a strong electric field in a wide range of the meniscus region. This eliminates the need to precisely arrange the counter electrode in order to form a strong electric field in a wide range of meniscus regions, as in the related art, and reduces the dielectric constant and thickness of the recording medium in the arrangement of the counter electrode. No longer affected.
[0070]
Therefore, in the electrostatic suction type fluid jet device, the degree of freedom for the arrangement of the counter electrode is increased. That is, the degree of freedom in designing the electrostatic suction type fluid jet device is increased. As a result, printing can be performed on a recording medium that has been conventionally difficult to use without being affected by the dielectric constant or thickness, and a fluid jet device with high versatility can be realized.
[0071]
Therefore, according to the electrostatic suction type fluid jet device having the above configuration, it is possible to realize a highly versatile device that satisfies both high resolution and safety.
[0072]
Here, in addition to pure water and oil, the above-mentioned fluid may be ink that is a colored liquid containing dyes and pigments as fine particles, or a wiring material (a conductive material such as silver or copper) for forming a circuit board. And the like containing fine particles of the above) can be used.
[0073]
For example, when ink is used as a fluid, high-definition printing can be performed, and when a solution containing a wiring material for forming a circuit board is used as a fluid, ultra-high-definition wiring having a very narrow line width is used. Circuit can be formed, and in any case, the fluid can be stably discharged.
[0074]
In addition, the applied voltage control means controls the amount of charge induced in the fluid droplet immediately after being discharged from the fluid discharge hole to be 90% or less of the charge amount corresponding to the Rayleigh limit of the droplet. By controlling the voltage applied to the fluid, it is possible to prevent discharge due to the phenomenon of the surface area of the droplets upon drying of the discharged droplets, and to prevent a decrease in vapor pressure due to charging of the droplets.
[0075]
As a result, it is possible to reduce a decrease in the drying time of the ejected droplets (the time until all the solvent of the droplets evaporates), and it is possible to eliminate the variation in the dot diameter of the landed droplets.
[0076]
Further, since the drying time of the ejected droplets becomes longer, the change in the diameter of the droplets, that is, the amount of the droplets before the droplets land can be reduced. As a result, environmental conditions such as air resistance and ambient humidity received by the droplet during flight become uniform for each droplet, so that the landing accuracy of the droplet is improved, that is, the variation of the droplet at the time of landing is suppressed. be able to.
[0077]
Furthermore, since the drying time of the discharged droplets becomes longer, even if the diameter of the discharged droplets is as small as about 5 μm, the droplets can be landed without drying. .
[0078]
Therefore, if the electrostatic suction type fluid jet device having the above-described configuration is used, fine droplets can be stably ejected and landed with high precision.
[0079]
To ensure that the amount of charge induced in the fluid droplet immediately after being discharged from the fluid discharge hole is 90% or less of the amount of charge corresponding to the Rayleigh limit of the droplet, the following should be performed. Can be considered.
[0080]
That is, in order to solve the above-described problems, the electrostatic suction type fluid jet device of the present invention converts a fluid charged by voltage application into a droplet state by electrostatic suction from a fluid discharge hole of a nozzle made of an edge material. An electrostatic suction type fluid jet device that discharges the fluid by means of an applied voltage control means for controlling a voltage applied to the fluid in the nozzle, wherein the diameter of the fluid discharge hole of the nozzle is equal to the droplet diameter of the fluid immediately after the discharge. Is set to be equal to or less than, the applied voltage control means, the amount of charge induced in the fluid droplets immediately after being discharged from the fluid discharge hole, the droplet diameter immediately after fluid discharge by the maximum electric field strength of the meniscus The voltage applied to the fluid is controlled so as to be equal to or less than the electric charge corresponding to the Rayleigh limit of the above.
[0081]
The applied voltage control means controls the fluid so that the amount of charge induced in the droplet of the fluid immediately after being discharged from the fluid discharge hole is 60% or more of the amount of charge corresponding to the Rayleigh limit of the droplet. It is characterized in that the voltage applied to is controlled.
[0082]
In general, a charged droplet has a reduced vapor pressure due to the amount of charge (charge amount) charged on the surface of the droplet. Therefore, if the charge amount is too small, it does not affect relaxation of evaporation. Specifically, when the charge amount is less than 60% of the charge amount corresponding to the Rayleigh limit of the droplet, the effect of reducing the evaporation of the droplet is not affected.
[0083]
Therefore, it is preferable that the amount of charge induced in the fluid droplet immediately after being discharged from the fluid discharge hole be set to 60% or more and 90% or less of the charge amount corresponding to the Rayleigh limit of the droplet.
[0084]
To ensure that the amount of charge induced in the fluid droplet immediately after being discharged from the fluid discharge hole is 60% or more of the amount of charge corresponding to the Rayleigh limit of the droplet, the following should be performed. Can be considered.
[0085]
That is, the applied voltage control means determines that the amount of charge induced in the fluid droplet immediately after being discharged from the fluid discharge hole is a Rayleigh limit at the droplet diameter immediately after the fluid discharge due to the maximum electric field strength of the meniscus of the fluid. The voltage applied to the fluid is controlled so as to be 0.8 times or more the charge amount corresponding to the above.
[0086]
It is preferable that the diameter of the fluid discharge hole of the nozzle is set to φ5 μm or less, and the diameter of the fluid discharge hole of the nozzle is set to φ0.2 μm or more and φ4 μm or less.
[0087]
In this case, by setting the diameter of the fluid discharge hole of the nozzle to φ5 μm or less, the electric field intensity is concentrated, the electric field concentration becomes extremely large, and the maximum electric field intensity can be increased. Efficiency can be improved. Further, in order to improve the charging efficiency of the droplet, the diameter of the fluid discharge hole of the nozzle may be set to be not less than 0.2 μm and not more than 4 μm. In this case, the electric field concentration becomes extremely large, and the maximum electric field intensity can be increased. As a result, it is possible to stably eject fine droplets having a small diameter.
[0088]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described below. In this embodiment, an electrostatic suction type ink jet apparatus using ink as a fluid will be described.
[0089]
FIG. 1 is a diagram illustrating a structure of an inkjet apparatus according to an embodiment of the present invention.
[0090]
As shown in FIG. 1, the ink jet apparatus includes a nozzle 4 for discharging an ink 2 as a fluid stored in an ink chamber 1. The nozzle 4 is connected to the ink chamber 1 via a packing 5. Thereby, the ink 2 in the ink chamber 1 is sealed so as not to leak outside from the connection portion between the nozzle 4 and the ink chamber 1.
[0091]
Further, the nozzle 4 has a shape narrowed toward the opposite side to the connection portion with the ink chamber 1, that is, toward the tip 4a on the ink ejection side, so that the inner diameter becomes smaller. The inner diameter (diameter) of the ink ejection hole 4b at the tip 4a of the nozzle 4 is set in relation to the particle size of the ink 2 immediately after ejection.
[0092]
In addition, in order to distinguish the ink 2 discharged from the nozzle 4 from the ink 2 stored in the ink chamber 1, the ink 2 discharged from the nozzle 4 will be hereinafter referred to as a droplet 3. Details of the relationship between the diameter of the ink ejection hole 4b and the droplet diameter of the droplet 3 immediately after ejection will be described later.
[0093]
Further, inside the nozzle 4, an electrostatic field applying electrode 9 for applying an electrostatic field to the ink 2 is provided. The electrode 9 for applying a static electric field is connected to a process control unit 10, and the process control unit 10 controls an electric field intensity by a voltage applied from a drive circuit (not shown). By controlling the electric field strength, the droplet diameter of the droplet 3 discharged from the nozzle 4 is adjusted. That is, the process control unit 10 has a function as an applied voltage control unit that controls the voltage applied to the ink 2 via the electrostatic field applying electrode 9.
[0094]
An opposing electrode 7 is disposed at a position separated by a predetermined distance on the side of the nozzle 4 opposite to the ink ejection hole 4b. The counter electrode 7 changes the surface of the recording medium 8 conveyed between the nozzle 4 and the counter electrode 7 to a potential having a polarity opposite to the charging potential of the droplet 3 ejected from the ink ejection hole 4 b of the nozzle 4. It is charged. Thus, the droplet 3 discharged from the ink discharge hole 4b of the nozzle 4 is stably landed on the surface of the recording medium 8.
[0095]
As described above, since the droplet 3 needs to be charged, it is desirable that at least the ink ejection surface of the tip 4a of the nozzle 4 is formed of an insulating member, and that the nozzle 3 has a fine nozzle diameter (the ink ejection hole 4b). In this embodiment, a glass capillary tube is used as the nozzle 4.
[0096]
Therefore, in the process of electrostatic suction of the ink 2, which is a fluid, the nozzle 4 forms a tailor cone-shaped ink meniscus formed to discharge a droplet having a diameter smaller than the diameter of the ink discharge hole of the nozzle. The diameter of the ink ejection hole 4b of the nozzle 4 is set to be substantially the same as the diameter of the tip of the meniscus immediately before ink ejection, and the diameter of the droplet 3 immediately after ejection is formed. It is set to less than or equal.
[0097]
In the ink jet apparatus having the above configuration, the voltage applied to the ink 2 via the electrostatic field applying electrode 9 is controlled by the process control unit 10 so that the amount of the ink 2 to be ejected is 1 pl or less. .
[0098]
In addition to the nozzles 4, an ink supply path 6 for supplying the ink 2 from an ink tank (not shown) is connected to the ink chamber 1. Here, since the ink 2 is held in a state where the ink 2 is filled in the ink chamber 1 and the nozzle 4, a negative pressure is applied to the ink 2.
[0099]
Here, the behavior of the meniscus portion (meniscus region) 14 formed near the ink ejection hole 4b when the ink 2 is ejected from the nozzle 4 as the droplet 3 will be described below. FIGS. 2A to 2C are model diagrams showing the behavior of the meniscus portion 14 in the vicinity of the ink ejection hole 4b.
[0100]
First, in a state before the ink 2 is discharged, as shown in FIG. 2A, a negative pressure is applied to the ink, so that the meniscus portion 14 has a concave shape inside the tip 4a of the nozzle 4. A meniscus 14a is formed.
[0101]
Next, in order to discharge the ink 2, the voltage applied to the ink 2 via the electrostatic field applying electrode 9 is controlled by the process control unit 10, and when a predetermined voltage is applied to the ink 2, the nozzle As shown in FIG. 2B, the electric charge is induced on the surface of the ink 2 in the ink 4, and as a meniscus portion 14, the surface of the ink discharge hole 4 b of the tip 4 a of the nozzle 4, that is, the counter electrode side ( (Not shown), the pulled meniscus 14b is formed. At this time, since the diameter of the nozzle 4 is very small, the meniscus 14b is pulled outward while forming a tailor cone shape from the beginning.
[0102]
Subsequently, as shown in FIG. 2C, the meniscus 14b pulled outward becomes a meniscus 14c having a shape further discharged to the counter electrode side (not shown) as the meniscus portion 14, and the induced meniscus 14c is formed. When the electric charge on the surface and the force of the electric field (electric field intensity) formed in the nozzles 4 exceed the surface tension of the ink 2, a discharged droplet is formed.
[0103]
Here, the inner diameter (hereinafter, referred to as the nozzle diameter) of the ink ejection hole 4b of the nozzle 4 used in the present embodiment is φ5 μm. As described above, when the nozzle diameter of the nozzle 4 is very small, the radius of curvature of the tip of the meniscus can be regarded as substantially constant without gradually changing to a small value due to the concentration of surface charges as in the related art.
[0104]
Therefore, if the physical properties of the ink are constant, the surface tension at the time of droplet separation is almost constant in the state of ejection by applying a voltage, and the amount of surface charge that can be concentrated also exceeds the surface tension of the ink, that is, The maximum is uniquely defined because it is less than the Rayleigh split value.
[0105]
Since the nozzle diameter is very small, the electric field intensity has a very strong value only in the very vicinity of the meniscus portion. Thus, the discharge breakdown strength at a high electric field in an extremely small region has a very high value, which poses a problem. No.
[0106]
As the ink used in the inkjet apparatus according to the present embodiment, a dye-based ink including pure water and an ink containing fine particles can be used. Here, as for the ink containing the fine particles, the particle size of the fine particles to be contained must be small because the nozzle portion is very small conventionally. Less likely to clog.
[0107]
Therefore, if the nozzle diameter of the nozzle 4 used in the present embodiment is φ5 μm as described above, the fine particle diameter of the ink corresponding to the nozzle diameter is 50 nm or less. At this time, as in the principle of ejecting ink containing fine particles disclosed in Patent Document 2, the method of concentrating the charges in the meniscus portion by the movement due to the charging of the fine particles and discharging by the electrostatic repulsive force of the concentrated fine particles mutually. However, since the diameter is smaller than the conventionally used minimum particle diameter of φ100 nm, the moving speed of the charged fine particles in the ink is reduced, and the response speed of ejection and the recording speed are reduced.
[0108]
On the other hand, in the present invention, instead of using the electrostatic repulsion between the charged fine particles, the discharge is performed by the electric charge on the meniscus surface as in the case of the ink containing no fine particles. In this case, the charge amount of the fine particles in the ink is much smaller than the charge on the meniscus surface in order to eliminate ejection instability due to the influence of the charge of the fine particles in the ink affecting the charge on the meniscus surface. Shape is desirable.
[0109]
This is because if the amount of electric charge per unit mass of the fine particles in the ink is 10 μC / g or less, the electrostatic repulsion and the response speed between the fine particles are reduced, and the mass of the ink fine particles is reduced. By reducing the diameter of the fine particles, the total charge amount of the fine particles in the ink can be reduced.
[0110]
Table 1 below shows the ejection stability when the average particle diameter in the ink is from φ3 nm to φ50 nm.
[0111]
[Table 1]

[0112]
The symbols in Table 1 indicate the ejection stability of each nozzle, x: non-ejection due to clogging, etc., Δ: unstable ejection in continuous ejection, and ：: stable ejection.
[0113]
From Table 1, it was found that the diameter of the fine particles is preferably 30 nm or less. In particular, when the particle diameter is 10 nm or less, the influence of the charge on one particle in the ink as the charge in the ink ejection can be almost ignored, and the moving speed due to the charge is very slow, so that the particles do not concentrate on the meniscus center. Further, when the nozzle diameter is φ3 μm or less, the maximum electric field intensity becomes extremely high due to the electric field concentration in the meniscus portion, and the electrostatic force of each particle becomes large. Therefore, it is preferable to use an ink containing fine particles of φ10 nm or less. However, if the diameter of the fine particles is less than φ1 nm, the aggregation of the fine particles and the occurrence of non-uniformity of the concentration are increased, and therefore, the diameter of the fine particles is preferably in the range of φ1 nm to φ10 nm.
[0114]
In the present embodiment, a paste containing fine silver particles having an average particle diameter between φ3 nm and φ7 nm is used, and the fine particles are coated with an anti-aggregation coating.
[0115]
Here, the relationship between the nozzle diameter of the nozzle 4 and the electric field intensity will be described below with reference to FIGS. 3 (a) and 3 (b) to FIGS. 8 (a) and 8 (b). Corresponding to FIGS. 3 (a) and 3 (b) to FIGS. 8 (a) and 8 (b), the nozzle diameter is φ0.2, 0.4, 1, 8, 20 μm and the nozzle diameter conventionally used for reference. The electric field intensity distribution in the case of φ50 μm is shown.
[0116]
Here, in each drawing, the nozzle center position indicates the center position of the ink ejection surface of the ink ejection hole 4b of the nozzle 4. (A) of each figure shows the electric field intensity distribution when the distance between the nozzle and the counter electrode is set to 2000 μm, and (b) shows the electric field intensity distribution when the distance between the nozzle and the counter electrode is set to 100 μm. 5 shows the electric field intensity distribution of the first embodiment. Note that the applied voltage was kept constant at 200 V under each condition. The distribution line in the figure indicates that the electric field strength is 1 × 10 ⁶ From V / m to 1 × 10 ⁷ The range up to V / m is shown.
[0117]
Table 2 below shows the maximum electric field strength under each condition.
[0118]
[Table 2]

[0119]
From FIGS. 3 (a) (b) to 8 (a) (b), it was found that when the nozzle diameter was φ20 μm or more (FIGS. 7 (a) and 7 (b)), the electric field intensity distribution spread over a wide area. . Table 2 also shows that the distance between the nozzle and the counter electrode affects the electric field strength.
[0120]
From these facts, when the nozzle diameter is φ8 μm or less (FIGS. 6A and 6B), the electric field intensity is concentrated, and the variation in the distance between the opposing electrodes hardly affects the electric field intensity distribution. Therefore, if the nozzle diameter is φ8 μm or less, stable ejection can be performed without being affected by the positional accuracy of the opposing electrode and the material characteristics and thickness of the recording medium. Here, in order to discharge the ink 2 having a droplet amount of 1 pl, the nozzle diameter needs to be 10 μm. Therefore, as described above, if the nozzle diameter is 8 μm or less, the droplet amount is 1 pl or less. It is possible.
[0121]
Next, the relationship between the nozzle diameter of the nozzle 4, the maximum electric field intensity of the meniscus portion 14, and the strong electric field region is shown in FIG.
[0122]
From the graph shown in FIG. 9, it was found that when the nozzle diameter becomes φ4 μm or less, the electric field concentration becomes extremely large and the maximum electric field intensity can be increased. As a result, the initial ejection speed of the ink can be increased, so that the flying stability of the ink (droplets) increases, and the speed of movement of the electric charges in the meniscus portion increases, thereby improving the ejection responsiveness.
[0123]
Next, the maximum chargeable amount of the discharged ink droplets 3 in the ink droplets 3 will be described. The amount of charge that can be charged to the droplet 3 is expressed by the following equation (5) in consideration of the Rayleigh splitting (Rayleigh limit) of the droplet 3.
[0124]
q = 8 × π × (ε0 × γ × r ³ ) ² (5)
Here, q is the amount of charge that gives the Rayleigh limit, ε0 is the dielectric constant of vacuum, γ is the surface tension of the ink, and r is the radius of the ink droplet.
[0125]
The closer the charge q obtained by the above formula (5) is to the Rayleigh limit value, the stronger the electrostatic force is, even at the same electric field strength, and the ejection stability is improved. The mist of the ink 2 is generated in the ink ejection hole 4b, and the ejection stability is lacking.
[0126]
Here, a discharge start voltage at which an initial discharge droplet having a diameter approximately twice as large as the nozzle diameter of the nozzle and the nozzle diameter discharged at the meniscus portion starts to fly, a voltage value at the Rayleigh limit of the initial discharge droplet, and a discharge start FIG. 10 is a graph showing the relationship between the voltage and the ratio of the Rayleigh limit voltage value.
[0127]
From the graph shown in FIG. 10, in the range of the nozzle diameter of φ0.2 μm to φ4 μm, the ratio of the discharge start voltage to the Rayleigh limit voltage value exceeded 0.6, and the charging efficiency of the droplet was good. It was found that stable ejection can be performed in the range.
[0128]
For example, the nozzle diameter and the strong electric field (1 × 10 ⁶ (V / m or more) indicates that the area where the electric field is concentrated becomes extremely narrow when the nozzle diameter becomes φ0.2 μm or less. This indicates that the ejected droplet cannot receive sufficient energy for acceleration and the flight stability is deteriorated. Therefore, it is necessary to set the nozzle diameter to be larger than φ0.2 μm.
[0129]
Next, the initial discharge of the meniscus part induced from the maximum electric field strength when the applied voltage when the inkjet apparatus having the above configuration is actually driven, that is, when the optimum voltage value is changed at a voltage equal to or higher than the discharge start voltage of the droplet. FIG. 12 is a graph showing the relationship between the charge amount of a droplet and the Rayleigh limit value derived from the surface tension of the droplet when the droplet is fixed.
[0130]
In the graph shown in FIG. 12, point A is the intersection of the charge amount of the droplet and the Rayleigh limit value derived from the surface tension of the droplet, and if the voltage applied to the ink is higher than point A, The maximum amount of electric charge is almost formed near the Rayleigh limit in the ejected droplet, and if the voltage is lower than the point A, it indicates that the amount of electric charge below the Rayleigh limit and necessary for ejection is formed.
[0131]
Here, paying attention only to the equation of motion of the ejected droplet, the flying is performed under optimal conditions as a strong electric field and the ejection energy of the maximum charge amount. Therefore, the applied voltage is preferably higher than the point A.
[0132]
FIG. 13 is a graph showing the relationship between the initial ejection droplet diameter of ink (here, pure water) and the drying time (the time when all the solvent of the droplets evaporates) when the environmental humidity is 50%. Is shown. From this graph, it can be seen that when the initial ejected droplet diameter is small, the change in the ink droplet diameter due to evaporation is very fast, and drying proceeds even in a short time during flight.
[0133]
For this reason, if the maximum charge amount is formed in the droplet at the time of initial ejection, the droplet diameter decreases due to drying, that is, the surface area of the droplet in which the charge is formed decreases, and Rayleigh splitting occurs during the flight of the ink. When the excess charge is generated and released, the charge is released along with a part of the droplet, so that the flying droplet is reduced more than evaporation.
[0134]
Accordingly, the dispersion of the droplet diameter and the landing accuracy at the time of landing are deteriorated, and the divided mist floats between the nozzle and the recording medium, thereby contaminating the recording medium. For this reason, in consideration of the formation of stable ejection dots, it is necessary to reduce the amount of charge induced to the initially ejected droplet to a certain extent from the amount of charge corresponding to the Rayleigh limit. In this case, when the charge amount is about 95% of the charge amount corresponding to the Rayleigh limit value, the accuracy of the variation of the landing dot diameter cannot be improved, and as a result, it is preferable to set the charge amount to 90% or less.
[0135]
As a specific numerical value, the Rayleigh limit of the initial ejection droplet diameter based on the maximum electric field intensity of the meniscus when the nozzle hole diameter is regarded as the tip shape of the needle electrode is calculated, and the landing is performed by setting the range equal to or less than the calculated value. The variation of the droplet at the time was able to be suppressed. This is because the surface area immediately before separation of the discharged droplet is smaller than that of the droplet immediately after discharge, and the amount of charge induced in the actual initial discharged droplet is obtained by the above calculation due to the time lag of the moving time of the charge. This is probably because the charge amount is smaller than the charge amount.
[0136]
Under such conditions, Rayleigh splitting during flight can be prevented, and stable discharge such as mist formation due to a large amount of charge at the time of separation of discharged droplets at the meniscus portion can be reduced.
[0137]
Note that the charged droplets have a reduced vapor pressure and are less likely to evaporate. This can be seen from the following equation (6).
[0138]
RTρ / M × log (P / P0) = 2γ / d−q ² / (8πd ⁴ ) (6)
Here, R is a gas constant, M is the molecular weight of the gas, T is the temperature of the gas, ρ is the density of the gas, P is the vapor pressure of the microdroplet, P0 is the vapor pressure of the plane, and γ is the surface tension of the ink. , D is the radius of the ink droplet.
[0139]
As shown in the above equation (6), the charged droplet has a reduced vapor pressure due to the amount of charge of the droplet, and if the amount of charge is too small, there is little effect on relaxation of evaporation. And 60% or more of the electric field strength and the voltage value corresponding to the above were preferable results. As a result, the Rayleigh limit of the initial ejected droplet diameter based on the maximum electric field strength of the meniscus when the nozzle hole diameter is regarded as the tip shape of the needle electrode is calculated as described above, and the calculated value is 0.8 times or more. It is the same as indicating a range.
[0140]
In particular, as shown in FIG. 13, when the diameter of the initially discharged droplets is less than φ5 μm, the drying time becomes extremely short and is easily affected by evaporation. It can be seen that the effect is more effective from the viewpoint. The ambient humidity in the case of obtaining the relationship between the drying time and the initial droplet diameter shown in FIG. 13 was set to 50%.
[0141]
In addition, in consideration of drying of the discharged droplet, it is necessary to shorten the time for discharging the liquid to the recording medium.
[0142]
Here, the average flying speed until the discharged droplet separates from the meniscus portion and lands on the recording medium from the nozzle is 5 m / s, 10 m / s, 20 m / s, 30 m / s, 40 m / s, and 50 m / s. Table 3 below compares the ejection stability and the position accuracy of the landing dots.
[0143]
[Table 3]

[0144]
The symbols of the ejection stability in Table 3 indicate that: X: almost no ejection, Δ: non-ejection with continuous ejection, ○: no ejection, and the symbol of impact accuracy: ×: landing deviation > Landing dot diameter, Δ: Landing deviation> Landing dot diameter × 0.5, ○: Landing deviation <Landing dot diameter × 0.5, ◎: Landing deviation <Landing dot diameter × 0.2.
[0145]
As can be seen from Table 3 above, at an average flight speed of 5 m / s, the landing accuracy is poor and the ejection stability is also poor. In particular, when the nozzle diameter is φ1 μm or less, if the ejection speed is low, the air resistance applied to the droplet is largely caused, and the droplet diameter may be further reduced due to evaporation. Conversely, at an average flight speed of 50 m / s, the applied voltage must be increased, so that the electric field intensity at the meniscus portion becomes extremely strong, and mist of the discharged droplets frequently occurs, and stable discharge is performed. Turned out to be difficult.
[0146]
From the above, it was found that the average flying speed from when the ejected droplets separated from the meniscus portion and landed on the recording medium was preferably between 10 m / s and 40 m / s.
[0147]
By the way, FIG. 13 shows the relationship between the initial ejection droplet diameter and the drying time when the ambient humidity is set to 50%. In FIG. The relationship between the ambient humidity and the drying time when the distance of the medium is 0.2 mm is shown.
[0148]
From the graph shown in FIG. 14, it was found that the value of the drying rate did not fluctuate significantly when the ambient humidity was 60% or less. However, when the ambient humidity exceeds 70%, it is possible to extremely suppress the evaporation of the ink. When the ambient humidity is 70% or more, the influence of the above conditions and the like becomes low. %, The influence of drying can be almost neglected, and it was found that the degree of freedom of the design conditions of the ink jet device of the present invention could be widened and the applicable range could be widened.
[0149]
Here, Table 4 shows the ejection stability and the ejection dot diameter variation (landing variation) when the initial ejection droplet diameter is varied with the nozzle diameters of φ1 and φ3 μm. Note that the initial discharge diameter of the nozzle can be controlled by changing the applied voltage value, and can also be controlled by adjusting the pulse width of the applied voltage pulse. In order to eliminate the influence of the intensity, the initial discharge diameter is adjusted by changing the pulse width.
[0150]
[Table 4]

[0151]
In the symbols of the ejection stability in Table 4, x: almost no ejection, Δ: no ejection after 10 minutes continuous ejection, ○: no ejection after 10 minutes continuous ejection, ◎: after 30 minutes continuous ejection No ejection failure is shown. In the symbol of variation, Δ: variation of landing dot> landing dot diameter × 0.2, ○: variation of landing dot ≦ landing dot diameter × 0.2, ◎: variation of landing dot ≤ landing dot diameter x 0.1.
[0152]
From Table 4, it was found that the ejection stability was good when the nozzle diameter was about 1.5 to 3 times the diameter, and that the dispersion of the landing dot diameter was extremely suppressed particularly when the nozzle diameter was 1.5 to 2 times. This is because, when the shape of the ink drawn out from the meniscus portion is regarded as a liquid column, droplet separation under the condition that the surface area of the liquid column is larger than the surface area of the sphere corresponding to the volume of the liquid column is most stable. Conceivable.
[0153]
According to the above configuration, in the electrostatic suction type inkjet apparatus that discharges minute ink droplets having a droplet amount of 1 pl or less immediately after the ink is discharged, the diameter of the ink discharge hole 4b of the nozzle 4 is set to Since the electric field for ejection can be concentrated on the meniscus portion 14 of the nozzle 4 by making the droplet diameter equal to or less than the droplet diameter of the ink droplet, the applied voltage required for ejecting the ink can be greatly reduced. In this way, it is possible to realize stable discharge with small variation in diameter of droplets to be separated and discharged.
[0154]
In addition, the application of a bias voltage, which was conventionally required, becomes unnecessary, and the drive voltage can be applied alternately in the positive and negative directions, and the influence on the landing accuracy due to the increase in the surface potential of the recording medium can be reduced. .
[0155]
In addition, by setting the diameter of the nozzle hole to a range of φ8 μm or less, the electric field can be concentrated on the meniscus portion of the nozzle, and the influence of the positional accuracy of the counter electrode and the material characteristics and thickness variation of the recording medium. Stable discharge was made possible without receiving.
[0156]
In particular, when the diameter of the ink discharge hole 4b of the nozzle 4 is in the range of φ0.2 μm or more and φ4 μm or less, the electric field concentration becomes extremely large. As described above, increasing the maximum electric field strength increases the initial ejection speed of the ink, so that the flight stability is increased, and the movement speed of the electric charges in the meniscus portion is increased, so that the ejection response is improved. At the same time, it is possible to suppress variations in the dot diameter due to the influence of Rayleigh splitting.
[0157]
Further, by setting the diameter of the droplet immediately after the ejection of the ink from the nozzle 4 to be 1.5 times to 3 times or less the diameter of the ink ejection hole 4b of the nozzle 4, the ejection stability can be improved. By setting the diameter of the droplet immediately after the ejection of the ink to 1.5 times to 2 times or less the diameter of the nozzle, the variation in the diameter of the ejection dot can be extremely suppressed.
[0158]
In the present embodiment, as described above, an example in which a negative pressure is applied to the ink in the ink chamber 1 has been described. However, a case where a positive pressure is applied to the ink may be used. To apply a positive pressure to the ink in the ink chamber 1, for example, as shown in FIG. 15, a pump 12 is provided on an ink tank side (not shown) of the ink supply path 6, and the pump 12 is used to It is conceivable to apply a positive pressure to the ink. In this case, the drive of the pump 12 may be controlled using the process control unit 13 so that the pump 12 is driven in accordance with the timing of ink ejection from the ink chamber 1. As described above, if a positive pressure is applied to the ink in the ink chamber 1, the trouble of forming the convex shape of the meniscus portion with electrostatic force can be omitted, and the applied voltage can be reduced and the response speed can be improved. .
[0159]
In the present embodiment, an ink jet apparatus having a single nozzle has been described for simplicity of description. However, the present invention is not limited to this, and the design is made in consideration of the influence of the electric field strength on adjacent nozzles. Is applicable to an ink jet apparatus having a multi-head having a plurality of nozzles.
[0160]
Further, in this embodiment, as shown in FIGS. 1 and 15, an ink jet apparatus in which the opposing electrode 7 is always provided has been described. However, as can be seen from Table 2, the opposing electrode 7 and the ink ejection holes 4 b of the nozzle 4 are provided. Has little effect on the electric field strength between the recording medium and the nozzle, if the distance between the recording medium and the nozzle is short and the surface potential of the recording medium is stable No counter electrode is required.
[0161]
【The invention's effect】
As described above, the electrostatic suction type fluid jet apparatus according to the present invention is configured to discharge the fluid charged by voltage application from the fluid discharge holes of the nozzle made of an insulating material in the form of droplets by electrostatic suction. The fluid jet device of the type, further comprising applied voltage control means for controlling a voltage applied to the fluid in the nozzle, a diameter of a fluid discharge hole of the nozzle is set to φ8 μm or less, and the applied voltage control means A structure in which a voltage applied to the fluid is controlled such that the amount of charge induced in the liquid droplet immediately after being discharged from the fluid discharge hole is 90% or less of the amount of charge corresponding to the Rayleigh limit of the liquid droplet. It is.
[0162]
Therefore, in the process of the conventional electrostatic suction of the fluid, the tip of the tip where the Taylor cone-shaped charges formed to discharge the fluid having a smaller droplet diameter than the diameter of the fluid discharge hole of the conventional nozzle are concentrated. In the present invention, by setting the nozzle diameter to be substantially the same as the diameter, it is possible to narrow the formation of the electric field which is required in a wide range.
[0163]
From the above, it is possible to greatly reduce the voltage required for moving the electric charge, that is, the voltage required for applying the charge amount required for electrostatically attracting the fluid to the fluid. This eliminates the need for a high voltage of 2000 V as in the related art, so that safety when using the fluid jet device can be improved.
[0164]
In addition, since the diameter of the fluid discharge hole of the nozzle is set to φ8 μm or less, the electric field intensity distribution is concentrated near the discharge surface of the fluid discharge hole, and the distance from the counter electrode to the fluid protrusion hole of the nozzle is reduced. The fluctuation does not affect the electric field intensity distribution.
[0165]
This makes it possible to discharge the fluid stably without being affected by the positional accuracy of the opposing electrode and the variations in the material properties and thickness of the recording medium.
[0166]
Further, as described above, since the electric field can be narrowed, a strong electric field can be formed in a narrow region, and as a result, a droplet that can be formed can be minute. This makes it possible to increase the resolution of a printed image when ink droplets are used.
[0167]
Further, as described above, since the charge concentration region and the meniscus region of the fluid have substantially the same size, the movement time of the charge in the meniscus region does not affect the ejection response, and the droplet Can be improved (the printing speed when the droplets are ink).
[0168]
Further, since the charge concentration region and the fluid meniscus region have substantially the same size, it is not necessary to form a strong electric field in a wide range of the meniscus region. This eliminates the need to precisely arrange the counter electrode in order to form a strong electric field in a wide range of meniscus regions, as in the related art, and reduces the dielectric constant and thickness of the recording medium in the arrangement of the counter electrode. No longer affected.
[0169]
Therefore, in the electrostatic suction type fluid jet device, the degree of freedom for the arrangement of the counter electrode is increased. That is, the degree of freedom in designing the electrostatic suction type fluid jet device is increased. As a result, printing can be performed on a recording medium that has been conventionally difficult to use without being affected by the dielectric constant or thickness, and a fluid jet device with high versatility can be realized.
[0170]
Therefore, according to the electrostatic suction type fluid jet device having the above-described configuration, there is an effect that a device with high versatility that satisfies both high resolution and safety can be realized.
[0171]
In addition, the applied voltage control means controls the amount of charge induced in the fluid droplet immediately after being discharged from the fluid discharge hole to be 90% or less of the charge amount corresponding to the Rayleigh limit of the droplet. By controlling the voltage applied to the fluid, it is possible to prevent discharge due to the phenomenon of the surface area of the droplets upon drying of the discharged droplets, and to prevent a decrease in vapor pressure due to charging of the droplets.
[0172]
As a result, it is possible to reduce a decrease in the drying time of the ejected droplets (the time until all the solvent of the droplets evaporates), and it is possible to eliminate the variation in the dot diameter of the landed droplets.
[0173]
Further, since the drying time of the ejected droplets becomes longer, the change in the diameter of the droplets, that is, the amount of the droplets before the droplets land can be reduced. As a result, environmental conditions such as air resistance and ambient humidity received by the droplet during flight become uniform for each droplet, so that the landing accuracy of the droplet is improved, that is, the variation of the droplet at the time of landing is suppressed. be able to.
[0174]
Furthermore, since the drying time of the discharged droplets becomes longer, even if the diameter of the discharged droplets is as small as about 5 μm, the droplets can be landed without drying. .
[0175]
Therefore, the use of the electrostatic suction type fluid jet device having the above-described configuration has an effect that a minute droplet can be stably ejected and landed with high accuracy.
[0176]
The applied voltage control means controls the fluid so that the amount of charge induced in the droplet of the fluid immediately after being discharged from the fluid discharge hole is 60% or more of the amount of charge corresponding to the Rayleigh limit of the droplet. The voltage to be applied to is controlled.
[0177]
In general, a charged droplet has a reduced vapor pressure due to the amount of charge (charge amount) charged on the surface of the droplet. Therefore, if the charge amount is too small, it does not affect relaxation of evaporation. Specifically, when the charge amount is less than 60% of the charge amount corresponding to the Rayleigh limit of the droplet, there is an effect that the evaporation of the droplet is not affected.
[0178]
It is preferable that the diameter of the fluid discharge hole of the nozzle is set to φ5 μm or less, and the diameter of the fluid discharge hole of the nozzle is set to φ0.2 μm or more and φ4 μm or less.
[0179]
In this case, by setting the diameter of the fluid discharge hole of the nozzle to φ5 μm or less, the electric field intensity is concentrated, the electric field concentration becomes extremely large, and the maximum electric field intensity can be increased. Efficiency can be improved. Further, in order to improve the charging efficiency of the droplet, the diameter of the fluid discharge hole of the nozzle may be set to be not less than 0.2 μm and not more than 4 μm. In this case, the electric field concentration becomes extremely large, and the maximum electric field intensity can be increased. As a result, there is an effect that minute droplets having a small diameter can be stably ejected.
[Brief description of the drawings]
FIG. 1 is a schematic configuration cross-sectional view of an inkjet apparatus according to an embodiment of the present invention.
FIGS. 2A to 2C are diagrams for explaining the behavior of meniscus of ink in the ink jet device shown in FIG.
3A is a graph showing the relationship between the distance from the nozzle center and the distance from the counter electrode when the distance between the nozzle and the counter electrode is 2000 μm, and FIG. 3B is a graph showing the relationship between the nozzle and the counter electrode. 4 is a graph showing the relationship between the distance from the center of the nozzle and the distance from the counter electrode when the distance from the nozzle is 100 μm.
FIG. 4A is a graph showing the relationship between the distance from the nozzle center and the distance from the counter electrode when the distance between the nozzle and the counter electrode is 2000 μm, and FIG. 4B is a graph showing the relationship between the nozzle and the counter electrode. 4 is a graph showing the relationship between the distance from the center of the nozzle and the distance from the counter electrode when the distance from the nozzle is 100 μm.
5A is a graph showing the relationship between the distance from the nozzle center and the distance from the counter electrode when the distance between the nozzle and the counter electrode is 2000 μm, and FIG. 5B is a graph showing the relationship between the nozzle and the counter electrode. 4 is a graph showing the relationship between the distance from the center of the nozzle and the distance from the counter electrode when the distance from the nozzle is 100 μm.
6A is a graph showing the relationship between the distance from the nozzle center and the distance from the counter electrode when the distance between the nozzle and the counter electrode is 2000 μm, and FIG. 6B is a graph showing the relationship between the nozzle and the counter electrode. 4 is a graph showing the relationship between the distance from the center of the nozzle and the distance from the counter electrode when the distance from the nozzle is 100 μm.
FIG. 7A is a graph showing the relationship between the distance from the nozzle center and the distance from the counter electrode when the distance between the nozzle and the counter electrode is 2000 μm, and FIG. 7B is a graph showing the relationship between the nozzle and the counter electrode. 4 is a graph showing the relationship between the distance from the center of the nozzle and the distance from the counter electrode when the distance from the nozzle is 100 μm.
FIG. 8A is a graph showing the relationship between the distance from the nozzle center and the distance from the counter electrode when the distance between the nozzle and the counter electrode is 2000 μm, and FIG. 4 is a graph showing the relationship between the distance from the center of the nozzle and the distance from the counter electrode when the distance from the nozzle is 100 μm.
FIG. 9 is a graph showing a relationship between a nozzle diameter and a maximum electric field intensity.
FIG. 10 is a graph showing a relationship between a nozzle diameter and various voltages.
FIG. 11 is a graph showing a relationship between a nozzle diameter and a strong electric field region.
FIG. 12 is a graph showing a relationship between an applied voltage and a charge amount.
FIG. 13 is a graph showing a relationship between an initial ejection droplet diameter and a drying time.
FIG. 14 is a graph showing a relationship between ambient humidity and drying time.
FIG. 15 is a schematic sectional view of an ink jet apparatus according to another embodiment of the present invention.
FIG. 16 is a diagram illustrating the principle of the present invention.
FIG. 17 is a schematic sectional view of a configuration of a conventional electrostatic suction type inkjet apparatus.
FIGS. 18A to 18C are diagrams illustrating the meniscus behavior of the ink in the ink jet device shown in FIG.
FIG. 19 is a schematic configuration diagram of another conventional electrostatic suction type inkjet apparatus.
20 is a schematic sectional perspective view of a nozzle portion of the ink jet device shown in FIG.
21 is a diagram illustrating the principle of ink ejection of the ink jet device shown in FIG.
22 is a diagram illustrating a state of fine particles when a voltage is applied to a nozzle portion of the ink jet device shown in FIG.
FIG. 23 is a view for explaining the principle of fine particle formation at the nozzle portion of the ink jet apparatus shown in FIG.
FIGS. 24A to 24C are diagrams illustrating the behavior of the meniscus of the ink in the ink jet device shown in FIG. 19;
[Explanation of symbols]
1 Ink chamber
2 Ink (fluid)
3 droplets
4 nozzles
4a Tip
4b Ink ejection hole (fluid ejection hole)
5 Packing
6 Ink supply path
7 Counter electrode
8 Recording medium
9 Electrode for applying electrostatic field
10. Process control unit (applied voltage control means)
12 pumps
13 Process control unit
14 Meniscus part
14a Meniscus
14b Meniscus
14c Meniscus

Claims (6)

In an electrostatic suction type fluid jet device, a fluid charged by voltage application is ejected in the form of droplets by electrostatic suction from a fluid ejection hole of a nozzle made of an insulating material,
An applied voltage control means for controlling a voltage applied to the fluid in the nozzle,
The diameter of the fluid discharge hole of the nozzle is set to φ8 μm or less,
The applied voltage control means controls the fluid so that the amount of charge induced in the droplet of the fluid immediately after being discharged from the fluid discharge hole is 90% or less of the amount of charge corresponding to the Rayleigh limit of the droplet. An electrostatic suction type fluid jet device characterized by controlling a voltage applied to a fluid jet device. In an electrostatic suction type fluid jet device that discharges a fluid charged by voltage application from a fluid discharge hole of a nozzle made of an edge material in a state of droplets by electrostatic suction,
An applied voltage control means for controlling a voltage applied to the fluid in the nozzle,
The diameter of the fluid discharge hole of the nozzle is set to be equal to or less than the droplet diameter of the fluid immediately after the discharge,
The applied voltage control means may be configured such that the amount of charge induced in the fluid droplet immediately after being discharged from the fluid discharge hole is a charge corresponding to the Rayleigh limit in the droplet diameter immediately after fluid discharge due to the maximum electric field strength of the meniscus. An electrostatic suction type fluid jet device, wherein a voltage applied to the fluid is controlled so as to be equal to or less than an amount. The applied voltage control means controls the fluid so that the amount of charge induced in the droplet of the fluid immediately after being discharged from the fluid discharge hole is 60% or more of the amount of charge corresponding to the Rayleigh limit of the droplet. The electrostatic suction type fluid jet apparatus according to claim 1, wherein a voltage applied to the fluid jet apparatus is controlled. The applied voltage control means may be configured such that the amount of charge induced in the fluid droplet immediately after being discharged from the fluid discharge hole corresponds to the Rayleigh limit in the droplet diameter immediately after fluid discharge due to the maximum electric field strength of the meniscus of the fluid. 3. The electrostatic suction type fluid jet device according to claim 2, wherein the voltage applied to the fluid is controlled so as to be 0.8 times or more the amount of electric charge generated. The electrostatic suction type fluid jet device according to claim 1, wherein a diameter of the fluid discharge hole of the nozzle is set to φ5 μm or less. 4. The electrostatic suction type fluid jet device according to claim 1, wherein a diameter of the fluid discharge hole of the nozzle is set to be not less than 0.2 μm and not more than 4 μm.

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