Non-conductive fluid droplet forming apparatus and method

A method and apparatus for forming fluid droplets includes a nozzle channel, a pressurized source of a non-conductive fluid in fluid communication with the nozzle channel, and a stimulation electrode. The pressurized source is operable to form a jet of the non-conductive fluid through the nozzle channel. At least one portion of the stimulation electrode is electrically conductive and contactable with a portion of the non-conductive fluid jet. The at least one electrically conductive and contactable portion of the stimulation electrode is operable to transfer an electrical charge to a region of the portion of the non-conductive fluid jet with the electrical charge stimulating the non-conductive fluid jet to form a non-conductive fluid droplet.

FIELD OF THE INVENTION

This invention relates generally to the field of digitally controlled fluid drop forming devices, and in particular to devices that form drops with non-conductive fluids.

BACKGROUND OF THE INVENTION

The use of ink jet printers for printing information on a recording media is well established. Printers employed for this purpose may be grouped into those that continuously emit a stream of fluid droplets, and those that emit droplets only when corresponding information is to be printed. The former group is generally known as continuous inkjet printers and the latter as drop-on-demand inkjet printers. The general principles of operation of both of these groups of printers are very well recorded. Drop-on-demand inkjet printers have become the predominant type of printer for use in home computing systems, whereas continuous inkjet systems find major application in industrial and professional environments. Typically, continuous inkjet systems produce higher quality images at higher speeds than drop-on-demand systems.

Continuous inkjet systems typically have a print head that incorporates a fluid supply system for fluid and a nozzle plate with one or more nozzles fed by the fluid supply. The fluid is jetted through the nozzle plate to form one or more thread-like streams of fluid from which corresponding streams of droplets are formed. Within each of the streams of droplets, some droplets are selected to be printed on a recording surface, while other droplets are selected not to be printed, and are consequently guttered. A gutter assembly is typically positioned downstream from the nozzle plate in the flight path of the droplets to be guttered.

In order to create the stream of droplets, a droplet generator is associated with the print head. The droplet generator stimulates the stream of fluid within and just beyond the print head, by a variety of mechanisms known in the art, at a frequency that forces continuous streams of fluid to be broken up into a series of droplets at a specific break-off point within the vicinity of the nozzle plate. In the simplest case, this stimulation is carried out at a fixed frequency that is calculated to be optimal for the particular fluid, and which matches a characteristic drop spacing of the fluid jet ejected from the nozzle orifice. The distance between successively formed droplets, S, is related to the jet velocity, v, and the stimulation frequency, f, by the relationship: v=fS. U.S. Pat. No. 3,596,275, issued to Sweet, discloses three types of fixed frequency generation of droplets with a constant velocity and mass for a continuous inkjet recorder. The first technique involves vibrating the nozzle itself. The second technique imposes a pressure variation on the fluid in the nozzle by means of a piezoelectric transducer placed typically within the cavity feeding the nozzle. A third technique involves exciting a fluid jet electrohydrodynamically (EHD) with an EHD droplet stimulation electrode.

Additionally, continuous inkjet systems employed in high quality printing operations typically require small closely spaced nozzles with highly uniform manufacturing tolerances. Fluid forced under pressure through these nozzles typically causes the ejection of small droplets, on the order of a few pico-liters in size, traveling at speeds from 10 to 50 meters per second. These droplets are generated at a rate ranging from tens to many hundreds of kilohertz. Small, closely spaced nozzles, with highly consistent geometry and placement can be constructed using micro-machining technologies such as those found in the semiconductor industry. Typically, nozzle channel plates produced by these techniques are typically made from materials such as silicon and other materials commonly employed in micromachining manufacture (MEMS). Multi-layer combinations of materials can be employed with different functional properties including electrical conductivity. Micro-machining technologies may include etching. Therefore through-holes can be etched in the nozzle plate substrate to produce the nozzles. These etching techniques may include wet chemical, inert plasma or chemically reactive plasma etching processes. The micro-machining methods employed to produce the nozzle channel plates may also be used to produce other structures in the print head. These other structures may include ink feed channels and ink reservoirs. Thus, an array of nozzle channels may be formed by etching through the surface of a substrate into a large recess or reservoir which itself is formed by etching from the other side of the substrate.

FIG. 1schematically illustrates a prior art conventional electrohydrodynamic (EHD) stimulation means used to excite a jet of conductive fluid into a stream of droplets. Fluid supply10contains conductive fluid12under pressure which forces ink through nozzle channel20in the form of a conductive fluid jet22. Conductive fluid12is grounded or otherwise connected through an electrical pathway. A prior art droplet stimulation electrode15is approximately concentric with an exit orifice21of nozzle channel20as shown in cross-section inFIG. 1A. Droplet stimulation electrode15typically includes a conductive electrode structure13produced from a variety of conductive materials, including a surface metallization layer, or from one or more layers of a semiconductor substrate doped to achieve certain conductivity levels. Prior art conductive electrode structure13is electrically connected to a stimulation signal driver17that produces a potential waveform of chosen voltage amplitude, period and functional relationship with respect to time in accordance to a stimulation signal19. InFIG. 1, an example of a stimulation signal19comprises a uni-polar square wave with a 50% duty cycle. The resulting EHD stimulation is a function of the square of field strength created at the surface of the conductive fluid12near exit orifice21. The resulting EHD stimulation induces charge in the conductive fluid jet22and creates pressure variations along the jet. Conductive electrode structure13is covered by one or more insulating layers24which are necessary to isolate droplet stimulation electrode15from conductive fluid12in order to prevent field collapse, excessive current draw and/or resistive heating of conductive fluid12. The conductive fluid12must be sufficiently conductive to allow charge to move through the fluid from the grounded fluid supply10in order to electrohydrodynamically stimulate conductive fluid jet22to form droplets that subsequently form at break-off point26. Since conductive fluids are employed, a non-uniform distribution of charge cannot be supported in the fluid jet column outside of the stimulating electric field. The electrohydrodynamic stimulation effect occurs due to the momentary induction of charge in conductive fluid12at nozzle orifice20that creates the pressure variation in fluid jet22. For a correctly chosen frequency of the stimulation signal19, the perturbation arising from the pressure variations will grow on the conductive fluid jet22until break-off occurs at the break-off point26.

Various means for distinguishing or characterizing printing droplets from non-printing droplets in the continuous stream of droplets have been described in the art. One commonly used practice is that of electrostatic charging and electrostatic deflecting of selected droplets as described in U.S. Pat. No. 1,941,001, issued to Hansell, and U.S. Pat. No. 3,373,437, issued to Sweet et al. In these patents, a charge electrode is positioned adjacent to the break-off point of fluid jet. Charge voltages are applied to this electrode thus generating an electric field in the region where droplets separate from the fluid. The function of the charge electrode is to selectively charge the droplets as they break off from the fluid jet.

Referring back toFIG. 1, a typical prior art electrostatic droplet characterizing means includes charging electrode30. Conductive fluid12is employed such that a current return path exists through the fluid supply10(e.g. through grounding). A charge is induced in a specific droplet under the influence of the field generated by charge electrode30. This droplet charge is locked in on the droplet when it separates from the fluid jet22. Charging electrode30is electrically connected to charge electrode driver32. The charging electrode30is driven by a time varying voltage. The voltage attracts charge through conductive fluid12to the end of the fluid stream where it becomes locked-in or captured on charged droplets34once they break-off from the jet22.

A high level of conductivity of fluid12is required to effectively charge droplets formed in these prior art systems. Prior art inkjet print heads that employ electrostatic droplet characterizing means typically use conductive fluid12conductivities on the order of 5 mS/cm. These conductivity levels permit induction of sufficient charge on charged droplets34to allow downstream electrostatic deflection. The conductivity required for droplet charging is typically much greater than that for droplet stimulation. Typically, a conductive fluid suitable for charging can also be stimulated using EHD principles. The selective charging of the droplets in conventional electrostatic prior art inkjet systems allows each droplet to be characterized. That is, the conductive inks permit charges of varying levels and polarities to be selectively induced on the droplets such that they can be characterized for different purposes. Such purposes may include selectively characterizing each of the droplets to be used for printing or to not be used for printing.

Again referring to the prior art system shown inFIG. 1, a potential waveform produced by the charging electrode driver32will determine how the formed droplets will be characterized. The potential waveform will determine which of the formed droplets will be selected for printing and which of the formed droplets will not be selected for printing. Droplets in this example are characterized by charging as shown by charged droplets34and uncharged droplets36. Since a specific droplet characterization is dependant upon whether that droplet is printed with or not, the potential waveform will typically be based at least in part on a print-data stream provided by one or more systems controllers (not shown). The print-data stream typically comprises instructions as to which of the specific droplets within the stream of droplets are to be printed with, or not printed with. The potential waveform will therefore vary in accordance with the image content of the specific image to be reproduced.

Additionally, the potential waveform may also be based on methods or schemes employed to improve various printing quality aspects such as the placement accuracy of droplets selected for printing. Guard drop schemes are an example of these methods. Guard drop schemes typically define a regular repeating pattern of specific droplets within the continuous stream of droplets. These specific droplets, which may be selected to print with if required by the print-data stream, are referred to as “print-selectable” droplets. The pattern is additionally arranged such that additional droplets separate the print-selectable droplets. These additional droplets cannot be printed with regardless of the print-data stream and are referred to as “non-print selectable” droplets. This is done so as to minimize unwanted electrostatic field effects between the successive print-selectable droplets. Guard drop schemes may be programmed into one or more systems controllers (not shown) and will therefore alter the potential waveform so as to define the print-selectable droplets. The voltage waveform will therefore characterize printing droplets from non-printing droplets by selectively charging individual droplets within the stream of droplets in accordance with the print data stream and any guard drop scheme that is employed.

Again referring to the prior art system shown inFIG. 1, electrostatic deflection plates38placed near the trajectory of the characterized droplets interact with charged droplets34by steering them according to their charge and the electric field between the plates. In this example, charged droplets34that are deflected by deflection plates38are collected on a gutter40while uncharged droplets36pass through substantially un-deflected and are deposited on a receiver surface42. In other systems, this situation may be reversed with the deflected charged droplets being deposited on the receiver surface42. In either case, further complications arise from the fact that the charging electrode driver32must be synchronized with stimulation signal driver17to ensure that optimum charge levels are transferred to droplets, thus ensuring accurate droplet printing or guttering as the architecture of the recorder may dictate. These synchronization constraints arise as result of charging or characterizing those conductive fluid droplets at a place and time separate from their stimulation. Although prior art electrostatic characterization and deflection systems are advantageous in that they permit large droplet deflection, they have the disadvantage that they have been used primarily only with conductive fluids, thus limiting the applications of these systems.

A wide range of fluid properties is desirable in commercial inkjet applications. Jetted inks may be made with pigments or dyes suspended or dissolved in fluid mediums comprised of oils, solvents, polymers or water. These fluids typically have a large range of physical properties including viscosity, surface tension and conductivity. Some of these fluids are considered to be non-conductive fluids, and thus have insufficient levels of conductivity so as to be employed in continuous inkjet systems that rely on the selective electrostatic charging and deflection of conductive fluid droplets.

Various systems and methods for stimulating a non-conductive fluid medium to form a series of droplets and for characterizing the series of droplets to form “printing” droplets and “non-printing” droplets have been proposed. For example, U.S. Pat. No. 3,949,410, issued to Bassous et al., teaches use of a monolithic structure useful for the EHD stimulation of conductive fluid droplets in a jet stream emitted from a nozzle.

U.S. Pat. No. 6,312,110, issued to Darty, and U.S. Pat. No. 6,154,226, issued to York et al., teach the construction of various inkjet print heads wherein droplets are not stimulated from a stream of non-conductive fluid. Rather, the print heads comprises EHD pumps within the print head nozzles themselves. Droplets are ejected from the fluid supply in a similar fashion to drop-on-demand printers.

U.S. Pat. No. 4,190,844, issued to Taylor, teaches a use of a first pneumatic deflector for deflecting non-printing ink droplets towards a droplet catcher. A second pneumatic deflector either creates an “on-off” basis for line-at-a-time printing, or a continuous basis for character-by-character printing.

U.S. Pat. No. 6,079,821, issued to Chwalek et al., teaches a use of asymmetric heaters to both create and deflect individual droplets formed in a continuous inkjet recorder. Deflection of the droplets occurs by the asymmetrical heating of the jetted stream.

U.S. Pat. No. 4,123,760, issued to Hou, teaches the use of deflection electrodes upstream of a break-off point from which droplets are formed from a corresponding jetted fluid stream. Droplets produced by the stream are steered to different laterally separated printing locations by applying a cyclic differential charging signal to the deflection electrodes. This causes a deflection of the unbroken fluid stream which directs the droplets towards their desired printing positions.

It can be seen that there is a need to provide an apparatus and method of stimulating or forming a non-conductive fluid droplet or droplets from a jet of non-conductive fluid.

SUMMARY OF THE INVENTION

According to a feature of the present invention, an apparatus for forming fluid droplets includes a nozzle channel, a pressurized source of a non-conductive fluid in fluid communication with the nozzle channel, and a stimulation electrode. The pressurized source is operable to form a jet of the non-conductive fluid through the nozzle channel. At least one portion of the stimulation electrode is electrically conductive and contactable with a portion of the non-conductive fluid jet. The at least one electrically conductive and contactable portion of the stimulation electrode is operable to transfer an electrical charge to a region of the portion of the non-conductive fluid jet with the electrical charge stimulating the non-conductive fluid jet to form a non-conductive fluid droplet.

According to another feature of the present invention, a method of forming fluid droplets includes providing a jet of a non-conductive fluid; providing an electrical charge on an electrically conductive portion of a stimulation electrode; and stimulating the non-conductive fluid jet to form a non-conductive fluid droplet by transferring the electrical charge from the electrically conductive portion of the stimulation electrode to a portion of the non-conductive fluid jet.

According to another feature of the present invention, a stimulation electrode for forming a fluid droplet from a non-conductive fluid jet includes at least one electrically conductive portion contactable with a portion of the non-conductive fluid jet operable to transfer an electrical charge to a region of the portion of the non-conductive fluid jet such that the electrical charge stimulates the non-conductive fluid jet to form a non-conductive fluid droplet.

According to another feature of the present invention, a droplet or a stream of droplets is formed from a corresponding jet of non-conductive fluid. A droplet stimulation electrode is used to stimulate the jet of non-conductive fluid to form each of the droplets in the stream. The droplet stimulation electrode transfers charge to one or more regions of the non-conductive fluid jet. The transferred charges cause the jet to be stimulated such that a given droplet is typically formed from the corresponding regions of the jet. The specific droplet can include at least in part of the charge that has been transferred to the corresponding region or regions from which it was formed. One or more systems controllers are used create and provide a droplet stimulation signal. The droplet stimulation signal includes a waveform that is structured in accordance with the required sequence of droplets to be formed. The droplet stimulation signal is provided to a droplet stimulation driver that in turn provides a potential waveform to the droplet stimulation electrode so as to selectively transfer charge the various regions of the non-conductive fluid jet. This transfer of charge is used electrohydrodynamically stimulate the various regions of the jet to form corresponding droplets.

In addition to the exemplary features and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and the detailed description.

DETAILED DESCRIPTION OF THE INVENTION

The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus and method in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.

FIG. 2schematically shows a printing apparatus50including an example embodiment of the present invention. Printing apparatus50comprises a housing52that can comprise any of a box, closed frame, continuous surface or any other enclosure defining an interior chamber54. In the embodiment ofFIG. 2, interior chamber54of housing52holds an inkjet print-head56, a translation unit58that positions a receiver surface42relative to inkjet print-head56, and system controller60. System controller60may comprise a micro-computer, micro-processor, micro-controller or any other known arrangement of electrical, electromechanical and electro-optical circuits and systems that can reliably transmit signals to inkjet print-head56and translation unit58to allow the pattern-wise disposition of non-conductive donor fluid62onto receiver surface42. System controller60may comprise a single controller or it can comprise a plurality of controllers.

As shown inFIG. 2, inkjet print-head56includes a source of pressurized non-conductive donor fluid64such as a pressurized reservoir or a pump arrangement and a nozzle channel20allowing the pressurized non-conductive donor fluid62to form a non-conductive fluid jet63traveling in a first direction65toward receiver surface42. A droplet generation circuit66is in electrical communication with a droplet stimulation (or formation) electrode100of the present invention. In response to a droplet stimulation (or formation) signal72, droplet stimulation electrode100applies a force to non-conductive fluid jet63to perturb fluid jet63to form a stream of droplets70at a break-off point26. Discrete or integrated components within the droplet generation circuit66such as timing circuits of a type well known to those of skill in the art may be used or adapted for use in generating the droplet stimulation signal72to form droplets.

Selected droplets within the stream of droplets70may be characterized to be printed with or not to be printed with. A droplet separation means74is used to separate droplets selected for printing from the other droplets based on this characterization. Droplet separation means74may include any suitable means that can separate the droplets based on the characterization scheme that is employed. Without limitation, droplet separation means74may include one or more electrostatic deflection plates operable for applying an electrostatic force to separate droplets within the stream of droplets70when the characterization scheme involves the selective charging of droplets. When the droplets are characterized by selectively forming them with different sizes or volumes, droplet separation means74may include a lateral gas deflection apparatus as taught, for example, by Jeanmaire et al., in U.S. Pat. No. 6,554,410. In U.S. Pat. No. 6,554,410, a continuous gas source is positioned at an angle with respect to a stream of droplets. The stream of droplets is composed of a plurality of volumes. The gas source is operable to interact with the stream of droplets thereby separating droplets consisting of one the plurality of volumes from droplets consisting of another plurality of volumes. As shown inFIG. 2, droplet separation means74is employed to deposit some characterized droplets onto receiver surface42while the remaining droplets are deposited on gutter40.

Droplets70can also be characterized using other devices and methods, see, for example, U.S. patent application Ser. No. 11/240,826 entitled Non-conductive Fluid Droplet Characterization Apparatus and Method, filed Sep. 30, 2005.

In the embodiments described with reference toFIGS. 3 through 6a, at least one apparatus and method are described for stimulating non-conductive donor fluid62in inkjet print-head56. It will be understood that non-conductive donor fluid62is not limited to an ink and may comprise any non-conductive fluid that can be caused to form a jet and droplets as described herein. Typically, non-conductive donor fluid62includes a colorant, ink, dye, pigment, or other image forming material. However, donor fluid62can also carry dielectric material, electrically insulating material, or other functional material.

Further, in the embodiment illustrated inFIG. 2, receiver surface42is shown as comprising a generally paper type receiver medium, however, the invention is not so limited and receiver surface42may comprise any number of shapes and forms and may be made of any type of material upon which a pattern of non-conductive donor fluid62may be imparted in a coherent manner. Accordingly, in the embodiment illustrated inFIG. 2, translation unit58has been shown as having a motor76and arrangement of rollers78that selectively positions a paper type receiver surface42relative to a stationary inkjet print-head56. This too is done for convenience and it will be appreciated, that receiver surface42may comprise any type of receiver surface42and translation unit58will be adapted to position either one of the receiver surface42and inkjet print-head56relative to each other.

FIG. 3schematically shows droplet stimulation electrode100for stimulating a stream of droplets70from a non-conductive fluid jet63as per an example embodiment of the present invention. Fluid supply64contains non-conductive donor fluid62under pressure which forces non-conductive donor fluid62through nozzle channel20in the form of a jet. Droplet stimulation electrode100is preferably made from an electrically conductive material, and is preferably concentric with an exit orifice21. Droplet stimulation electrode100, along with droplet stimulation driver102electrohydrodynamically are operable for stimulating a jet of non-conductive fluid into a stream of droplets.

Droplet stimulation electrode100is configured such that it is in direct electrical communication with non-conductive donor fluid62. As such, droplet stimulation electrode100is electrically conductive, or includes at least one electrically conductive electrical contact layer112or portion that is in intimate contact with non-conductive donor fluid62. Electrical contact layer112should be produced from materials that have appropriate wear resistance and chemical resistance with respect to the composition of non-conductive donor fluid62.

Droplet stimulation electrode100may be constructed by a variety of micromachining methods, and may be formed on or from a substrate110. Electrical contact layer112may be made from a surface metallization layer. The surface metallization layer is typically deposited on one or more insulating layers114, especially when substrate110possesses conductive properties. Substrates110suitable for the embodiments of the present invention may include, but are not limited to materials such as glass, metals, polymers, ceramics and semiconductors doped to various conductivity levels.

FIG. 4shows a cross-sectional view of a substrate110that includes a plurality of droplet stimulation electrodes100as per another example embodiment of the present invention. Each of the droplet stimulation electrodes100includes an electrical contact layer112that surrounds the exit orifices21of the nozzle channels. In this embodiment of the present invention, the electrical contact layers are formed as a metal layer115which is deposited on an insulating layer114. Insulating layer114isolates the metal layer115from substrate110, which in this embodiment of the invention is a conductive substrate. The nozzle channels20and their corresponding exit orifices21may be formed by etching, preferably by a reactive ion etch. Insulating layer114that is preferably made from silicon dioxide, may also be applied to the inner surfaces of nozzle channels20to add further electrical isolation between metal layer115and substrate110. Optionally, metal layer115may also be applied over portions of insulating layer114that may cover the inner surfaces of nozzle channels21. Referring back toFIG. 3, nozzle channel20may be defined by corresponding openings in substrate110, insulating layer114and electrical contact layer112which are formed into an integrated assembly. InFIG. 4, electrical contact layer112defines exit orifice21from which jet63is emitted.

As shown inFIG. 5, electrical contact layer112may be patterned around nozzle channels20to form various isolated electrical pathways130to each of the droplet stimulation electrodes100positioned at each of the nozzle orifices20. Electrical contacts135may be made to each independent pathway. Electrical leads may be attached to the electrical pathways by a means such as wire bonding. A separate droplet stimulation driver102(like the one shown inFIG. 3, for example) may be connected to each electrical lead in order to independently drive each of the electrodes surrounding the nozzle bores. Alternatively, droplet stimulation drivers102may be incorporated into substrate110. In other embodiments of the present invention, electrical contact layer112is not patterned to form independent electrical pathways. In such embodiments, all the nozzles are driven in tandem with a single common droplet stimulation driver102. In yet other embodiments of the present invention, the electrical contact layer112may be patterned to drive a group of nozzle simultaneously while driving one or more additional nozzles independently.

InFIG. 5, two parallel rows of nozzles are arranged on a substrate. Nozzle channels20within each row are separated from each other by a fixed spacing, A and the rows themselves are separated from one another by a distance, B. In this embodiment of the present invention, the nozzle channels20in each of the two rows both have the same center-to-center spacing A, but the rows themselves may be offset from one another by a portion of this spacing. This construction allows two rows of nozzles with greater spacing (i.e. a lower resolution) to form a system with combined smaller effective spacing (a higher resolution). The separation of both the rows by spacing B, and the nozzles within a given row by a spacing A will typically permit more room for electrical contacts135on the substrate surface and thereby reduced interaction between the electrically conductive pathways130, as well as reduced electrostatic interactions between droplets generated by different nozzles channels20. Other embodiments of the present invention may incorporate different arrangements of nozzles channels20and droplet stimulation electrodes100.

Referring back toFIG. 4, when electrical contact layer112includes metal layer115, one or more nozzles channels20can be etched in substrate110prior to patterning a metal layer115around the nozzle channels20. Alternatively, metal layer115can be first patterned onto substrate110such that the pattern is suitably registered with the intended location of the nozzle channels20. Using the patterned metal layer as a mask, nozzles channels110may then be etched through substrate110.

Although the electrical contact layer112is a metal layer in the example embodiment described inFIG. 4, other materials that are sufficiently conductive and possess properties that are compatible with a desired non-conductive fluid to be jetted may be used. When MEMS fabrication techniques are employed, droplet stimulation electrode100may be made from suitable semiconductor substrates that provide the necessary properties including conductivity. Additionally, although it is preferable that the droplet stimulation electrodes described herein be produced using MEMS fabrication techniques, these are not the only fabrication techniques that can be used. As such, additional embodiments of the invention may include droplet stimulation electrodes produced from any appropriate materials using any appropriate fabrication techniques known in the art.

In the example embodiments of the present invention shown inFIGS. 3,4and5, openings in the electrical contact layer112are positioned and sized around each of the exit orifices21so that the electrical contact layer is in direct intimate contact with the non-conductive donor fluid62as it is jetted from the exit orifices21. The position of electrical contact layer112is not limited to the embodiments shown these figures. Alternate embodiments of the present invention may include, but are not limited to, positioning an electrical contact layer112on an inner surface of the nozzle channel20itself. Placement of droplet stimulation electrode100may vary so long as the electrical contact layer112intimately contacts the non-conductive donor fluid62such that a charge can be transferred to non-conductive donor fluid62in order to stimulate non-conductive fluid jet63to form stream droplets70.

Under the influence of the droplet stimulation driver102, droplet stimulation electrode100is typically driven to a potential that is relative to a ground point located at some point on the apparatus. One possible location of the ground point may be a portion of a conductive substrate that makes up the nozzle plate comprising the one or more nozzles channels20as shown inFIG. 3. The amount of charge transferred to the fluid jet62at a given stimulation potential will vary depending on the location of the ground and will be typically become smaller as the ground point is moved further away from the droplet stimulation electrode.

In the example embodiment of the present invention shown inFIG. 3, an electrohydrodynamic stimulation of non-conductive fluid jet63forms the stream of droplets70. The forming of droplets may result from an outward radial pressure buildup that arises from the repulsion of “like” charges that are transferred to the surface of the jet63by droplet stimulation electrode100. Although this embodiment of the invention describes a build up of electrohydrodynamic pressures due to a transfer of charge to the jet of non-conductive fluid, these electrohydrodynamic pressures may be generated by several mechanisms. A primary mechanism may arise from a coulomb force that acts on a free charge in an electric field. Free charge is typically injected or directly transferred to the fluid from an electrode at high potential in contact with the fluid. Secondary mechanisms of generating electrohydrodynamic pressures in non-conductive fluids may involve charge polarization and the electrostriction effect. Although establishing a charge in the non-conductive fluid to induce EHD pressure effects will typically arise from the primary mechanism of direct charge transfer, it is to be understood that other EHD mechanisms may contribute to the establishment of these effects.

It is also be possible to stimulate a jet of non-conductive fluid to form a stream of droplets by transferring charges of opposite polarity to different regions located around the perimeter of the jet. In such a case, droplets may be formed by a pinching effect that is created by an attraction of the transferred opposite polarity charges. In these cases a droplet stimulation electrode may be spilt into a plurality of corresponding electrodes portions. Each portion of the droplet stimulation electrode may be driven by a separate droplet stimulation driver to charge each respective region of the jet with a charge comprising a desired polarity. Such a case may produce droplets that have a neutral net charge.

FIGS. 6 and 6Ashow another example embodiment of droplet stimulation electrode100according to the present invention. Droplet stimulation electrode100includes a plurality of electrically conductive portions112A and112B. In this embodiment, droplet stimulation electrode100is divided into two electrical contact layer portions112A and112B, with each layer being arranged to be in intimate contact with opposing regions of non-conductive fluid jet63. Separate droplet stimulation drivers102A and102B are electrically connected to the separate electrical contact layer portions112A and112B. Droplet stimulation drivers102A and102B are driven with by two droplet stimulation signals72A and72B. Each of the droplet stimulation signals can comprise, for example, uni-polar square signal waveforms with a 50% duty cycle. Although the two signal waveforms have substantially equivalent amplitudes and wavelengths, they differ from one another in that they have opposite polarity when compared to each other.

Under the influence of droplet stimulation signals72A and72B, corresponding potential waveforms are created in which positive charge is applied to a first region138of a portion of non-conductive fluid jet63while negative charge is applied to a second region139of a portion of non-conductive fluid jet63. Preferably, the regions are located on opposing sides of each other. With equal and different polarities applied to the opposing regions of non-conductive fluid jet63, the net charge on the jet segment comprising the two regions is substantially zero. However, an attraction between these opposite charges creates an electrohydrodynamic pinching effect on the non-conductive fluid jet63at these regions. Droplets subsequently form from at least the regions of the jet located between the dissimilarly charged regions. Further, since an equal distribution of positive and negative charges is transferred to droplets after break-off, the droplets70are substantially neutral in total charge. The formed droplets are substantially equally charged and substantially equally sized. Preferably, both droplet stimulation signals72A and72B are synchronized such that the opposing regions of unlike charge distribution are positioned to create the pinching effect.

It should be noted that the stimulation effect illustrated by the droplet stimulation electrode100embodiment shown inFIG. 3can also be substantially recreated with the electrode embodiment shown inFIG. 6by simply synchronously providing droplet stimulation signals with the same identical waveforms (polarity included) to each of the droplet stimulation drivers102A and102B.

Referring back toFIG. 3, droplet stimulation driver102generates a potential waveform (not shown) of chosen voltage amplitude, period and functional relationship with respect to time. This potential waveform will alternately charge various regions of non-conductive fluid jet63As herein described, a region of a non-conductive fluid jet may comprise any area of the jet that is intimately contacted by an electrical contact surface of a droplet stimulation electrode, regardless of whether charge is, or is not transferred to the region. As such, a region may comprise a complete surface area that extends around the perimeter of the jet, or a portion of the complete surface area. In accordance with the droplet generation characteristics that are desired, charged regions120represent various charged portions of fluid jet63while uncharged regions125represent other uncharged portions of the jet. For a correctly chosen frequency of the potential waveform, a perturbation resulting from these charged and uncharged regions will grow on non-conductive fluid jet63until droplets break-off from the jet at a point further downstream.

The break-off of droplets from the non-conductive fluid jet63occurs at break-off point26. For the sake of clarity, this droplet break-off is exaggerated inFIG. 3and the start of break-off may take on the order of many droplet spacings; typically 20 S wherein “S” is a center-to-center separate distance between the formed droplets. During the electrohydrodynamic formation of droplets in prior art continuous inkjet printers, any local charge redistribution due to the stimulation quickly vanishes because a conductive fluid is used. In the present invention, charges that are transferred to the non-conductive fluid jet63as a consequence of the EHD stimulation of that jet are not quickly dissipated. As shown inFIG. 3, droplets will form as the non-conductive fluid jet63separates in the areas between the charged regions.120. A non-limiting example of droplet stimulation signal72includes a uni-polar square wave with a 50% duty cycle. As shown inFIG. 3, each of the resulting droplets will be of substantially equal size or volume and will be equally spaced from one another by an equal center-to-center distance, S, since the stimulation signal72waveform is uniform and cyclical in nature. The formed droplets will each have substantially the same charges since each of the charges transferred to charged regions120are subsequently isolated within each of the droplets that break off from a corresponding charged region120. Droplet charge levels and uniformity of charging is controlled by the potential waveform that is applied to the droplet stimulation electrode100and any leakage of charge through fluid jet63prior to droplet break-off. This embodiment of the present invention discloses a droplet stimulation means that gives rise to the simultaneous stimulation and charging of droplets from a non-conductive fluid jet.

Embodiments of the present inventions allow for a charge that induces droplet stimulation from a non-conductive fluid jet to get “locked-in” the subsequently formed droplets. This “locking-in” of charge may allow the formed droplets to be characterized for different purposes that may include being printed with, or not being printed with. Characterization typically requires modifying the droplet stimulation signal72such that various portions of its waveform will not necessarily be identical during the formation of selected droplets formed from stimulated non-conductive jet63. Portions of the droplet stimulation signal72waveform may be varied in some form including, but not limited to, amplitude, duration, duty cycle and polarity. Portions of the droplet stimulation signal72waveform may be varied to characterize selected droplets within the stream of droplets70with different charge levels or different sizes. Such modification of droplet stimulation signal72may potentially vary the time to break-off of differently characterized droplets, but does not fundamentally affect the droplet stimulation mechanism as taught by embodiments of the present invention.

Non-conductive fluids suitable for droplet stimulation according to embodiments of the present invention may be defined by a range of resistivities whose numerical values may be determined by parameters including, but not limited to, the time to droplet break-off, the fluid jet diameter, and the center-to-center distance S between the formed droplets. According to the embodiments of the invention described herein, droplet stimulation of a non-conductive fluid jet is made possible since once charges are transferred to the various regions of the jet, the charges have exceptionally limited capability to dissipate or to migrate along the length of the jet. Preferably, transferred charges should not be able to discharge or migrate more than the center-to-center distance S of the subsequently formed droplets. A time required for a discharge or migration of the transferred charges preferably should not be greater than the cumulative time required to transfer a charge to a charged region120of the fluid jet62and then incorporate that charged region120into a corresponding droplet at break-off point26.

Estimates of the non-conductive fluid resistivity range required for droplet stimulation may be determined by requiring that a discharge time constant, TRCof the transferred charges be of the same duration, or longer than a droplet time-to-break-off interval, Tb. Therefore, TRC≧Tb. Time-to-break-off interval, Tbmay be measured from the time charge is transferred from electrical contact layer112to a given charged region120to the time a specific droplet is formed at break-off point26from that given region. Time-to break-off interval Tbwill typically vary as a function of the electrohydrodynamic stimulation strength, the diameter of fluid jet62, and the non-conductive fluid properties themselves.

Estimates of the discharge time constant, TRC, may be made by modeling a non-conductive fluid jet as a fluid column in free space surrounded by a grounded cylindrical surface. A capacitance per unit length, CLof the fluid column may be estimated by the relationship:
CL=2π∈/|1n(rj/rg)|, where:rjis a radius of the non-conductive fluid jet,rgis a radius of the surrounding cylindrical grounding surface, and∈ is the permittivity of the medium surrounding the non-conductive fluid jet.

When the non-conductive fluid jet is surrounded by air, the value of ∈ in the above relationship differs only marginally from the permittivity in free space or vacuum denoted as ∈0. Accordingly, ∈=∈air=1.0006 ∈0(at atmospheric pressure, 20 degrees Celsius). Other types surrounding mediums may alter the effective permittivity such that ∈=∈eff*∈0, wherein ∈eff>1. For the purpose of making an estimate of capacitance per unit length, ∈=∈0may be used to calculate a lower limit of capacitance. As previously stated, various ground points may be located on an apparatus defined by the present invention. Although these ground points may be located proximate to non-conductive fluid jet63, modeling the reference ground as a distantly positioned surrounding grounded cylindrical surface may be used to provide a lower limit for the capacitance per unit length and hence, a lower limit for the discharge time constant TRC.

For embodiments of the invention in which charge dissipation over a maximum jet length of one droplet-to-droplet spacing, S is acceptable, the total capacitance C for a length of the non-conductive fluid jet equal to droplet-to-droplet spacing S may be estimated by the relationship: C=CL·S. The resistance R of a length S of the non-conductive fluid jet may be estimated by the relationship:
R=ρf·S/(π·rj2), wherevariables S and rjare as previously defined, andvariable ρfis the resistivity of the non-conductive fluid.

The discharge time constant is given by the relationship: TRC=RC. Accordingly, a minimum resistivity, ρfof a non-conductive fluid required for droplet stimulation as described by embodiments of the present invention may be estimated by the following relationship:
ρf≧|Tb(½∈)(rj2/S2)1n(rj/rg)|, where:rgis a radial distance from the jet to the surrounding cylindrical grounding surface, and variables Tb, ∈, rjand S are as previously defined with ∈ being substantially equal to ∈0when an air atmosphere is present.

As an example, for a jet radius rj=5 um, a grounding radius rg=1 m, a droplet center-to-center distance, S=50 um, and a time to break-off, Tb=0.1 msec, a required non-conductive fluid resistivity, ρfwould be in excess of ˜70 MΩ-cm. This value is on the order of the resistivity of ultra pure water (approximately 18 MΩ-cm). This exemplified estimated level of resistivity may be considered to be an approximate lower limit, which may or may not preclude using numerous aqueous inks in embodiments of the present invention. However, inks made with low viscosity high resistivity fluids have resistivity levels that are typically many orders of magnitude above the estimated minimum. An example of such a fluid is isoparaffin with a resistivity of 2·1013Ω-cm. It is to be noted that the above exemplified estimated resistivity level is very conservative since it was based on a model that specified a non-conductive fluid jet-to-ground distance of 1 meter. In practical applications of embodiments of the present invention, non-conductive fluid jet-to-ground distances are likely to be much closer thereby allowing for a lower non-conductive fluid resistivity limit. Practical lower limits for the resistivity of a non-conductive fluid employed in embodiments of the present invention may be as low as 1 MΩ-cm depending on the grounding configuration used.

Embodiments of the present invention have described methods of transferring charge to a non-conductive fluid jet to form a stream of droplets. This transfer of charge may also include a transfer of charge to characterize a droplet with a certain charge polarity. The transfer of charge may also include the transfer of charge to stimulate the jet to selectively form droplets of a desired shape, volume or size characteristic. The charge transferred to a non-conductive fluid jet is typically locked-in, unlike a charge that is applied to a conductive fluid jet. For a given level of charging, the arising electrohydrodynamic stimulation as described in embodiments of the present invention, is typically stronger than that of prior art techniques involving an electrohydrodynamic stimulation of conductive fluids.

The strength of the droplet forming stimulation is typically proportional to the internal radial pressure created by the electrohydrodynamic effect on charged regions of non-conductive fluid jet63. A radial pressure, P due to a charge transferred to a region of jet63may be estimated by the following relationship:
P=1/(2∈)·σ2, wherevariable ∈ is as previously defined and is substantially equal to ∈0when an air atmosphere is present, andσ is a charge density, that in turn may be derived by the relationship:
σ=q/(2πrj·S), wherevariable q is a resulting droplet charge, andvariables rjand S are as previously defined.

By example, for a resulting droplet charge on the order of q=100 fC, a droplet center-to-center distance, S=50 um, and a jet radius, rj=5 um, the radial pressure P on the jet may be estimated to be approximately 230 Pa. This radial pressure value is similar to induced pressures created by prior art EHD droplet stimulation electrodes employed to stimulate conductive fluid jets. However, the stimulation of non-conductive fluid jets as per embodiments of the present invention typically acts on a jet for a greater duration of time than would occur with a similar stimulation of a conductive fluid jet. This extended duration is due to the relative immobility of transferred charge on the non-conductive fluid jet. Therefore, the non-conductive EHD stimulation provided by embodiments of the present invention may be considered to be stronger than that of prior art conductive fluid EHD stimulators.

A corresponding upper limit of a potential, V required for the transfer of charge during droplet stimulation of the various embodiments of the present invention may be estimated by the following relationship:
V=q/C, wherevariables q and C are as previously defined.

The potential V may be estimated to be 430 volts for the previously example in which q=100 fC, S=50 um, rj=5 um, and wherein rgis additionally taken to equal 1 m. The capacitance value C used to obtain this estimate was based upon the derived capacitance per unit length of the non-conductive fluid jet located in free space inside a large diameter grounded cylindrical surface. Accordingly, this capacitance value may be considered to be a lower limit, and consequently an upper limit for the potential estimated by the above relationship. In actual practice, the capacitance of non-conductive fluid jet63with respect to the droplet stimulation electrode100is a function of the geometry of the electrode shape, and the position of the electrode100near the non-conductive fluid jet63. The actual capacitance value is typically higher than that of the above estimated capacitance value. Hence, the potential may be much lower than estimated above, especially with a suitable choice of electrode geometry and with an added placement of a nearby ground electrode to further increase the capacitance.

The example embodiment of the present invention illustrated inFIG. 3discloses a single nozzle channel. Other example embodiments of the present invention may also include a group or row of multiple nozzles or multi-jet or multi-rows of nozzles. Various apparatus incorporating embodiments of the preset invention may include without limitation, continuous inkjet and multi-jet continuous inkjet apparatus.

PARTS LIST

13prior art conductive electrode structure

15prior art droplet stimulation electrode

17prior art stimulation signal driver

22prior art conductive fluid jet

64source of pressurized non-conductive donor fluid

70stream of droplets

74droplet separation means

112electrically conductive electrical contact layer

112A electrical contact layer portion

112B electrical contact layer portion

137conductive ground ring

138a first region of a portion of non-conductive fluid jet63

139a second region of a portion of non-conductive fluid jet63