Abstract:
An electrohydrodynamic printing system includes a nozzle that dispenses a printing fluid and a substrate support. The nozzle includes a conductive portion. A voltage source applies a voltage differential between the conductive portion of the nozzle and the substrate support. A controller is configured to provide a burst mode waveform to the voltage source such that a drop of the printing fluid is caused to form from the conductive nozzle and travel toward the substrate support.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned, U.S. patent application Ser. No. 13/939,249, entitled “BURST MODE ELECTROHYDRODYNAMIC PRINTING”, filed concurrently herewith. 
     FIELD OF THE INVENTION 
     This invention relates generally to the field of fluid dispensers and, in particular, dispensers that produce a flow of liquid drops using electrohydrodynamic printing techniques and systems. 
     BACKGROUND OF THE INVENTION 
     Electrohydrodynamic jet (E-jet)printing uses electric-field induced fluid flows through micro capillary nozzles to cause a fine stream of drops to be formed and ejected. Typically, these electric fields are created by establishing a potential difference between the nozzle carrying the ink (the print head) and the receiving print substrate. A DC voltage is applied to the nozzle, causing the mobile ions in the ink to gather near the surface. This causes the meniscus at the nozzle tip to change into a conical shape, typically referred to as a Taylor cone, due to the tangential stress and attraction to the substrate. This is an unstable state that eventually results in a periodic drop release from the apex of the cone. 
     Other ways of forming a continuous stream of drops include drop-on-demand ink-jet printing using thermal and piezo-excitation and continuous inkjet printing using electrostatic or air deflection to direct the drops to a gutter or the receiver selectively. Among these, traditional ink jet printing systems are limited to low viscosity inks (say, less than 5 cP). Electrohydrodynamic jet printing has demonstrated superior resolution, printing of micron and sub-micron scale drops using a wide variety of inks. E-jet has been shown to work with fluids as high as 90 cP which makes it possible to use a much greater range of printing inks. 
     These developments are still inadequate, however, to open up a much greater range of applications to ink jet printing such as 3-D printing fluids and functional fluids whose viscosity is in the range of 15-100 cP or greater because the pulsed conditions lack sufficient control to print uniform drops (both in size and period between drops) in the kHz range. 
     Improved control of the drop formation of an E-jet system can be achieved by using a pulsed voltage on the capillary nozzle. In particular, control of the timing of the drop formation and the regularity of the drop size can be achieved by using a voltage profile, shown in  FIG. 1  that has successive pulses at a fixed periodicity. This technique has proven effective with higher viscosity fluids in the sub kHz range, but not with consistent control of drop period and size in the kHz range.  FIG. 2 , adapted from High-speed and drop-on-demand printing with a pulsed electrohydrodynamic jet, S Mishra et al., J. Micromech. Microeng. 20 (2010), pages 1-8, shows the 1 kHz printing of Norland Optical Adhesive 74. The magnified edge of the image has an indeterminate period and drops size. It is unclear from the image what the true fundamental period that corresponds to the 1 kHz input is. The larger drops have a larger period. It does not appear as if the drops moved and merged on the surface, rather they are a result of missing every other print. This resulted in twice as larger of a drop at half the printing rate. 
     What is needed is a way to precisely control drop size and period with a range of higher viscosity fluids in the kHz range. Additionally, enhanced process controls to independently regulate process outputs such as drop size and delivery frequency also is desired. 
     SUMMARY OF THE INVENTION 
     According to another aspect of the invention, an electrohydrodynamic printing system includes a nozzle that dispenses a printing fluid and a substrate support. The nozzle includes a conductive portion. A voltage source applies a voltage differential between the conductive portion of the nozzle and the substrate support. A controller is configured to provide a burst mode waveform to the voltage source such that a drop of the printing fluid is caused to form from the conductive nozzle and travel toward the substrate support. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the example embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1  is a graph of a conventional waveform presently used for drop formation; 
         FIG. 2  is a prior art image showing an example of a 1 kHz printed pattern of NOA 73 which has been adapted by removing its intermediate magnification; 
         FIG. 3  is a schematic view of an E-jet printing system made in accordance with an example embodiment of the present invention including an E-jet ejector apparatus; 
         FIG. 4  is a schematic view of the E-jet printing system shown in  FIG. 2  including an E-jet waveform control system and the E-jet ejector apparatus; 
         FIG. 5  a graph of a burst mode waveform according to a first example embodiment of present invention; 
         FIG. 6  shows three images of drops produced with the waveform of  FIG. 5 , in each image the frequency is doubled while maintaining constant energy; 
         FIG. 7  is an image of drops produced at 5.0 kHz with a 120 cP material using the waveform of  FIG. 5 ; 
         FIG. 8  is a graph of a burst mode waveform according to a first example embodiment of present invention; and 
         FIG. 9  is an image of drops produced using the waveform of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, an apparatus in accordance with the present invention. It is to be understood that elements not specifically shown, labeled, or described can take various forms well known to those skilled in the art. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements. It is to be understood that elements and components can be referred to in singular or plural form, as appropriate, without limiting the scope of the invention. 
     The example embodiments of the present invention are illustrated schematically and not to scale for the sake of clarity. One of ordinary skill in the art will be able to readily determine the specific size and interconnections of the elements of the example embodiments of the present invention. 
     Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” Additionally, directional terms such as “on”, “over”, “top”, “bottom”, “left”, “right” are used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration only and is in no way limiting. 
     As described herein, the example embodiments of the present invention provide components typically used in inkjet printing systems. However, many other applications are emerging which use these components to emit liquids (other than inks) that need to be finely metered and deposited with high spatial precision. Such liquids include inks, both water based and solvent based, that include one or more dyes or pigments. These liquids also include various substrate coatings and treatments, various medicinal materials, and functional materials useful for forming, for example, various circuitry components or structural components. As such, as described herein, the terms “liquid” and “ink” refer to any material that is ejected by the system components described below. 
     Inkjet printing is commonly used for printing on paper. However, there are numerous other materials in which inkjet is appropriate. For example, vinyl sheets, plastic sheets, textiles, paperboard, and corrugated cardboard can comprise the print media. Additionally, although the term inkjet is often used to describe the printing process, the term jetting is also appropriate wherever ink or other liquids is applied in a consistent, metered fashion, particularly if the desired result is a thin layer or coating. 
     E-jet systems can be used to eject small drops of fluid using electrohydrodynamic forces. It has been found that under a sufficiently high constant applied potential difference drop are formed and ejected periodically as charges accumulate and relax with the ejection of a drop. In this case, drop size and frequency are determined by the viscosity, surface force, needle tip diameter, back pressure, and mobility of the ionic charge in the fluid. For printing there is a need to control the drop size and frequency of the drop ejection. Prior art practice has been to apply a pulsed potential difference as shown in  FIG. 1 . Such a waveform acts as a forcing function to provide some control over the drop parameters, volume and frequency. As shown in  FIG. 1 , the voltage amplitude, the difference between V high  and V low ,  1  and  2 , respectively, can be controlled and is typically chosen to provide acceptable drop formation. Too low of a difference, however, and drop formation is intermittent while too high of a difference can cause the drops to break up into a misty spray. In addition, the waveform period  3  can be specified thus controlling the drop frequency. The V high  pulse width  4  also is controlled (PWM) to control the drop size, albeit over a limited range. 
     An example embodiment of an E-jet deposition system  30  for e-jet printing is shown in  FIGS. 3 and 4 .  FIG. 3  shows an E-jet ejector apparatus  5 . E-jet ejector apparatus  5  includes a nozzle  6 , a grounded surface  7 , a liquid reservoir  9 , and a high voltage connection  10 . The potential difference is applied between the nozzle  6  and the grounded surface  7 . The actual electric field is determined by the separation gap between the electrode and ground, in the case of a conductive nozzle it is the distance  11  between the nozzle tip  12  and the closest approach  13  of the ground  7  to the nozzle  12 . Alternatively, an electrode can be threaded inside a nonconductive nozzle. In this latter case the potential difference would be between the base of the electrode and the grounded surface. When the potential difference is applied and the ions flow in the liquid supplied by the liquid reservoir  9 , for example, a conductive liquid or ink, a Taylor cone  14  is formed and drops  15  are formed and ejected. Once formed, the drops  15  fall and make contact to a receiving substrate  8 . 
       FIG. 4  shows the E-jet ejector apparatus  5  and the E-jet control apparatus  16  of the E-jet deposition system  30 . In order to maintain a steady production of drops, liquid is supplied to the nozzle  6  by a liquid source that includes a constant back pressure  17 . The back pressure is supplied by the house compressed air line. The pressure assists with replenishing material depleted at the Taylor cone, it is not sufficient to jet the ink on its own. The high voltage is delivered by an in-house built and designed high voltage, high frequency switch  18 . The output of switch  18  is controlled by a waveform generator  19  and is switched between a low voltage rail  20  and a high voltage rail  21 . The switch  18  is designed to operate at frequencies up to and including 0.5 MHz and up to and including 1800V can be applied to the switch  18  from the high voltage rail  21 . MHz resolution is required to resolve individual pulse widths of a bust sequences where the individual peak widths can be in tens of nanosecond range. 
     System  30  includes a substrate support mechanism  38 . The relative location of the nozzle  6  and the receiving substrate  8  is controlled during a drop dispensing or deposition operation. A substrate conveyance mechanism  32 , a nozzle conveyance mechanism  28 , or a combination of both can be used to accomplish relative movement during deposition. In some example embodiments, the substrate support mechanism  38  also moves the substrate during the deposition operation. One example of a substrate conveyance mechanism  32  includes a rotating drum or an x-y translation table. One example of a nozzle conveyance mechanism  28  includes a linear motor that moves the nozzle in one direction or a plurality of motors configured to move the nozzle in more than one direction. As shown in  FIG. 4 , the receiving substrate  8  is positioned on a rotating drum  32  and a linear motor  28  translates the nozzle  6  back and forth across the receiving substrate  8 . Other conventional substrate conveyance mechanisms or nozzle conveyance mechanisms can be used to control the relative location of the nozzle  6  and the receiving substrate  8  during the drop dispensing or deposition operation. Drop spacing is determined by the rotation speed of the drum  32  on which the substrate is affixed, the amount of translation of the nozzle  6 , or a combination thereof. 
     A first example embodiment of a waveform of the present invention is shown in  FIG. 5 . It has been found that this waveform provides improved control of drop formation when compared to conventional waveforms. With this pulse sequence a steady stream of drops with uniform period and size are reproducibly generated. The waveform includes a burst  22  that includes a plurality, for example, a series, of smaller pulses  34 . Each of the pulses of the burst  22  takes place within an operational period  23 . The length of the burst  22  can be changed by a multiplication factor to either fill more or less of the time of the period  23 . The period  23  defines the desired drop generation rate. V high    1  and V low    2  are defined in the same manner as was described with reference to  FIG. 1 . The number of peaks, the peak widths, and the values of V high    1  and V low    2  are typically optimized to generate one drop within the given frequency  23 . The pulse widths  34  of the individual pulses can also be independently defined. The burst  22  waveform also includes one larger pulse  36  that follows the plurality of smaller pulses  34 . The sizes of pulses  34  and  36  are relative to each other and the pulse width  36  of the individual pulse can also be independently defined. As shown in  FIG. 4 , burst  22  includes five, smaller, pulses  34  with the final pulse of the burst sequence defined by one larger pulse width  36 . For a given operational frequency  23  there are different combinations, through changing the number of peaks, peak widths, and V high    1  and V low    2 , that can yield the same print result. The combination selected usually depends on the specific contemplated. 
       FIG. 6  shows three different examples of printed dots of Norland Optical Adhesive 74 (NOA 74) on thermal paper using the burst waveform shown in  FIG. 5 . At 25° C., the test fluid NOA74 had a measured viscosity of ˜120 cps. Thermal paper was selected as the receiving substrate both because it is thin (thin support act as less of a capacitor, allowing for the tip to better sense the under-laying ground), and because NOA chemically reacts with the papers by turning dark upon UV curing, allowing for direct imaging of an otherwise transparent material. Other substrates were tested but this combination was preferred because direct imaging of the NOA on the surface was possible. It should be noted that the thermal paper causes large spreading of the printed material. The drops in-flight are of smaller diameters and printing onto appropriately surface treated receiving substrates will result in a significant reduction of the print drop diameter. The drops in  FIG. 6  were printed with a burst  22  that filled ⅓ of the period  23 . The burst included 5 pulses with a variable operating frequency. The first 4 pulses in this example are identical and the last is 50% longer. The final pulse does not have to be larger but was found to be beneficial in assisting with drop break off. In each image of  FIG. 6  the operational frequency is doubled. Doubling the frequency cuts the widths of the pulses within the burst in half (constant percent duty); however the total energy is maintained because there are now twice as many busts within the same time frame.  FIG. 6  is important in highlighting the effectiveness of the bust sequence. At 133 Hz groups of 4 drops are generated for each burst, successive doubling of the frequency to 266 and again to 533 reduces the number to two and finally one drop per period, respectively. The length of the period (distance between a single drops or the distance between repeating groups of drops for the case of multiple drops generated) is also cut in half for each doubling of the frequency. The print volume is roughly constant; this is primarily controlled by the pack pressure (constant feed rate of material). In all cases multiple waveforms within a single burst are needed to generate a print drop. Drop spacing was determined by the rotation speed of the drum  32  to which the print media was attached. 
       FIG. 7  shows that controlled uniform printing of NOA 74 is possible in the kHz range utilizing a burst sequence. The drops, as with  FIG. 7 , were printed with a burst  22  that filled ⅓ of the period  23 . The burst included 5 pulses with an operating frequency of 5.0 kHz. The first 4 pulses are identical and the last is 50% longer. Drops were printed with a 10 μm I.D. nozzle and measured 71±4 μm on the thermal paper. The printing optimization is dependent upon the material. Factors including viscosity, surface tension, conductivity, and shear thinning, require unique waveform optimization per material. Higher printing rates with a 10 μm inner diameter nozzle diameter have been demonstrated. The print frequency can be further increased by reducing the inner nozzle diameter. Typically a 10 μm inner diameter nozzle was used because this is a size range that is generally accepted in inkjet printing nozzle heads as a diameter where the likelihood of nozzle clogging is minimal. This technique is ideally suited for drop formation with materials with a viscosity range of 4-200 cP but not inclusive. Lower viscosities materials can be printed. Materials at the other extreme (200-1000+ cP) have been tested but drop breakup, though while possible, requires more energy and the maximum printing rate drops when compared to the printing rate of a lower viscosity material. 
     A second embodiment of a waveform of the present invention is shown in  FIG. 8 . When compared to convention waveforms, this waveform provides improved control of the size and number of drops formed by varying the pulse burst. As shown, two differing burst waveforms  24  and  25  are provided in an alternating manner to form a single large drop and two smaller drops per sequence in  FIG. 9 . Each pulse burst  24  and  25  takes place over a constant period  26 . V high    1  and V low    2  are defined in the same manner as was described with reference to  FIG. 1 . 
     The first burst  24  includes a plurality, for example, a series, of smaller pulses  34 . Each of the pulses of the burst  24  takes place within the operational period  26 . The length of the burst  24  can be changed by a multiplication factor to either fill more or less of the time of the period  26 . The period  26  defines the desired drop generation rate. The number of peaks, the peak widths, and the values of V high    1  and V low    2  are typically optimized to generate one drop within the given frequency  26 . The pulse widths  34  of the individual pulses can also be independently defined. The burst  24  waveform also includes one larger pulse  36  that follows the plurality of smaller pulses  34 . The sizes of pulses  34  and  36  are relative to each other and the pulse width  36  of the individual pulse can also be independently defined. As shown in  FIG. 8 , burst  24  includes five, smaller, pulses  34  with the final pulse of the burst sequence defined by one larger pulse width  36 . For a given operational frequency  26  there are different combinations, through changing the number of peaks, peak widths, and V high    1  and V low    2 , that can yield the same print result. The combination selected usually depends on the specific contemplated. 
     The second burst  25  includes a plurality, for example, a series, of smaller pulses  34 . Each of the pulses of the burst  25  takes place within the operational period  26 . The length of the burst  25  can be changed by a multiplication factor to either fill more or less of the time of the period  25 . The period  26  defines the desired drop generation rate. The number of peaks, the peak widths, and the values of V high    1  and V low    2  are typically optimized to generate one drop within the given frequency  26 . This is not the case in  FIG. 9 . In  FIG. 9 , because two differing sized drops were desired, two drops were generated within the frequency  26 . The pulse widths  34  of the individual pulses can also be independently defined. The burst  25  waveform also includes one larger pulse  36  that follows the plurality of smaller pulses  34 . The sizes of pulses  34  and  36  are relative to each other and the pulse width  36  of the individual pulse can also be independently defined. As shown in  FIG. 8 , burst  25  includes three, smaller, pulses  34  with the final pulse of the burst sequence defined by one larger pulse width  36 . For a given operational frequency  26  there are different combinations, through changing the number of peaks, peak widths, and V high    1  and V low    2 , that can yield the same print result. The combination selected usually depends on the specific contemplated. 
       FIG. 9  shows an example of printed dots of NOA 74 on thermal paper using the waveform shown in  FIG. 8 . The first burst  24  included six pulses and the second burst  25  included four pulses. The net operation frequency was 500 Hz, 1.0 kHz for each burst  24  and  25 . At 25° C., NOA74 was measured to have viscosity of ˜120 cP. Drops were printed with a 10 μm I.D. nozzle and measured 71±4 μm on the thermal paper. The large drop was formed by burst waveform  24  and the pair of smaller drops was formed by burst waveform  25 . Drop spacing was determined by the rotation speed of the drum  32  on which the receiving substrate was positioned. The net spacing of the repeating pattern is consistent for a 500 Hz period. The conditions of each individual burst  24  and  25  are stable as long as a constant volume of liquid is ejected between each burst. Depending on the application contemplated, the large and small drops shown in  FIG. 9  can be thought of as print drops and catch drops with the catch drops being collected using a conventional deflection and catcher technology. Alternatively, both the large and small drops can be used to create the image or pattern on the substrate 
     By way of background, the liquid is supplied to the nozzle having an inner diameter, D. The shape of the liquid on the nozzle tip is defined by the material&#39;s surface tension. A back pressure feeds the material. This pressure, however, is not sufficient to impart a velocity to the fluid. In the absence of external stimuli (voltage) the material will ooze out of the nozzle. When the pulses are applied to the stimulation device charge is added to the material. Charge buildup leads to perturbation of the material at the nozzle tip. The tip deformation is known as a Taylor cone and the charge buildup moves the material with a velocity V that is material dependant. As the material elongates, eventually a threshold will be reached at which point a charged drop will separate from the Taylor cone. On the other hand, if the charge stops building it will not cause a drop to break off from the Taylor cone. 
     Referring back to  FIGS. 4-9 , generally described, the present invention provides improved control of drop formation, drop size, or drop numbers by the introduction of a higher frequency burst of stimulations pulses during the time interval that is to form a drop. Comparing  FIG. 5  or  FIG. 8  to  FIG. 1 , one sees that the single large pulse in  FIG. 1  has been replaced by a series of smaller, or narrow, pulses followed by a single larger pulse. The time period between each “burst mode” pulse and energy is sufficiently low that these individual pulses don&#39;t induce drop break off. 
     In accordance with the present invention,  FIGS. 5 and 8  show examples pulse configurations that can be used to generate either drops of the same size or drops of different sizes. One skilled in the art will recognize and understand that any number of drops can be formed in succession. 
     The burst pulses, the closely spaced pulses in  FIGS. 4 and 6 , have the same duty cycle as the other pulses shown in each figure, respectively, but only one-half the period. Therefore, the burst pulses are generated at twice the frequency as the other pulses. The burst pulses do, however, have an effect on the drop generation, as shown in  FIGS. 7 and 9 . In accordance with this example embodiment of the invention, the last pulse in the burst of pulses has a larger duty cycle than the other pulses in the burst of pulses but is not a necessary requirement. 
     There are several differences to note between the drops generated by the standard waveform shown in  FIG. 1  and the burst waveforms shown in  FIGS. 5 and 8 . The burst waveform shown in  FIG. 6  has benefits including, for example, a more energy efficient manor to generate drops; allows for a larger portion of the total waveform to be populated; and, at higher frequencies, it allows for a more distributed energy within the period. 
     The invention has been described in detail with particular reference to certain example embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. Even though specific embodiments of the invention have been described herein, it should be noted that the application is not limited to these embodiments. In particular, any features described with respect to one embodiment may also be used in other embodiments, where compatible. The features of the different embodiments can be exchanged, where compatible. 
     PARTS LIST 
     
         
           1  the voltage amplitude V high    
           2  the voltage amplitude V low    
           3  the waveform period 
           4  the V high  pulse width 
           5  the E-jet ejector apparatus 
           6  the conductive nozzle 
           7  the grounded surface 
           8  the receiving substrate 
           9  the ink reservoir 
           10  the high voltage connection 
           11  the separation gap 
           12  the nozzle tip 
           13  the closest approach of the ground to the nozzle 
           14  the Taylor cone 
           15  ejected drops 
           16  the E-jet control apparatus 
           17  constant back pressure 
           18  high frequency, high voltage switch 
           19  waveform generator 
           20  low voltage rail 
           21  high voltage rail 
           22  burst waveform 
           23  burst waveform operational period 
           24  first drop size burst waveform 
           25  second drop size burst waveform 
           26  burst waveform operational period 
           28  a nozzle conveyance mechanism, for example, a linear motor 
           30  the E-jet deposition system 
           32  substrate conveyance mechanism, for example, a rotating drum 
           34  small pulse(s) 
           36  large pulse(s) 
           38  substrate support mechanism