Abstract:
The present subject matter relates to a method and system for pulsed air-actuated micro-droplet on demand jetting, especially for jetting high viscosity liquids. A needle extends from a liquid chamber and terminates in a drop-forming orifice outlet from which micro-droplets are generated. At least two air jets direct a timed pulse of air at the drop-forming orifice outlet of the needle. The pulsed air is synchronized with the formation of a desired volume of liquid at the orifice outlet to extract and propel a micro-droplet at high velocity to a substrate. The air jets are turned on prior to the forming of the desired volume at the orifice outlet of the needle, and turned off after the micro-droplet had been produced.

Description:
CLAIM OF PRIORITY 
     This application relates to and claims priority to U.S. Provisional Patent Application entitled “Pulsed Air-Actuated Micro-Droplet on Demand Ink Jet” filed Aug. 25, 2010 and assigned U.S. Application Ser. No. 61/376,942; the entire disclosure of which is hereby herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The exemplary teachings herein pertain to methods and systems for ink jet heads for ink jet printers or other liquid jetting devices, and in particular, to a pulsed air-actuated micro-droplet on demand ink jet head. Specifically, the present disclosure relates to an ink jet head using pulsed air to extract and propel micro-droplets from a needle-shaped orifice at high velocity, for jetting high viscosity liquids with drop-on-demand requirements. 
     BACKGROUND 
     As is known in the art, ink jet printers use one or more ink jet heads for projecting drops of ink onto a printing medium (such as paper) to generate text, graphical images or other indicia. Drops are projected from a minute external orifice in each head to the printing medium so as to form the text, graphical images or other indicia on the printing medium. A suitable control system synchronizes the generation of ink drops. It is important that the ink drops be of substantially uniform size, and also that the drops are applied consistently onto the printing medium so that printing is not distorted. 
     Existing ink jet technology, whether it is thermal jet or piezo-jet, can only jet micro-droplets with low viscosity liquids (typically 2-15 centistokes), such as water based inks, and only for short printing distances. In such existing ink jet technology, a pressure pulse is applied to a fluid chamber with sufficient pressure to overcome surface tension forces, thereby forming and ejecting a droplet of fluid from the ink jet nozzle. However, for jetting higher viscosity liquids (greater than 100 centistokes) with drop-on-demand requirement, there is no known ink jetting method. 
     In one basic type of ink jet head, ink drops are produced on demand, for example as disclosed in U.S. Pat. No. 4,106,032 issued to Miura, et al. on Aug. 8, 1978, the entire disclosure of which is herein incorporated by reference. In such drop-on-demand ink jet heads, ink in an ink chamber in the ink jet head, in response to a pressure wave generated from an electric pulse applied to a piezoelectric crystal, flows through an ink passageway in an ink chamber wall and forms an ink drop at an internal drop-forming orifice outlet located at the outer surface of the ink chamber wall. The ink drop passes from the drop-forming orifice outlet, through an air chamber, and toward a main external orifice of the ink jet head leading to the print medium. Continuous air under pressure is delivered to the air chamber and propels the ink drop through the air chamber and to the print medium. 
     However, such prior art drop-on-demand ink jet heads suffer from numerous disadvantages, drawbacks and/or limitations, for example as discussed in U.S. Pat. No. 4,613,875 issued to Le et al. on Sep. 23, 1986, and in U.S. Pat. No. 4,728,969 issued to Le et al. on Mar. 1, 1988, the entire disclosures of these patents are herein incorporated by reference. In an attempt to improve upon such prior art drop-on-demand ink jet heads, Le et al. discloses in the &#39;875 patent an ink chamber with an ink drop-forming orifice outlet from which ink drops are generated in response to pressure waves caused by a piezoelectric crystal. This internal orifice outlet is centered in a projecting structure which extends toward an external orifice. The projecting structure is of a frustoconical or mesa-like shape. As stated therein, air flowing past the top (orifice outlet) of the projection prevents ink from wetting anything but the top of the projection, resulting in highly uniform ink drop formation with a single uniform dot being produced on the printing medium in response to each pressure wave. 
     Reproduced herein as  FIG. 1 , for the purpose of illustration, is the prior art  FIG. 2  from the &#39;875 patent to Le et al. showing this projecting structure of Le et al. As can be seen therein, the projection extends a length “D” into an annular air chamber, almost completely to the external orifice, with only a small spacing “E” there between. Le et al. discloses that the length of the projection is in the range of 50-90 μm with a preferred distance of 60 μm. 
     However, this configuration suffers from numerous disadvantages, drawbacks and/or limitations itself. For example, Le et al. uses continuous air flow to accelerate the ink drop. As such, if the velocity is too high, the continuous air flow will adversely affect the ink drop as it is propelled to the printing medium, resulting in a poor or failed printing result. If the velocity is too low, then the ink drop will not properly form and will not be propelled at a high enough velocity, again resulting in a poor or failed printing result. These limitations are particularly apparent with higher viscosity liquids. 
     Therefore, a need exists for an improved air assisted drop-on-demand ink jet head which is directed toward overcoming these and other disadvantages of prior art devices. Accordingly, to address the above stated issues, a method and system for jetting high viscosity liquids to form micro-droplets and at high velocity for achieving increased print distances is needed. The exemplary teachings herein fulfill such a need. It is desired that the methods and systems for providing the above benefits be applicable to any instances or applications wherein micro-droplets of high viscosity liquid are to be dispensed. 
     SUMMARY 
     The exemplary technique(s), system(s) and method(s) presented herein relate to a pulsed air-actuated micro-droplet on demand ink/liquid jet, and in particular for jetting higher viscosity liquids with drop-on-demand requirement. The exemplary method and system include utilizing a needle extending from an ink chamber, the needle terminating in an ink drop-forming orifice outlet from which ink drops are generated, and at least two air jets directing a non-continuous or pulsed air flow at the ink drop-forming orifice outlet of the needle. The pulsed air is synchronized with the generation of a desired volume of ink at the orifice outlet to extract and propel a micro-droplet at high velocity for printing. 
     In use, ink from an ink chamber is supplied through the needle to its ink drop-forming orifice outlet, and the desired volume is pushed by a suitable actuator to the exit of the orifice and exposed. At least two air jets on opposite side of the exposed ink at the exit of the orifice are used to extract the volume of ink to produce a micro-droplet. The air jets produce a timed pulse of high velocity air to break off the micro-droplet and propel it onto the printing medium. The air jets are turned on prior to or contemporaneously with the forming of the desired volume at the exit of the orifice outlet of the needle, and turned off after the micro-droplet had been produced. A suitable control system synchronizes both the generation of desired volume at the needle orifice and the timing of the pulse of high velocity of air from the air jets. In this manner, micro-droplets on demand can be produced for liquids in a wide range of viscosities, and for printing at greater print distances. 
     Additional objects, advantages and novel features will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by production or operation of the examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing figures depict one or more implementations in accord with the present teachings, by way of example only, not by way of limitation. In the drawing figures, like reference numerals refer to the same or similar elements. 
         FIG. 1  is an illustration of a prior art air assisted drop-on-demand ink jet head; 
         FIGS. 2A ,  2 B,  2 C and  2 D are schematic depictions of the formation of a micro-droplet by the method and system of the present disclosure; and 
         FIG. 3  is a schematic depiction of an exemplary embodiment of the needle extending from an ink chamber, and an actuator used to expose the desire volume of ink at the orifice outlet of the needle. 
     
    
    
     DETAILED DESCRIPTION 
     The following description refers to numerous specific details which are set forth by way of examples to provide a thorough understanding of the relevant teachings. It should be apparent to those skilled in the art that the present teachings may be practiced without such details. In other instances, well known methods, procedures, components, and circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present teachings. While the description refers by way of example to ink and ink jet printing, is should be understood that the method(s) and system(s) described herein may be used for jetting any type of liquid of various viscosities, and for application to any type of substrate, for example, wood, metal, plastic, textiles, etc. 
     The description now proceeds with a discussion of  FIGS. 2A-2D , which depict by way of example the following:  FIG. 2A  illustrates the initial formation of the desired volume of ink at the orifice outlet of the needle, and the beginning of the timed pulse of air from the air jets.  FIG. 2B  illustrates the initial formation of the micro-droplet by the pulsed air from the air jets.  FIG. 2C  illustrates the breaking away of the micro-droplet from the supply of ink and the orifice outlet of the needle, and the acceleration of the micro-droplet toward the printing medium.  FIG. 2D  illustrates the completed micro-droplet exiting the external orifice of the ink jet head, and the ending of the timed pulse of air. 
     Accordingly,  FIGS. 2A-2D  illustrate schematically a cross-sectional view of the ink drop-forming portion of an exemplary ink jet head of the system  10  of the present disclosure. As can be seen, the end of a needle  20  terminates in an orifice outlet  30 . A first air jet  40  and a second air jet  50  are positioned at opposite sides of the orifice outlet  30  for directing a timed pulse of air at the orifice outlet  30 . An external orifice  60  of the exemplary ink jet head is located under the orifice outlet  30  of needle  20 . The external orifice  60  is axially aligned with the needle  20  and its orifice outlet  30 . 
     Turning now to  FIG. 2A , an initial formation of the desired volume  70  of ink  80  is formed at the orifice outlet  30  of the needle  20  by a suitable actuator, such as a piezoelectric crystal, a piston, or any other suitable actuator capable of pulsing the ink from an ink chamber into and through the needle  20 . The actuator force need not be sufficient to fully eject a droplet from the needle outlet. The desired volume  70  is depicted as the generally semi-spherical projection of ink extending out from the orifice outlet  30  of the needle  20 . Prior to or simultaneously with the formation of the desired volume  70 , the first air jet  40  and the second air jet  50  are activated in concert to deliver a timed pulse of air at the desired volume  70 . 
     As a result, the force of the pulsed air from the air jets  40  and  50  squeezes the desired volume  70 , as illustrated in  FIG. 2B , until the desired volume  70  breaks free from the remainder of the ink  80  in the needle  20 , as illustrated in  FIG. 2C , thus creating the micro-droplet  70   a . As illustrated in  FIG. 2C , the force from of the pulsed air from the air jets  40  and  50  continues to accelerate the micro-droplet  70   a  out of the external orifice  60  and towards the printing medium. 
     Once the micro-droplet  70   a  is formed and propelled out of the external orifice  60 , the air jets  40  and  50  are deactivated, as illustrated in  FIG. 2D . By utilizing a timed pulse of air to create and accelerate the micro-droplet  70   a  using the method described above, a micro-droplet smaller than the diameter of the orifice outlet  30  can be created and accelerated at a high enough velocity for proper jetting, and at longer print distances. This is true even for high viscosity liquids. Ending the timed pulse of air at the time illustrated in  FIG. 2D  ensures that the micro-droplet  70   a  will maintain its integrity as it travels to the printing medium at high velocity. A continuous high velocity air flow will shear the ink drop, produce long “tails” of ink trailing the ink drop, or otherwise adversely affect the integrity of the ink drop and result in improper or otherwise flawed application to the printing medium. 
     Referring now to  FIG. 3 , an exemplary embodiment of the disclosed method and system is illustrated. A piston housing  100  is illustrated having an ink chamber  110  terminating in a piston housing orifice  120  having a diameter D o . A needle  20  is suitably attached to the piston housing orifice  120 . The needle  20  having the same or substantially the same diameter as the piston housing orifice D o . The needle  20  further defining a length or liquid length S o . A piston  130  is operatively positioned inside the piston housing  100  and is moveable therein at a piston stroke distance of S p . The piston  130  has a piston diameter of D p . It should be understood that the piston  130  is moved via a suitable piston control system which actuates the piston  130  on demand, wherein the piston  130  travels the piston stroke distance S p  to push a desired volume of ink out of the ink chamber  110 , into and through the needle  20  and out of the needle orifice outlet  30 , as illustrated in  FIG. 2A . 
     While the actuator in  FIG. 3  is illustrated as a piston  130 , it should be understood that any suitable actuator may be used to push a desired volume of ink out of the needle  20  as shown in  FIG. 2A . For example, a piezoelectric crystal may be used instead of a piston, as disclosed in the &#39;875 patent referenced above. 
     Numerous factors affect the size of the ink drop, i.e. droplet diameter, and the jetting of the ink drop to the print medium. Such factors include acceleration time Δ t , orifice area A o , piston area A p , piston acceleration a p , orifice diameter D o , piston diameter D P , force on piston F p , piston mass M p , liquid column length S o , piston stroke S p , average liquid velocity U o , liquid final velocity at orifice U o,f , liquid initial velocity at orifice U o,i , average piston velocity U p , piston final velocity U p,f , piston initial velocity U p,i , liquid density ρ, and surface tension σ. In accordance with the presently disclosed method and system, it has been determined that the size of the ink drop, or more specifically the diameter of the droplet D d , can be calculated using the following equation: 
     
       
         
           
             
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     It can thus be seen that droplet volume is proportional to the ratio of surface tension to liquid density. Droplet volume is inversely proportional to acceleration. Droplet volume is proportional to the ratio of the cube root of the orifice diameter or D o   3  to the square root of the piston diameter or D p   2 . It can also be seen that higher acceleration produces smaller droplet volume. To get higher acceleration, a longer stroke length is generally needed, otherwise a huge amount of energy must be supplied. However, a longer stroke generates a larger droplet volume. To get small droplet volume, a short stroke must be used, but then high acceleration cannot typically be obtained. The present method and system, however, is able to use a short stroke to produce a small droplet volume, while simultaneously achieving high droplet velocity. 
     In the presently disclosed method and system, only a micro-volume of ink is needed to be present at the needle orifice outlet, and the timed pulse of air from the air jets is used to extract the micro-volume. The timed pulse of air from the air jets is able to accelerate the droplet up to 340 m/s (sound velocity). Accordingly, the timed pulse of air from the air jets supplies the energy needed to extract the micro-droplets from high viscosity liquids and accelerate them to high velocities. The timed pulse of air also keeps the orifice clean, keeps the drop straight as it travels to the print medium, and adds extra detachment force. 
     By way of example, the embodiment illustrated in  FIG. 3  was used to jet micro-droplets of 30W Motor Oil. In this example, the orifice diameter D o  was 152 μm and the piston diameter D p  was 850 μm. Using a piston stroke S p  of 100 μm, the timed pulse of air from the air jets produced and accelerated at high velocity micro-droplets having a diameter D d  less than the orifice diameter D o . While the above stated dimensions are illustrative of the operation of the exemplary method and system, it should be understood that various modifications may be made to these dimensions with departing from the teachings herein. 
     While the foregoing discussion presents the teachings in an exemplary fashion with respect to the disclosed method and system for pulsed air-actuated, high velocity micro-droplets on demand for high viscosity liquids, it will be apparent to those skilled in the art that the teachings may apply to any type of device that produces and applies droplets of liquid to a substrate (e.g., painting, soldering, printing, etc.). Further, while the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that the teachings may be applied in numerous applications, only some of which have been described herein.