Abstract:
A method and apparatus for delivering solvent free marking material to a receiver is provided. A printhead includes a discharge device having an inlet and an outlet with a portion of the discharge device defining a delivery path. An actuating mechanism is moveably positioned along the delivery path. A material selection device has an inlet and an outlet with the outlet of the material selection device being connected in fluid communication to the inlet of the discharge device. The inlet of the material selection device is adapted to be connected to a pressurized source of a thermodynamically stable mixture of a fluid and a marking material, wherein the fluid is in a gaseous state at a location beyond the outlet of the discharge device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to commonly assigned U.S. Docket No. 83578, filed concurrently herewith, entitled Method And Apparatus For Printing, Cleaning, and Calibrating. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to printing and more particularly, to printing using solvent free materials.  
         BACKGROUND OF THE INVENTION  
         [0003]    Traditionally, digitally controlled printing capability is accomplished by one of two technologies. The first technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source which produces a continuous stream of ink droplets (typically containing a dye or a mixture of dyes). Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When print is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism.  
           [0004]    The second technology, commonly referred to as “drop-on-demand” ink jet printing, provides ink droplets (typically including a dye or a mixture of dyes) for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.  
           [0005]    Conventional “drop-on-demand” ink jet printers utilize a pressurization actuator to produce the ink jet droplet at orifices of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink droplet to be expelled. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.  
           [0006]    Conventional ink jet printers are disadvantaged in several ways. For example, in order to achieve very high quality images having resolutions approaching  900  dots per inch while maintaining acceptable printing speeds, a large number of discharge devices located on a printhead need to be frequently actuated thereby producing an ink droplet. While the frequency of actuation reduces printhead reliability, it also limits the viscosity range of the ink used in these printers. Typically, the viscosity of the ink is lowered by adding solvents such as water, etc. The increased liquid content results in slower ink dry times after the ink has been deposited on the receiver which decreases overall productivity. Additionally, increased solvent content can also cause an increase in ink bleeding during drying which reduces image sharpness negatively affecting image resolution and other image quality metrics.  
           [0007]    Conventional ink jet printers are also disadvantaged in that the discharge devices of the printheads can become partially blocked and/or completely blocked with ink. In order to reduce this problem, solvents, such as glycol, glycerol, etc., are added to the ink formulation, which can adversely affect image quality. Alternatively, discharge devices are cleaned at regular intervals in order to reduce this problem. This increases the complexity of the printer.  
           [0008]    Another disadvantage of conventional inkjet printers is their inability to obtain true gray scale printing. Conventional ink jet printers produce gray scale by varying drop density while maintaining a constant drop size. However, the ability to vary drop size is desired in order to obtain true gray scale printing.  
           [0009]    Other technologies that deposit a dye onto a receiver using gaseous propellants are known. For example, Peeters et al., in U.S. Pat. No. 6,116,718, issued Sep. 12, 2000, discloses a print head for use in a marking apparatus in which a propellant gas is passed through a channel, the marking material is introduced controllably into the propellant stream to form a ballistic aerosol for propelling non-colloidal, solid or semi-solid particulate or a liquid, toward a receiver with sufficient kinetic energy to fuse the marking material to the receiver. There is a problem with this technology in that the marking material and propellant stream are two different entities and the propellant is used to impart kinetic energy to the marking material. When the marking material is added into the propellant stream in the channel, a non-colloidal ballistic aerosol is formed prior to exiting the print head. This non-colloidal ballistic aerosol, which is a combination of the marking material and the propellant, is not thermodynamically stable/metastable. As such, the marking material is prone to settling in the propellant stream which, in turn, can cause marking material agglomeration, leading to discharge device obstruction and poor control over marking material deposition.  
           [0010]    Technologies that use supercritical fluid solvents to create thin films are also known. For example, R. D. Smith in U.S. Pat. No. 4,734,227, issued Mar. 29, 1988, discloses a method of depositing solid films or creating fine powders through the dissolution of a solid material into a supercritical fluid solution and then rapidly expanding the solution to create particles of the marking material in the form of fine powders or long thin fibers, which may be used to make films. There is a problem with this method in that the free-jet expansion of the supercritical fluid solution results in a non-collimated/defocused spray that cannot be used to create high-resolution patterns on a receiver. Further, defocusing leads to losses of the marking material.  
           [0011]    As such, there is a need for a technology that permits high speed, accurate, and precise delivery of marking materials to a receiver to create high resolution images. There is also a need for a technology that permits delivery of ultra-small (nano-scale) marking material particles of varying sizes to obtain gray scale. There is also a need for a technology that permits delivery of solvent free marking materials to a receiver.  
         SUMMARY OF THE INVENTION  
         [0012]    According to one feature of the present invention, a printhead for delivering marking material to a receiver includes a discharge device having an inlet and an outlet with a portion of the discharge device defining a delivery path. An actuating mechanism is moveably positioned along the delivery path. A material selection device has an inlet and an outlet with the outlet of the material selection device being connected in fluid communication to the inlet of the discharge device. The inlet of the material selection device is adapted to be connected to a pressurized source of a thermodynamically stable mixture of a fluid and a marking material, wherein the fluid is in a gaseous state at a location beyond the outlet of the discharge device.  
           [0013]    According to another feature of the present invention, a printing apparatus includes a pressurized source of a thermodynamically stable mixture of a compressed fluid and a marking material and a pressurized source of a compressed fluid. A material selection device has a plurality of inlets and an outlet with one of the plurality of inlets being connected in fluid communication to the pressurized source of compressed fluid and another of the plurality of inlets being connected in fluid communication to the thermodynamically stable mixture of the compressed fluid and the marking material. A printhead, portions of which define a delivery path has an inlet and an outlet with the inlet of the delivery path being connected in fluid communication to the outlet of the material selection device. An actuating mechanism is moveably positioned along the delivery path and the compressed fluid is in a gaseous state at a location beyond the outlet of the delivery path.  
           [0014]    According to another feature of the present invention, a method of printing includes providing a printhead with portions of the printhead defining a delivery path having an inlet and an outlet with the printhead being connected in fluid communication to a source of compressed fluid and a first marking material, a source of compressed fluid and a second marking material, and a source of compressed fluid at the inlet; delivering the first marking material to a receiver; cleaning the delivery path; and delivering the second marking material to the receiver.  
           [0015]    According to another feature of the present invention, a method of printing includes providing a printhead with portions of the printhead defining a delivery path having an inlet and an outlet with the printhead being connected in fluid communication to a source of compressed fluid and a marking material, and a source of compressed fluid at the inlet; delivering a first predetermined amount of the marking material to a receiver; calibrating to a second predetermined amount of the marking material; and delivering the second predetermined amount of the marking material to the receiver.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:  
         [0017]    FIGS.  1 A- 1 C are schematic views of a first embodiment made in accordance with the present invention;  
         [0018]    FIGS.  2 A- 3 B are schematic views of a discharge device and actuating mechanism made in accordance with the present invention;  
         [0019]    [0019]FIG. 4 is a schematic view of a second embodiment made in accordance with the present invention;  
         [0020]    [0020]FIG. 5 is a schematic view of a third embodiment made in accordance with the present invention;  
         [0021]    [0021]FIG. 6 is a schematic view of a fourth embodiment made in accordance with the present invention;  
         [0022]    FIGS.  7 A- 7 B is a schematic view of a fifth embodiment made in accordance with the present invention; and  
         [0023]    FIGS.  8 A- 8 C are schematic views of printed pixel color density charts. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus 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. Additionally, materials identified as suitable for various facets of the invention, for example, marking materials, solvents, equipment, etc. are to be treated as exemplary, and are not intended to limit the scope of the invention in any manner.  
         [0025]    Referring to FIGS.  1 A- 1 C, and  4 - 7 B, a printing apparatus  20  is shown. The printing apparatus  20  includes a marking material delivery system  22  and a receiver retaining device  24 . The marking material delivery system has a pressurized source of a thermodynamically stable mixture of a fluid and a marking material, herein after referred to as a formulation reservoir(s)  102   a ,  102   b ,  102   c , connected in fluid communication to a delivery path  26  at least partially formed in/on a printhead  103 . The printhead  103  includes a discharge device  105  positioned along the delivery path  26  configured (as discussed below) to produce a shaped beam of the marking material. An actuating mechanism  104  is also positioned along the delivery path  26  and is operable to control delivery of the marking material though the printhead  103 .  
         [0026]    The formulation reservoir(s)  102   a ,  102   b ,  102   c  is connected in fluid communication to a source of fluid  100  and a source of marking material  28  (shown with reference to formulation reservoir  102   c  in FIG. 1A). Alternatively, the marking material can be added to the formulation reservoir(s)  102   a ,  102   b ,  102   c  through a port  30  (shown with reference to formulation reservoir  102   a  in FIG. 1A).  
         [0027]    One formulation reservoir  102   a ,  102   b , or  102   c  can be used when single color printing is desired. Alternatively, multiple formulation reservoirs  102   a ,  102   b , or  102   c  can be used when multiple color printing is desired. When multiple formulation reservoirs  102   a ,  102   b ,  102   c  are used, each formulation reservoir  102   a ,  102   b ,  102   c  is connected in fluid communication through delivery path  26  to a discharge device(s)  105 . A material selection device  160  is appropriately positioned along delivery path  26  such that each discharge device(s)  105  can selectively eject marking material from each formulation reservoir  102   a ,  102   b ,  102   c  depending on the position of material selection device  160 . Additionally, at least one inlet of the material selection device  160  is connected to the source of fluid  100 .  
         [0028]    A discussion of illustrative embodiments follows with like components being described using like reference symbols.  
         [0029]    Referring to FIGS.  1 A- 1 C, printhead  103 , which includes at least one discharge device  105  and actuating mechanism  104  as described below with reference to FIGS.  5 A- 5 C, is moveable (arrow A) between a first position where printing occurs (as shown in FIGS. 1A and 1B) and a second position where cleaning and/or calibration occurs (as shown in FIG. 1C). Printhead  103  translates in a first direction while receiver retaining device  24  translates in at least one other direction. A rotatable drum  150  that rotates in a second direction relative to printhead  103  during printing is shown in FIGS  1 A- 1 C. Alternatively, other types of receiver retaining devices  24  can be used with the printing system of the present invention, for example, x, y, z translation stages, rollers, individual receiver trays, etc.  
         [0030]    Printhead  103  is connected to material selection device  160  through flexible tubing  110  which allows printhead  103  to translate between the first position over receiver retaining device  24  and the second position over a cleaning station  162  and/or a calibrating station  163 . Any suitable flexible tubing  110  can be used, for example, a Titeflex extra high-pressure hose P/N R157-3 (0.110 inside diameter, 4000 psi rated with a 2 in bend radius) commercially available from Kord Industrial, Wixom, Mich. In this embodiment, rigid tubing  101  connects material selection device  160  to formulation reservoir  102   a ,  102   b ,  102   c  and fluid source  100 .  
         [0031]    Alternatively, flexible tubing  110  can be replaced with rigid tubing  101  with appropriate modifications to the receiver retaining device  24  and the cleaning station  162  and calibrating station  163 . When rigid tubing  101  replaces flexible tubing  110 , the receiver retaining device  24  should be able to translate in at least two directions during printing. This can be accomplished using, for example, x, y translation stages in any known manner. Alternatively, printhead  103  can be a page width type printhead with receiver retaining device  24  being moveable in at least one direction. Additionally, the cleaning station  162  and/or the calibrating station  163  can be modified such that cleaning station  162  and/or calibrating station  163  can be positioned in the material delivery path of printhead  103 . This can be accomplished using, for example, a solenoid mechanism that extends and retracts cleaning station  162  and/or calibrating station  163  into and from the material delivery path.  
         [0032]    During a multicolor printing operation, each color is printed sequentially, rather than in parallel. As such, each discharge device  105  of printhead  103  is used to eject each printed color which helps to maximize the resolution of printhead  103 . For example, material selection device  160  is positioned to allow a marking material (for example, a first color) from formulation reservoir  102   a  to be ejected through discharge devices  105  on printhead  103 . Printhead  103  and receiver retaining device  24  move together in one of the ways described above to print the marking material from formulation reservoir  102   a  on receiver  106 . Actuating mechanism  104  is actuated in order to deliver the correct amount of material at the appropriate time and receiver location. When this process is complete, printhead  103  translates to cleaning station  162 , as shown in FIG. 1C. Any marking material from formulation reservoir  102   a  remaining in line  110  is purged at the cleaning station  162  by positioning the material selection device  160  to allow fluid from source  100  to be ejected from discharge devices  105  and actuating mechanism  104 . The above described process is then repeated in order to eject material from formulation reservoirs  102   b  and  102   c.    
         [0033]    Typically, the purging operation is performed for a predetermined amount of time and can be calculated using characteristics of the printing system  20  such as material mass flow rates, length of line  110 , etc. Alternatively, a material sensing system  164  positioned in cleaning station  162  can be used to verify that the marking material from one formulation reservoir  102   a ,  102   b ,  102   c  has been removed from the line  110  prior to ejecting material from another of formulation reservoirs  102   a ,  102   b ,  102   c.    
         [0034]    When material sensing system  164  is used to determine whether material from one formulation reservoir  102   a ,  102   b ,  102   c  has been purged from line  110 , a closed loop sensing operation is generally preferred. In this operation, purging continues until sensing system  164  indicates that an acceptable level of marking material remains in line  110 . Sensing systems  164  of this type typically analyze ejected streams of marking material having individual particle sizes ranging from approximately 10 microns to approximately 100 microns and usually include a CCD sensor or camera with appropriate optics and a light source positioned away from the sensor or camera on the opposite side of the marking material stream. Suitable equipment for this type of marking material stream analysis is, for example, a Sony model #XC-75 camera, a Navitar Zoom lens P/N 60135, and a fiber-optic illuminator model A-3000 from Dolan Jenner.  
         [0035]    Alternatively, an off line sensing system  164  can be used. Typically, off line sensing systems measure the amount of marking material present on a receiver sample. An example of a sensing system  164  suitable to perform this type of measurement is a spectrodensitometer, model number  530 , commercially available from X-rite Inc. of Grandville Mich.  
         [0036]    Material sensing system  164  can also be used to calibrate printing system  20 . Typically, system calibration is performed when the printing system  20  is starting up, when the marking material or media type is changed, before critical printing jobs are performed, or when the printing system  20  is otherwise out of calibration. During calibration, printhead  103  can be translated to a calibration station  163  including material sensing system  164 . Calibration station  163  can be positioned next to cleaning station  162 . Alternately, cleaning and calibration can be performed in a single cleaning/calibration station  165  as shown in FIG. 1B.  
         [0037]    Any known print scanning and correction algorithm for performing printer system calibration can be used in conjunction with the present invention. For example, calibration station  163  can scan a printed test target and form a lookup table containing data that can be used to adjust the length of time each actuating device  104  remains open. Using this data, color densities can be varied as discussed below with reference to FIGS.  8 A- 8 C.  
         [0038]    Referring to FIGS.  2 A- 3 B, the discharge device  105  of the print head  103  includes a first variable area section  118  followed by a first constant area section  120 . A second variable area section  122  diverges from constant area section  120  to an end  124  of discharge device  105 . The first variable area section  118  converges to the first constant area section  120 . The first constant area section  118  has a diameter substantially equivalent to the exit diameter of the first variable area section  120 . Alternatively, discharge device  105  can also include a second constant area section  125  positioned after the variable area section  122 . Second constant area section  125  has a diameter substantially equivalent to the exit diameter of the variable area section  122 . Discharge devices  105  of this type are commercially available from Moog, East Aurora, N.Y.; and Vindum Engineering Inc., San Ramon, Calif.  
         [0039]    The actuating mechanism  104  is positioned within discharge device  105  and moveable between an open position  126  and a closed position  128  and has a sealing mechanism  130 . In closed position  128 , the sealing mechanism  130  in the actuating mechanism  104  contacts constant area section  120  preventing the discharge of the thermodynamically stable mixture of supercritical fluid and marking material. In open position  126 , the thermodynamically stable mixture of supercritical fluid and marking material is permitted to exit discharge device  105 .  
         [0040]    The actuating mechanism  104  can also be positioned in various partially opened positions depending on the particular printing application, the amount of thermodynamically stable mixture of fluid and marking material desired, etc. Alternatively, actuating mechanism  104  can be a solenoid valve having an open and closed position. When actuating mechanism  104  is a solenoid valve, it is preferable to also include an additional position controllable actuating mechanism to control the mass flow rate of the thermodynamically stable mixture of fluid and marking material.  
         [0041]    In a preferred embodiment of discharge device  105 , the diameter of the first constant area section  120  of the discharge device  105  ranges from about 20 microns to about 2,000 microns. In a more preferred embodiment, the diameter of the first constant area section  120  of the discharge device  105  ranges from about 10 microns to about 20 microns. Additionally, first constant area section  120  has a predetermined length from about 0.1 to about 10 times the diameter of first constant area section  120  depending on the printing application. Sealing mechanism  130  can be conical in shape, disk shaped, etc.  
         [0042]    Referring back to FIGS.  1 A- 1 C, the marking material delivery system  22  takes a chosen solvent and/or predetermined marking materials to a compressed liquid/compressed gas and/or supercritical fluid state, makes a solution and/or dispersion of a predetermined marking material or combination of marking materials in the chosen compressed liquid/compressed gas and/or supercritical fluid, and delivers the marking materials as a collimated and/or focused beam onto a receiver  106  in a controlled manner. In a preferred printing application, the predetermined marking materials include cyan, yellow and magenta dyes or pigments.  
         [0043]    In this context, the chosen materials taken to a compressed liquid/compressed gas and/or supercritical fluid state are gases at ambient pressure and temperature. Ambient conditions are preferably defined as temperature in the range from −100 to +100° C., and pressure in the range from 1×10 −8 -1000 atm for this application.  
         [0044]    A fluid carrier, contained in the fluid source  100 , is any material that dissolves/solubilizes/disperses a marking material. The fluid source  100  delivers the fluid carrier at predetermined conditions of pressure, temperature, and flow rate as a supercritical fluid, or a compressed liquid/compressed gas. Materials that are above their critical point, as defined by a critical temperature and a critical pressure, are known as supercritical fluids. The critical temperature and critical pressure typically define a thermodynamic state in which a fluid or a material becomes supercritical and exhibits gas like and liquid like properties. Materials that are at sufficiently high temperatures and pressures below their critical point are known as compressed liquids. Materials that are at sufficiently high critical pressures and temperatures below their critical point are known as compressed gasses. Materials in their supercritical fluid and/or compressed liquid/compressed gas state that exist as gases at ambient conditions find application here because of their unique ability to solubilize and/or disperse marking materials of interest when in their compressed liquid/compressed gas or supercritical state.  
         [0045]    Fluid carriers include, but are not limited to, carbon dioxide, nitrous oxide, ammonia, xenon, ethane, ethylene, propane, propylene, butane, isobutane, chlorotrifluoromethane, monofluoromethane, sulphur hexafluoride and mixtures thereof. In a preferred embodiment, carbon dioxide is generally preferred in many applications, due its characteristics, such as low cost, wide availability, etc.  
         [0046]    The formulation reservoir(s)  102   a ,  102   b ,  102   c  in FIG. 1A is utilized to dissolve and/or disperse predetermined marking materials in compressed liquid/compressed gas or supercritical fluids with or without dispersants and/or surfactants, at desired formulation conditions of temperature, pressure, volume, and concentration. The combination of marking materials and compressed liquid/compressed gas/supercritical fluid is typically referred to as a mixture, formulation, etc.  
         [0047]    The formulation reservoir(s)  102   a ,  102   b ,  102   c  in FIG. 1A can be made out of any suitable materials that can safely operate at the formulation conditions. An operating range from 0.001 atmosphere (1.013×10 2  Pa) to 1000 atmospheres (1.013×10 8  Pa) in pressure and from −25 degrees Centigrade to 1000 degrees Centigrade is generally preferred. Typically, the preferred materials include various grades of high pressure stainless steel. However, it is possible to use other materials if the specific deposition or etching application dictates less extreme conditions of temperature and/or pressure.  
         [0048]    The formulation reservoir(s)  102   a ,  102   b ,  102   c  in FIG. 1 should be adequately controlled with respect to the operating conditions (pressure, temperature, and volume). The solubility/dispersibility of marking materials depends upon the conditions within the formulation reservoir(s)  102   a ,  102   b ,  102   c . As such, small changes in the operating conditions within the formulation reservoir(s)  102   a ,  102   b ,  102   c  can have undesired effects on marking material solubility/dispensability.  
         [0049]    Additionally, any suitable surfactant and/or dispersant material that is capable of solubilizing/dispersing the marking materials in the compressed liquid/compressed gas/supercritical fluid for a specific application can be incorporated into the mixture of marking material and compressed liquid/compressed gas/supercritical fluid. Such materials include, but are not limited to, fluorinated polymers such as perfluoropolyether, siloxane compounds, etc.  
         [0050]    The marking materials can be controllably introduced into the formulation reservoir(s)  102   a ,  102   b ,  102   c . The compressed liquid/compressed gas/supercritical fluid is also controllably introduced into the formulation reservoir(s)  102   a ,  102   b ,  102   c . The contents of the formulation reservoir(s)  102   a ,  102   b ,  102   c  are suitably mixed, using a mixing device to ensure intimate contact between the predetermined imaging marking materials and compressed liquid/compressed gas/supercritical fluid. As the mixing process proceeds, marking materials are dissolved or dispersed within the compressed liquid/compressed gas/supercritical fluid. The process of dissolution/dispersion, including the amount of marking materials and the rate at which the mixing proceeds, depends upon the marking materials itself, the particle size and particle size distribution of the marking material (if the marking material is a solid), the compressed liquid/compressed gas/supercritical fluid used, the temperature, and the pressure within the formulation reservoir(s)  102   a ,  102   b ,  102   c . When the mixing process is complete, the mixture or formulation of marking materials and compressed liquid/compressed gas/supercritical fluid is thermodynamically stable/metastable, in that the marking materials are dissolved or dispersed within the compressed liquid/compressed gas/supercritical fluid in such a fashion as to be indefinitely contained in the same state as long as the temperature and pressure within the formulation chamber are maintained constant. This state is distinguished from other physical mixtures in that there is no settling, precipitation, and/or agglomeration of marking material particles within the formulation chamber, unless the thermodynamic conditions of temperature and pressure within the reservoir are changed. As such, the marking material and compressed liquid/compressed gas/supercritical fluid mixtures or formulations of the present invention are said to be thermodynamically stable/metastable. This thermodynamically stable/metastable mixture or formulation is controllably released from the formulation reservoir(s)  102   a ,  102   b ,  102   c  through the discharge device  105  and actuating mechanism  104 .  
         [0051]    In the embodiment shown in FIGS.  1 A- 1 C, material selection device  160  is a valve having four inputs  166  connected through rigid tubing  101  to formulation reservoirs  102   a ,  102   b ,  102   c , and fluid source  100 . Additionally, material selection device  160  has one output  168  connected to printhead  103  through flexible tubing  110 . Alternatively, material selection device  160  can include four individual two-position valves with the outputs of theses valves being connected through a plenum to flexible tubing  110 . Suitable valves, for example, valves having a pressure rating of 3000 psi (model EH121G7DCCM) are available from Peter Paul electronics, New Britain Conn.  
         [0052]    During the discharge process, the marking materials are precipitated from the compressed liquid/compressed gas/supercritical fluid as the temperature and/or pressure conditions change. The precipitated marking materials are preferably directed towards a receiver  106  by the discharge device  105  through the actuating mechanism  104  as a focussed and/or collimated beam. The invention can also be practiced with a non-collimated or divergent beam provided that the diameter of first constant area section  120  and printhead  103  to receiver  106  distance are appropriately small. For example, in a discharge device  105  having a 10 um first constant area section  120  diameter, the beam can be allowed to diverge before impinging receiver  106  in order to produce a printed dot size of about 60 um (a common printed dot size for many printing applications).  
         [0053]    Discharge device  105  diameters of these sizes can be created with modern manufacturing techniques such as focused ion beam machining, MEMS processes, etc. Alternatively, capillary tubing made of PEEK, polyimide, etc. having a desired inner diameter (ca. 10 microns) and a desired outer diameter (ca. 15 microns) can be bundled together in order to form printhead  103  (for example, a rectangular array of capillaries in a 4×100, a 4×1000, or a 4×10000 matrix). Each capillary tube is connected to an actuating mechanism  104  thereby forming discharge device  105 . Printing speed for a printhead formed in this fashion can be increased for a given actuating mechanism frequency by increasing the number of capillary tubes in each row.  
         [0054]    The particle size of the marking materials deposited on the receiver  105  is typically in the range from 1 nanometers to 1000 nanometers. The particle size distribution may be controlled to be uniform by controlling the rate of change of temperature and/or pressure in the discharge device  105 , the location of the receiver  106  relative to the discharge device  105 , and the ambient conditions outside of the discharge device  105 .  
         [0055]    The print head  103  is also designed to appropriately change the temperature and pressure of the formulation to permit a controlled precipitation and/or aggregation of the marking materials. As the pressure is typically stepped down in stages, the formulation fluid flow is self-energized. Subsequent changes to the formulation conditions (a change in pressure, a change in temperature, etc.) result in the precipitation and/or aggregation of the marking material, coupled with an evaporation of the supercritical fluid and/or compressed liquid/compressed gas. The resulting precipitated and/or aggregated marking material deposits on the receiver  106  in a precise and accurate fashion. Evaporation of the supercritical fluid and/or compressed liquid/compressed gas can occur in a region located outside of the discharge device  105 . Alternatively, evaporation of the supercritical fluid and/or compressed liquid/compressed gas can begin within the discharge device  105  and continue in the region located outside the discharge device  105 . Alternatively, evaporation can occur within the discharge device  105 .  
         [0056]    A beam (stream, etc.) of the marking material and the supercritical fluid and/or compressed liquid/compressed gas is formed as the formulation moves through the discharge device  105 . When the size of the precipitated and/or aggregated marking materials is substantially equal to an exit diameter of the discharge device  105 , the precipitated and/or aggregated marking materials have been collimated by the discharge device  105 . When the sizes of the precipitated and/or aggregated marking materials are less than the exit diameter of the discharge device  105 , the precipitated and/or aggregated marking materials have been focused by the discharge device  105 .  
         [0057]    The receiver  106  is positioned along the path such that the precipitated and/or aggregated predetermined marking materials are deposited on the receiver  106 . The distance of the receiver  106  from the discharge device  105  is chosen such that the supercritical fluid and/or compressed liquid/compressed gas evaporates from the liquid and/or supercritical phase to the gas phase prior to reaching the receiver  106 . Hence, there is no need for a subsequent receiver drying processes. Alternatively, the receiver  106  can be electrically or electrostatically charged, such that the location of the marking material in the receiver  106  can be controlled.  
         [0058]    It is also desirable to control the velocity with which individual particles of the marking material are ejected from the discharge device  105 . As there is a sizable pressure drop from within the printhead  103  to the operating environment, the pressure differential converts the potential energy of the printhead  103  into kinetic energy that propels the marking material particles onto the receiver  106 . The velocity of these particles can be controlled by suitable discharge device  105  with an actuating mechanism  104 . Discharge device  105  design and location relative to the receiver  106  also determine the pattern of marking material deposition.  
         [0059]    The temperature of the discharge device  105  can also be controlled. Discharge device temperature control may be controlled, as required, by specific applications to ensure that the opening in the discharge device  105  maintains the desired fluid flow characteristics.  
         [0060]    The receiver  106  can be any solid material, including an organic, an inorganic, a metallo-organic, a metallic, an alloy, a ceramic, a synthetic and/or natural polymeric, a gel, a glass, or a composite material. The receiver  106  can be porous or non-porous. Additionally, the receiver  106  can have more than one layer. The receiver  106  can be a sheet of predetermined size. Alternately, the receiver  106  can be a continuous web.  
         [0061]    Referring to FIG. 4, an alternative embodiment is shown. An onboard reservoir  114  positioned on printhead  103  releasably mates with a docking station  161  connected to material selection device  160  through rigid tubing  101 . Material selection device  160  is connected through rigid tubing  101  to fluid source  100  and formulation reservoirs  102   a ,  102   b ,  102   c . Again, using material selection device  160  allows all discharge devices  105  to be used during each pass of the printing operation.  
         [0062]    During operation, printhead  103  translates to docking station  161  and receives a quantity of marking material from one of formulation reservoirs  102   a ,  102   b ,  102   c  depending on the positioning of material selection device  160 . The marking material is ejected onto receiver  106 . Excess marking material, if any, is purged over cleaning station  162 . Alternatively, printhead  103  can be calibrated, if necessary, over calibrating station  163 . The process is then repeated until printing is complete.  
         [0063]    Printhead  103  can translate back to docking station  161  (for example, to receive an additional quantity of fluid from fluid source  100 ) at any time during operation. This allows onboard reservoir  114  to be recharged as needed. For example, reservoir  114  can be recharged as a function of remaining pressure or weight of the formulation in reservoir  114 , after a known volume of formulation has been ejected through printhead  103 , after a predetermined number of translations over receiver  106 , etc. Reservoir  114  is equipped with the appropriate known sensing mechanisms  116  in order to determine when reservoir  114  should be recharged.  
         [0064]    Alternatively, reservoir  114  can be equipped with a pressure increasing device  115  that forces unused marking material and/or fluid back through docking station  161  and material selection device  160  and into the appropriate formulation reservoir  102   a ,  102   b ,  102   c , of fluid source  100  when the marking material and/or fluid is no longer needed. An example of a suitable pressure-increasing device  115  is a variable volume piston having a regulated fluid pressure source sufficient to force the marking material and/or fluid back through the marking material delivery system  22 . Alternatively a mechanical force can be applied to the piston to force the marking material and/or fluid back through marking material delivery system  22 .  
         [0065]    Referring to FIG. 5, another embodiment of the present invention is shown. In this embodiment, material selection device  160  is positioned on printhead  103  such that material selection device  160  and printhead  103  travel as a unit during operation. This embodiment helps to reduce waste and time associated with the cleaning process described above, for example when material selection device  160  is positioned to allow a different marking material to be ejected through printhead  103 .  
         [0066]    Referring to FIG. 6, a premixed tank(s)  124   a ,  124   b ,  124   c , containing premixed predetermined marking materials and the supercritical fluid and/or compressed liquid/compressed gas are connected in fluid communication through tubing  110  to printhead  103 . Premixed tank  124   d , containing fluid only, is also connected in fluid communication through tubing  110  to printhead  103 . The premixed tank(s)  124   a ,  124   b ,  124   c ,  124   d  can be supplied and replaced either as a set  125 , or independently in applications where the contents of one tank are likely to be consumed more quickly than the contents of other tanks. The size of the premixed tank(s)  124   a ,  124   b ,  124   c ,  124   d  can be varied depending on anticipated usage of the contents. The premixed tank(s)  124   a ,  124   b ,  124   c ,  124   d  are connected to the discharge devices  105  of printhead  103  through material selection device  160  positioned on printhead  103 . When multiple color printing is desired, each discharge device  105  can be utilized to eject a marking material from a particular premixed tank  124   a , for example, and then utilized to eject a marking material from another premixed tank  124   b , for example. Cleaning and calibrating can be accomplished as described above.  
         [0067]    Referring to FIGS. 7A and 7B, another embodiment describing premixed canisters containing predetermined marking materials is shown. Premixed canister(s)  137   a ,  137   b ,  137   c ,  137   d  is positioned on the printhead  103 . When replacement is necessary, premixed canister  137   a ,  137   b ,  137   c ,  137   d  can be removed from the printhead  103  and replaced with another premixed canister(s)  137   a ,  137   b ,  137   c ,  137   d . Each of premixed canister(s)  137   a ,  137   b ,  137   c ,  137   d  is connected in fluid communication to discharge device  105  through material selection device  160 . When multiple color printing is desired, each discharge device  105  can be utilized to eject a marking material from a particular premixed canister  137   a , for example, and then utilized to eject a marking material from another premixed canister  137   b , for example. Cleaning and calibrating can be accomplished as described above.  
         [0068]    Referring back to FIGS.  1 A- 7 B, in addition to multiple color printing, additional marking material can be dispensed through printhead  103  in order to improve color gamut, provide protective overcoats, etc. When additional marking materials are included check valves and printhead design help to reduce marking material contamination.  
         [0069]    Each of the embodiments described above can be incorporated in a printing network for larger scale printing operations by adding additional printing apparatuses on to a networked supply of supercritical fluid and marking material. The network of printers can be controlled using any suitable controller. Additionally, accumulator tanks can be positioned at various locations within the network in order to maintain pressure levels throughout the network.  
         [0070]    In each of the embodiments described above, there are several methods for achieving appropriate gray scale levels for each color (commonly referred to as color density) used in a given printing operation. After a nominal color value for a marking material is determined during calibration of the printing system, the color value of the marking material can be altered, as desired depending on the particular printing operation, varying one or more of the control mechanisms of the printing system.  
         [0071]    For example, the duration that actuating mechanism  104  remains open can be varied causing the amount of marking material delivered to each printed pixel to vary. Alternatively, the duration that actuating mechanism  104  remains open can be held constant, while the flow rate of marking material through actuating mechanism  104  is varied. This can be accomplished by adjusting a marking material flow control device (for example, a valve positioned upstream from actuating mechanism  104 ) or by varying the open position of actuating mechanism  104 . System controller can retrieve the information required to make these adjustments in any known manner, for example, retrieving the data from a look up table created during system calibration. Alternatively, the duration and flow rate can be held constant while the concentration of marking material is varied causing the amount of marking material delivered to each printed pixel to vary. Adjusting printed pixel color density using any of these methods helps to maintain maximum printer system resolution.  
         [0072]    Referring to FIGS.  8 A- 8 C, representative gray scale levels for a printed pixel  119 - 123  are shown. In FIGS.  8 A- 8 C, five gray scale levels are shown for illustrative purposes only, as one of ordinary skill in the art is well aware that it is possible to create many gray scale levels for a printed pixel depending to the particular printing operation.  
         [0073]    Referring to FIG. 8A, pixel  119  has a lowest color density which, as is the case in most printing applications, occurs when no marking material is delivered that that pixel location on a receiver. Pixel  120  has a medium low color density which can be established, for example, by determining the concentration of marking material in the fluid necessary to create pixel  120 . The concentration of marking material can then be fixed with pixel  121  having medium color density, pixel  122  having a medium high color density and pixel  123  having a high color density being achieved during printing by increasing the duration that actuating mechanism  104  remains open, or increasing the flow rate of marking material through actuating  104 .  
         [0074]    Alternatively, pixel  120  can be established by determining the duration that actuating mechanism  104  remains open or the flow rate of marking material through actuating mechanism  104 . When duration of actuating mechanism  104  is used to establish pixel  120 , typically the most preferred duration is the minimum amount of time that actuating mechanism  104  remains open in order to establish pixel  120 . This is a function of the mechanical design of actuating mechanism  104 . Pixels  121 - 123  are then achieved by increasing the concentration of marking material in the fluid, increasing the other of the duration that actuating mechanism  104  remains open or the flow rate of marking material through actuating mechanism  104 .  
         [0075]    Referring to FIG. 8B, in some printing applications it can be advantageous to vary the size of the printed pixel  119 - 123  in order to achieve different color densities. This can be accomplished by varying additional control mechanisms of the printing system. For example, varying the diameter of the fluid stream exiting the discharge device can vary the size of the printed pixel  119 - 123 . This can be accomplished, for example, by controlling the pressure differential (fluid velocity) of the printing system; providing a discharge device  105  having an actuating mechanism  104  that can open to a plurality of diameters; varying the geometry of the discharge device  105  such that multiple exit orifice sizes are provided; providing a plurality of discharge devices  105  each having a predetermined exit diameter size; etc. Alternatively, varying the distance between the discharge device  105  and the receiver  106  can vary the size of the printed pixel  119 - 123 . This can be accomplished, for example, by positioning receiver  106  on an x, y, z translator; controlling the motion of the receiver  106  relative to the printhead  103  or the motion of the printhead  103  relative to the receiver  106 ; etc. Unlike conventional inkjet printing systems, printing with the present invention delivers a solvent free marking material to receiver  106 . As such, problems associated with bleeding of the image (which can occur with liquid and/or solvent based inks) are reduced.  
         [0076]    Referring to FIG. 8C, in some printing applications it can be advantageous to maintain a single actuating mechanism  104  duration and printed pixel size. In these situations, pixels  119 - 123  having the color densities described above can be achieved using methods known as digital half toning. In these methods, there is only one printed pixel size having one concentration of marking material, however, the multiple color densities of pixels  119 - 123  can be achieved by delivering a predetermined number of printed pixels to an area of the receiver that forms pixels  119 - 123 . This is because the human eye perceives high-density dots at less than 100% coverage as a uniform lower density area on a receiver. As such, pixel  123  is created by delivering four pixels of marking material to the receiver area that makes up pixel  123 . Pixel  122  is formed by delivering three pixels of marking material; pixel  121  is formed by delivering two pixels, pixel  120  is formed by delivering one pixel; and pixel  119  is formed by delivering no pixels of marking material.  
         [0077]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.