Abstract:
Material-handling systems are described that maximize the stability of materials, such as inks or clear overcoat materials. Some of the material handling systems oxygenate the material as it moves through the material-handling systems, reducing premature polymerization of the material and/or providing a stable viscosity.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to drop ejection apparatuses, and to related apparatuses and methods. 
       BACKGROUND 
       [0002]    Some radiation-curable, e.g., UV-curable, jetting materials are liquid at room temperature. To ensure correct jetting viscosity, these liquid radiation-curable materials, e.g., inks and clear overcoats, are often jetted above room temperature, e.g., 35° C. or more, e.g., 45° C. Such materials can be jetted onto substantially non-porous substrates, e.g., plastic pen barrels or circuit boards, or porous substrates, e.g., paper. When such liquid radiation-curable materials are jetted onto a substrate, e.g., paper or plastic, to form an image, phenomena such as bleed-through, pinhole wetting and fisheyes due to the wetting characteristics of the liquid can result in inadequate coverage and overall poor print quality. One solution that is often used to reduce wicking is to treat the substrate to make it less porous. However, some liquid radiation-curable materials do not perform well with such treatments. Another solution to minimizing wicking and bleed-through is to rapidly surface cure the liquid radiation-curable materials, but often this does not completely eliminate wicking and bleed-through, and can require cumbersome and expensive equipment. 
         [0003]    “Hybrid-F” radiation-curable jetting materials, i.e., those that polymerize by radical and/or cationic mechanisms to give polymer networks, are often described as “semi-solid materials,” and can be substantially more viscous at room temperature than at jetting temperature. Hybrid-F materials are available from Aellora™, e.g., under the tradename VistaSpec™ HB. Typically, these materials are jetted at elevated temperatures, e.g., above 60° C. or above 65° C., to lower material viscosity to an appropriate jetting viscosity. After jetting hybrid-F ink, e.g., through a piezoelectric drop-on-demand printhead, material viscosity rapidly increases as the material cools on contact with the substrate. Once cooled to about room temperature, the hybrid-F material does not flow without shear, allowing “wet-on-wet” printing without intermediate curing stages. Since the hybrid-F material does not substantially flow at room temperature, wetting defects can be reduced, often reducing or eliminating the need for substrate surface treatments. 
         [0004]    Liquid and hybrid-F radiation-curable materials typically contain one or more inhibitors, e.g., hydroquinone (HQ), hydroquinone monomethyl ether (MEHQ) or mixtures thereof, which help to stabilize the material, e.g., inhibit premature polymerization of the material. Premature polymerization is problematic since it can clog small and delicate flow pathways and/or jetting nozzles within a print engine. Oxygen often works in combination with inhibitors to reduce instabilities, e.g., premature thermal polymerization. In such systems, oxygen is used up in chemical reactions that occur in the material, e.g., during conveyance of the material from a supply to a printhead that jets the material. 
       SUMMARY 
       [0005]    Generally, material-handling systems are described that maximize the stability of a material such as an ink or a clear overcoat material. Some of the material handling systems oxygenate the material in the material-handling systems, e.g., as it moves through the material handling systems or as the material sits stationary in the material handling systems, reducing premature polymerization of the material. For example, apparatuses for printing on a substrate are described in which a jetting material is oxygenated during conveyance of the jetting material from a supply to a printhead. For example, an apparatus can include a printing module configured to print a material that includes a radiation-curable material and a material supply module that includes a conduit connecting the print module and a material supply. At least a portion of the printing module and/or the material supply module, e.g., the conduit, has a gas permeable device that is in fluid communication with the jetting material. The gas permeable device has a first side configured to contact the ink and a second side opposite the first side configured to be maintained at a pressure greater than the first side such that gas diffuses from the second side to the first side. Other apparatuses described herein can, e.g., include a device configured to inject gas bubbles into the material. 
         [0006]    In one aspect, the invention features drop ejecting apparatuses that include a jetting module configured to jet a material that includes a radiation-curable material, a material supply module connected to the jetting module by a conduit and a gas permeable device attached to a wall of the conduit. The device includes a partition and a gas-supply region; the partition includes a non-wetting layer adjacent the gas-supply region and a wetting layer opposite the non-wetting layer. One or more passageways extend through the wetting and non-wetting layers. 
         [0007]    Embodiments may include one or more of the following features. A thickness of the wetting layer and/or non-wetting layer is from about 0.5 micron to about 25 micron. The passageways are circular in transverse cross-section, e.g., having a diameter of from about 0.25 micron to about 5 micron. The non-wetting layer includes a fluoropolymer, such as poly(tetrafluoroethylene), PTFE. The wetting layer includes an oxide, such as a silicon oxide, e.g., silicon dioxide. The gas-supply region contains air or oxygen-enriched air (relative to air at sea level on earth). The gas-supply region is maintained at a pressure of from about 2 mm Hg to about 25 mm Hg higher than a pressure inside the conduit. 
         [0008]    In another aspect, the invention features methods of jetting materials that include providing a drop ejector that includes a jetting module configured to jet a material that includes a radiation-curable material, a material supply module connected to the jetting module by a conduit, and a gas permeable device attached to a wall of the conduit that includes a partition and a gas-delivery region. The partition includes a non-wetting layer adjacent the gas-delivery region and a wetting layer opposite the non-wetting layer, and one or more passageways extending through the wetting and non-wetting layers. The material that includes the radiation-curable material is conveyed through the conduit in such a manner that the material contacts the wetting layer of the gas permeable device; and gas is delivered to the gas-supply region. 
         [0009]    In another aspect, the invention features drop ejecting apparatuses that include a jetting module configured to jet a material that includes a radiation-curable material and a material supply module that includes a conduit connecting the jetting module and a supply. The conduit includes a material having an oxygen permeability coefficient of greater than 20×10 −11  cm 3 ·cm/cm 2 ·s·cm Hg at standard temperature and pressure. 
         [0010]    Embodiments may have one or more of the following features. The conduit includes a crosslinked material, such as a cross-linked polysiloxane. The oxygen permeability coefficient is greater than 1000×10 −11  cm 3 ·cm/cm 2 ·s·cm Hg, e.g., greater than 25000×10 −11  cm 3 ·cm/cm 2 ·s·cm Hg. 
         [0011]    In another aspect, the invention features methods of jetting materials that include providing a drop ejector that includes a jetting module configured to jet a material that includes a radiation-curable material and a material supply module connected to the jetting module by a conduit. The conduit includes a material having an oxygen permeability coefficient of greater than 20×10 −11  cm 3 ·cm/cm 2 ·s·cmHg at standard temperature and pressure. The material that includes the radiation-curable material is conveyed through the conduit. 
         [0012]    In another aspect, the invention features, methods of jetting that include providing a drop ejector that includes a jetting module configured to jet a material that includes a radiation-curable material and a material supply module housing the material and connected to the jetting module by a conduit; and delivering gas bubbles to the material. 
         [0013]    Embodiments may include one or more of the following features. The gas bubbles are delivered from a porous bubbler, such as one that includes sintered metal particles. The gas bubbles delivered have a diameter of less than about 100 micron, e.g., less than about 10 micron or less than about 1 micron. 
         [0014]    In another aspect, the invention features packages for holding a jetting material that include a hollow first container housing material including a radiation-curable material, an operable seal disposed on the first container and a hollow second container disposed inside the sealed hollow first container, the hollow second container having an aperture defined in a wall of the second container. 
         [0015]    Embodiments may include any one or more of the following features. Each aperture is circular in transverse cross-section, e.g., each having a diameter of between about 0.001 inch to about 0.025 inch. The hollow first container is sealed with air or oxygen-enriched air (relative to sea level air on earth). A pressure in an airspace of the first container is greater than 12 psi (gauge), e.g., greater than 20 psi, 30 psi, 50 psi or even greater than 100 psi. 
         [0016]    Embodiments may have one or more of the following advantages. Generally, the material, such as ink, in the material-handling systems has enhanced stability, e.g., a reduced tendency to polymerize. For example, the ink handling systems have a reduced tendency to thermally polymerize ink flowing through the ink flow pathways, which can result in a system having enhanced ink flow and jetting performance. Such ink handling systems have a reduced tendency for ink flow pathway blockage, nozzle clogging, and/or valve blockage. This in turn reduces cleaning downtime and improves printing efficiency. Keeping the often small and delicate flow paths and/or nozzles clear of containments allow materials to flow through the flow paths with reduced resistance. Lower resistance to flow enables, e.g., a more rapid refilling of the pumping chamber. For example, rapidly refilling the pumping chamber can translate into an ability to eject drops at a higher frequency, e.g., 10 kHz, 25 kHz, 50 kHz or higher, e.g., 75 kHz. Higher frequency printing can improve the resolution of ejected drops by increasing the rate of drop ejection, reducing size of the ejected drops, and enhancing velocity uniformity of the ejected drops. In addition, keeping nozzles and/or flow paths clear of polymerized ink can reduce ejection errors, such as mis-fires or trajectory errors, and thereby improve overall print quality. 
         [0017]    Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference herein in their entirety for all that they contain. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
         [0018]    Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a perspective schematic view of a printing apparatus including a printing module and ink supply module. 
           [0020]      FIGS. 1A and 1B  are perspective front and back views of a printhead, respectively. 
           [0021]      FIG. 2  is an enlarged perspective view of a portion of a printhead. 
           [0022]      FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 1 . 
           [0023]      FIG. 4  is a perspective schematic view of a printing apparatus that includes a conduit having a relatively high oxygen gas permeability. 
           [0024]      FIG. 5  is a schematic side view of a porous bubbler delivering bubbles of a gas to a jetting material. 
           [0025]      FIG. 5A  is a highly enlarged view of region  5 A of  FIG. 5 . 
           [0026]      FIGS. 6A and 6B  are perspective see-through views of a package for housing a jetting material;  FIG. 6A  being the package in a sealed state and  FIG. 6B  being the package in an open state. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Referring  FIG. 1 , an apparatus  10  for printing on a substrate  12  includes a material supply module  16  and a printing module  14  which is configured to jet a material  25  that includes a radiation-curable material. Material supply module  16  has a pathway  18  from a material supply  21  to the printing module  14 . The pathway  18  includes a first portion  20  that is configured to maintain the jetting material below a first temperature T 1 , and a second portion  22  downstream of first portion  20  that is configured to heat the jetting material above first temperature T 1 , e.g., at least about 25° C. above first temperature T 1 , as it is conveyed (indicated by arrow  19 ) through second portion  22 . 
         [0028]    Pathway  18  can be made gas permeable, allowing for oxygenation of jetting material  25 . Oxygenation replaces any oxygen lost, e.g., due to chemical reactions, in the jetting material during its conveyance from supply  21  to printing module  14 . Maintaining or enhancing an oxygen level of the jetting material from jetting material supply to printing module enhances the performance of the inhibition package added to the jetting material. In particular implementations, pathway  18  can include one or more (e.g., 2, 4, 6, 8, 10 or even 20) semi-permeable devices  41  that are disposed along the length of pathway  18 . The semi-permeable nature of the device  41  prevents jetting material from escaping from the flow pathway  18 , while allowing oxygen to pass through. Oxygen works synergistically, and in combination with, inhibitors to reduce instabilities, e.g., premature thermal polymerization of jetting material components, e.g., in flow pathway  18 . In addition, flow pathway  18  can include filters  17 , e.g., screen-type filters or sintered-type filters. Such filters can remove dust, debris and gels from the jetting material that can block ink flow pathways, nozzles, valves and/or filters, leading to a reduction in print quality. Such filters can also be located at other suitable locations along the ink flow pathways. 
         [0029]    In the embodiment of  FIG. 1 , jetting material  25  is conveyed through supply module  16  utilizing an auger  30 . Controller  32  manages the direction of rotation and the rotational speed of auger  30 . After exiting portion  22  of pathway  18 , the jetting material is delivered to a reservoir  40  in printing module  14 , where the temperature of the jetting material is maintained at a suitable jetting temperature, e.g., greater than 75° C. In some instances, the heating of the jetting material in the second portion  22  increases jetting material temperature exiting the second portion to a temperature that is within 15° C. of jetting material residing in the reservoir  40 . This minimizes the possibility that the ink in reservoir  40  is thermally shocked by the ink entering from the ink supply module  16 . The jetting material then travels along flow path  42  to printhead  44 . Controller  46  controls the jetting of material onto substrate  12 , which is traveling below the printhead (as indicated by arrow). Drop ejection is controlled by pressurizing the jetting material with an actuator, such as, e.g., a piezoelectric actuator, a thermal bubble jet generator, or an electrostatically deflected element. Typically, printhead  44  has an array of paths with corresponding nozzle openings and associated actuators, such that drop ejection from each nozzle opening can be independently controlled. U.S. Pat. No. 5,265,315 describes a printhead that has a semiconductor body and a piezoelectric actuator. Piezoelectric inkjet printheads are described in U.S. Pat. Nos. 4,825,227, 4,937,598, 5,659,346, 5,757,391, and in U.S. Patent Application No. 2004/0004649, now issued as U.S. Pat. No. 7,052,117. Jetting material such as ink on substrate  12 , e.g., in the form of text or graphics, is cured with a radiation source  47 , e.g., ultra-violet light from a UV lamp  49 , or e-beam radiation. If UV radiation is used to cure the radiation-curable material, a wavelength of the light that cures the radiation-curable material is between about 200 nm and about 400 nm, e.g., a typical output from a medium pressure, metal-doped lamp, e.g., an iron-mercury lamp. 
         [0030]    Referring to  FIGS. 1 ,  1 A,  1 B and  2 , piezoelectric printhead  44  includes jetting modules  50  and an orifice plate  52  with an array of orifice openings  53 . The orifice plate  52  is mounted on a manifold  54 , attached to a collar  56 . The inkjet printhead  44  is controlled by electrical signals conveyed by flexprint elements  60  that are in electrical communication with controller  46  of print module  14 . 
         [0031]    Referring particularly to  FIG. 2 , in operation, jetting material flows from a reservoir (not shown) into a passage  72 . The jetting material is then conveyed through passage  76  to a pressure chamber  77  from which it is ejected on demand through an orifice passageway  80  and a corresponding orifice  53  in the orifice plate  52  in response to selective actuation of an adjacent portion  82  of a piezoelectric actuator plate  84 . Commercial inkjet printheads are available from Dimatix, Inc. (Spectra Printing Division), Hanover, N.H. 
         [0032]    Referring again to  FIG. 1  and now to  FIG. 3 , semi-permeable device  41  includes a partition  100 , and a gas-delivery region  102 , which in operation encloses a gas, e.g., air or oxygen-enriched air, under pressure. During operation, gas is delivered to gas-delivery region  102  via a gas source  104 . Partition  100  includes passageways  106  between the pathway  18  and pressure region  102 . Partition  100  also includes a wetting layer  110  adjacent the pathway  12  and a non-wetting layer  112  adjacent the gas-delivery region  102 . Jetting material in region  120  along flow pathway  18  comes into contact with wetting layer  110  of partition  100 , and absorbs oxygen delivered through passageways  106  that communicate with gas-delivery region  102  (flow indicated by arrows). To facilitate flow of gas from the gas-delivery region, the gas-delivery region is typically maintained at a higher pressure (e.g., about 5 mm Hg to about 50 mm Hg, about 2 mm Hg to about 25 mm Hg or about 5 mm Hg to about 10 mm Hg) than pressure in flow pathway  18 . In particular, jetting material  25  in region  120  contacts partition  100  and enters passageways  106 , forming a meniscus  122  at the interface between the wetting and non-wetting layers  110 ,  112 . The jetting material in region  120  is exposed, through the passageways  106 , to the gas-delivery region  102 , absorbing gas as it passes. The size of the passageways  106 , magnitude of the gas pressure and the materials of the partition layer  100  are selected such that fluid is drawn into the passageways  106 , but not drawn beyond the passageways  106  and into the gas-delivery region  102 . 
         [0033]    In some embodiments, the passageways  106  are circular in transverse cross-section, having a radius of about 5 micron or less, e.g., between about 5 micron and about 0.1 micron, e.g., between about 1.0 micron and 0.5 micron. A partition layer having an exposed surface area of several square centimeters typically includes thousands of passageways. For example, between about 10% and about 90% (e.g., 20% to 80%, 30% to 70%, 40% to 50%) of the partition can be made up of open passageways. 
         [0034]    In some embodiments, the wetting layer  110  has a surface energy equal to or greater than 40 dynes/cm, as determined according to the dynes test. In general, the dynes test is used to determine the surface energy of a solid surface through the application of a series of fluids that each have a different surface energy level (e.g., 30 dynes/cm to 70 dynes/cm in +1 dynes/cm increments.) A drop of one of the fluids in the series is applied to the solid surface. If the drop wets the surface, then a drop of the next higher surface energy level fluid is applied to the solid surface. This process is continued until the drop of fluid does not wet the solid surface (i.e., cohesive forces are stronger than adhesive forces). The surface energy of the solid surface is determined to be the same as the surface energy of the first fluid in the series that does not wet the solid surface. Equipment and instructions for performing the dynes test are available from Diversified Enterprises, Claremont, N.H. An example of a suitable material for the wetting layer  110  is an oxide layer, such as silicon dioxide. In some embodiments, the wetting layer has a thickness of about 25 micron or less, e.g., 1 micron or less. In some embodiments, the non-wetting layer  112  has a surface energy of about 40 dynes/cm or less, such as 25 dynes/cm or less. An example of a suitable material for the non-wetting layer  112  is a polymer, such as a fluoropolymer, e.g., poly(tetrafluoroethylene) available under the tradename TEFLON®. In some embodiments, the non-wetting layer  112  has a thickness of about 2 micron, e.g. about 1 micron or about 0.5 micron. A deaerator having a wetting and a non-wetting layer is described in Hoisington et al., in U.S. Patent Application No. 2005/0185030, now issued as U.S. Pat. No. 7,052,122. 
         [0035]    Referring now to the embodiment shown in  FIG. 4 , in some embodiments, a jetting material is conveyed from supply  21  to printing module  14  through a conduit  130  that includes a wall  132  having an oxygen permeability coefficient of greater than 20×10 −11  cm 3 ·cm/cm 2 ·s·cm Hg, e.g., greater than 40×10 −11 , 100×10 −11 , 200×10 −11 , 500×10 −11 , 1000×10 −11 , 4000×10 −11 , 25000×10 −11 , or even greater than 40000×10 −11  cm 3 ·cm/cm 2 ·s·cm Hg at standard temperature and pressure (STP). 
         [0036]    Suitable conduit materials generally have good chemical resistance and a relatively high oxygen permeability coefficient. Materials include, e.g., crosslinked polyvinylchloride, crosslinked chlorinated polyvinylchloride, crosslinked polyurethane, and crosslinked silicone. 
         [0037]    Referring now to the embodiment shown in  FIGS. 5 and 5A , the jetting material can be saturated or even super-saturated with a gas such as air or oxygen-enriched air by delivering bubbles of a desired gas to the jetting material. In the particular embodiment illustrated in  FIGS. 5 and 5A , the bubbles are delivered to the jetting material from a source of pressurized gas  200 , which is controlled by actuating a valve  202  using a controller  204 . When valve  202  has been actuated to the “on” position, gas enters a porous bubbler  210 , which converts the bulk gas to small bubbles  212  of the gas. To increase efficiency of solubilizing the gas into the jetting material, bubbles smaller than 1000 micron are generally preferred, e.g., less than 500 micron, less than 250 micron, less than 100 micron, less than 50 micron, less than 25 micron, less than 10 micron, less than 1 micron, or even less than 0.5 micron. 
         [0038]    The bubbler can be made by sintering metal particles  222 , e.g., having diameters between about 0.1 micron and 10 micron, to form a porous material though which a gas may flow. Bubblers are available from Mott Corporation, and are described in Kerfoot, U.S. Pat. No. 6,827,861 and Mitani et al.,  Ozone: Science and Engineering,  27, 45-51 (2005). 
         [0039]    Gas bubbles can be delivered at any point or multiple points along the flow pathway from ink supply to jetting module. 
         [0040]    Referring now to the embodiment shown in  FIGS. 6A and 6B , a pressurized jetting material supply drum  230  is sealed via bung  231 . Prior to sealing and pressurizing the drum  230  with a desired gas, e.g., air or oxygen-enriched air, a hollow cylinder  232  that is closed except for an aperture  240  defined in a wall  234  of the hollow cylinder  232  is placed into the supply drum, followed by a desired jetting material  25 . Prior to use, the bung  231  is removed, exposing a relatively large aperture  241 , e.g., a circular aperture having a diameter of about 1 to about 5 inch. The pressure in the airspace  250  is rapidly equalized with atmospheric pressure. However, in order for the pressure to equalize in the hollow cylinder  232 , gas has to be transferred through relatively small aperture  240 , generating a stream of bubbles  251  exiting aperture  240 . A beverage employing this principle has been described by Forage et al., U.S. Pat. No. 4,832,968. 
         [0041]    In some embodiments, the supply drum  230  is pressurized to a pressure of 12 psi (gauge), e.g., 20 psi, 30 psi, 50 psi or even 100 psi. In some embodiments, the aperture  241  is circular in cross-section and has a diameter of less than 0.030 inch, e.g., less than 0.025 inch, less than 0.020 inch, less than 0.010 inch, less than 0.005 inch, less than 0.001 inch or even less than 0.0005 inch. 
         [0042]    Generally, suitable jetting materials include clear overcoats, colorants, polymerizable materials, e.g., monomers and/or oligomers, and photoinitiating systems. The polymerizable materials can be cross-linkable. 
         [0043]    Colorants in the jetting material can include pigments, dyes, or combinations thereof. In some implementations, inks include less than about 10 percent by weight colorant, e.g., less than 7.5 percent, less than 5 percent, less than 2.5 percent or less than 0.1 percent. 
         [0044]    The pigment can be black, cyan, magenta, yellow, red, blue, green, brown, or a mixture these colors. Examples of suitable pigments include carbon black, graphite and titanium dioxide. Additional examples are disclosed in, e.g., U.S. Pat. No. 5,389,133. Alternatively or in addition to the pigment, the inks can contain a dye. Suitable dyes include, e.g., Orasol Pink 5BLG, Black RLI, Blue 2GLN, Red G, Yellow 2GLN, Blue GN, Blue BLN, Black CN, and Brown CR, each being available from Ciba-Geigy. Additional suitable dyes include Morfast Blue 100, Red 101, Red 104, Yellow 102, Black 101, and Black 108, each being available from Morton Chemical Company. Other examples include, e.g., those disclosed in U.S. Pat. No. 5,389,133. Mixtures of colorants may be employed. 
         [0045]    Generally, the jetting materials contain a polymerizable material, e.g., one or more polymerizable monomers. The polymerizable monomers can be mono-functional, di-functional, tri-functional or higher functional, e.g., penta-functional. The mono-, di- and tri-functional monomers have, respectively, one, two, or three functional groups, e.g., unsaturated carbon-carbon groups, which are polymerizable by irradiating in the presence of photoinitiators. In some implementations, the jetting materials include at least about 40 percent, e.g., at least about 50 percent, at least about 60 percent, or at least about 80 percent by weight polymerizable material. Mixtures of polymerizable materials can be utilized, e.g., a mixture containing mono-functional and tri-functional monomers. The polymerizable material can optionally include diluents. 
         [0046]    Examples of mono-functional monomers include long chain aliphatic acrylates or methacrylates, e.g., lauryl acrylate or stearyl acrylate, and acrylates of alkoxylated alcohols, e.g., 2-(2-ethoxyethoxy)-ethyl acrylate. The di-functional material can be, e.g., a diacrylate of a glycol or a polyglycol. Examples of the diacrylates include the diarylates of diethylene glycol, hexanediol, dipropylene glycol, tripropylene glycol, cyclohexane dimethanol (Sartomer CD406), and polyethylene glycols. Examples of tri- or higher functional materials include tris(2-hydroxyethyl)-isocyanurate triacrylate (Sartomer SR386), dipentaerythritol pentaacrylate (Sartomer SR399), and alkoxylated acrylates, e.g., ethoxylated trimethylolpropane triacrylates (Sartomer SR454), propoxylated glyceryl triacrylate, and propoxylated pentaerythritol tetraacrylate. The jetting materials may also contain one or more oligomers or polymers, e.g., multi-functional oligomers or polymers. 
         [0047]    In some instances, the viscosity of the jetting material is between about 1 centipoise and about 50 centipoise, e.g., from about 5 centipoise to about 45 centipoise, or from about 7 centipoise to about 35 centipoise, at a temperature ranging from about 20° C. to about 150° C. 
         [0048]    A photoinitiating system, e.g., a blend, in the jetting materials is capable of initiating polymerization reactions upon irradiation, e.g., ultraviolet light irradiation. The photoinitiating system can include, e.g., an aromatic ketone photoinitiator, an amine synergist, an alpha-cleavage type photoinitiator, and/or a photosensitizer. Each component is fully soluble in the monomers and/or diluents described above. Specific examples of the aromatic ketones include, e.g., 4-phenylbenzophenone, dimethyl benzophenone, trimethyl benzophenone (Esacure TZT), and methyl O-benzoyl benzoate. 
         [0049]    An amine synergist can be utilized. For example, the amine synergist can be a tertiary amine. Specific examples of the amine synergists include, e.g., 2-(dimethylamino)-ethyl benzoate, ethyl 4-(dimethylamino) benzoate, and amine functional acrylate synergists, e.g., Sartomer CN384, CN373. 
         [0050]    An alpha-cleavage type photoinitiator can be an aliphatic or aromatic ketone. Examples of the alpha-cleavage type photoinitiators include, e.g., 2,2-dimethoxy-2-phenyl acetophenone, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, and 2-methyl-1-[4-(methylthio)phenyl-2-morpholino propan-1-one (Irgacure 907). 
         [0051]    A photosensitizer can be a substance that either increases the rate of a photoinitiated polymerization reaction or shifts the wavelength at which the polymerization reaction occurs. Examples of photosensitizers include, e.g., isopropylthioxanthone (ITX), diethylthioxanthone and 2-chlorothioxanthone. 
         [0052]    The jetting materials may contain an adjuvant such as a vehicle (e.g., a wax or resin), a stabilizer, an oil, a flexibilizer, or a plasticizer. The stabilizer can, e.g., inhibit oxidation of the ink. The oil, flexibilizer, and plasticizer can reduce the viscosity of the jetting material. 
         [0053]    Examples of waxes include, e.g., stearic acid, succinic acid, beeswax, candelilla wax, carnauba wax, alkylene oxide adducts of alkyl alcohols, phosphate esters of alkyl alcohols, alpha alkyl omega hydroxy poly(oxyethylene), allyl nonanoate, allyl octanoate, allyl sorbate, allyl tiglate, bran wax, paraffin wax, microcrystalline wax, synthetic paraffin wax, petroleum wax, cocoa butter, diacetyl tartaric acid esters of mono and diglycerides, alpha butyl omega hydroxypoly(oxyethylene)poly(oxypropylene), calcium pantothenate, fatty acids, organic esters of fatty acids, amides of fatty acids (e.g., stearamide, stearyl stearamide, erucyl stearamide (e.g., Kemamide S-221 from Crompton-Knowles/Witco), calcium salts of fatty acids, mono &amp; diesters of fatty acids, lanolin, polyhydric alcohol diesters, oleic acids, palmitic acid, d-pantothenamide, polyethylene glycol (400) dioleate, polyethylene glycol (MW 200-9,500), polyethylene (MW 200-21,000); oxidized polyethylene; polyglycerol esters of fatty acids, polyglyceryl phthalate ester of coconut oil fatty acids, shellac wax, hydroxylated soybean oil fatty acids, stearyl alcohol, and tallow and its derivatives. 
         [0054]    Examples of resins include, e.g., acacia (gum arabic), gum ghatti, guar gum, locust (carob) bean gum, karaya gum (sterculia gum), gum tragacanth, chicle, highly stabilized rosin ester, tall oil, manila copais, corn gluten, coumarone-indene resins, crown gum, damar gum, dimethylstyrene, ethylene oxide polymers, ethylene oxide/propylene oxide copolymer, heptyl paraben, cellulose resins, e.g., methyl and hydroxypropyl; hydroxypropyl methylcellulose resins, isobutylene-isoprene copolymer, polyacrylamide, functionalized or modified polyacrylamide resin, polyisobutylene, polymaleic acid, polyvinyl acetate, polyvinyl alcohol, polyvinyl pyrrolidone, rosin, pentaerythritol ester, purified shellac, styrene terpolymers, styrene copolymers, terpene resins, turpentine gum, zanthan gum and zein. 
         [0055]    Examples of stabilizers, oils, flexibilizers and plasticizers include, e.g., methylether hydroquinone (MEHQ), hydroquinone (HQ), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate, tert-butyl hydroquinone (TBHQ), ethylenediaminetetraacetic acid (EDTA), methyl paraben, propyl paraben, benzoic acid, glycerin, lecithin and modified lecithins, agar-agar, dextrin, diacetyl, enzyme modified fats, glucono delta-lactone, carrot oil, pectins, propylene glycol, peanut oil, sorbitol, brominated vegetable oil, polyoxyethylene 60 sorbitan monostearate, olestra, castor oil; 1,3-butylene glycol, coconut oil and its derivatives, corn oil, substituted benzoates, substituted butyrates, substituted citrates, substituted formats, substituted hexanoates, substituted isovalerates, substituted lactates, substituted propionates, substituted isobutyrates, substituted octanoates, substituted palmitates, substituted myristates, substituted oleates, substituted stearates, distearates and tristearates, substituted gluconates, substituted undecanoates, substituted succinates, substituted gallates, substituted phenylacetates, substituted cinnamates, substituted 2-methylbutyrates, substituted tiglates, paraffinic petroleum hydrocarbons, glycerin, mono- and diglycerides and their derivatives, polysorbates  20 ,  60 ,  65 ,  80 , propylene glycol mono- and diesters of fats and fatty acids, epoxidized soybean oil and hydrogenated soybean oil. 
         [0056]    Additional jetting materials are described by Woudenberg in U.S. Patent Application No. 2004/0132862, now issued as U.S. Pat. No. 6,896,937. 
       OTHER EMBODIMENTS 
       [0057]    While certain embodiments have been described, other embodiments are possible. While the embodiment of  FIG. 1  utilizes a single jetting material, more than one jetting material can be conveyed. For example, two, three, four, five, six, seven or more, e.g., ten jetting materials can be conveyed. Each jetting material may be a different color, for example. 
         [0058]    While oxygen or oxygen-enriched air has been used in some embodiments, other gases, e.g., inert gases such as nitrogen or argon, may be utilized. 
         [0059]    Any of the systems described herein can be combined. For example, a hybrid system can be produced by combining the bubbler of  FIG. 5  with the device of  FIG. 3  and/or the conduit of  FIG. 4 . 
         [0060]    Other embodiments are within the scope of the following claims.