Abstract:
A positive air system, for a fluid jetting device that jets a fluid in a fluid droplet path prevents the ingress of dust and debris to the fluid jetting device and further prevents the introduction of dust and debris into the fluid droplet path. The air system includes an enclosure having at least one wall defining a barrier and enclosing the fluid jetting device. The barrier defines a local environment. The at least one wall has a plurality of orifices formed therein that are configured to direct a stream of pressurized air therefrom in a direction that diverges from the fluid droplet path. The fluid droplet path and the pressurized air stream direction do not converge so that the pressurized air flowing from the orifices does not interfere with the fluid moving through the droplet path.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to air systems for fluid jet devices. More particularly, the present invention pertains to air systems to prevent debris from interfering with the proper operation of fluid jet devices, such as ink jet print systems. 
     Fluid jet devices are in wide spread use. One particular use for such devices is in ink jet printers. There area number of principle types of ink jet printers. One type of printer relies upon capillary action to move a working fluid (e.g., ink) to the print head. The ink is directed from the print head through one or more orifices toward a target substrate. Ink jet printers include an actuator for urging the ink through the orifice. Actuators can include piezzo electric elements, thermal devices and the like. An exemplary ink jet print head is disclosed in DeYoung et al., U.S. Pat. No. 4,418,355. 
     The ink is ejected from the print head as a droplet of fluid. These droplets are extremely small in volume and mass. In that many such operations are carried out in commercial or industrial environments the processes are potentially subjected to dust and debris. For example, the printing is often applied to boxes or other shipping containers carried on a conveyor or line within a manufacturing facility. To this end, the potential for dust and debris to disrupt or interfere with the printing operation is quite high. 
     A number of devices, configurations and methods have been proposed and are used to prevent the introduction of dirt and debris to the inkjet print head and into the ink droplet path. For example, air knives, air curtains, blow off nozzles and air blankets are designed to alleviate dust and debris around the print heads. However, these devices are manufactured as part of the print head. As such, they are relatively costly, and cannot be retrofitted to existing inkjet system. 
     Moreover known systems typically operate at high pressures, on the order of about 30 to 80 pounds per square inch (psi). Even the known lower pressure system, generally operate at pressures of about 30 psi or greater. These high pressure systems can adversely effect printing by action of the high pressure air interfering with the ink droplet pattern. 
     Accordingly, there exists a need for an air system for inkjetting devices that reduces the potential for dust and debris interfering with the jetting pattern. Desirably, such a system effectively forces debris from an article that is to have the jetted fluid applied thereto. More desirably, such a system effectively envelopes the environment around the jetted fluid to prevent the ingress of outside dust and debris into the local environment. Most desirably, such a system minimally, if at all, adversely interferes with the jetted fluid. 
     BRIEF SUMMARY OF THE INVENTION 
     A positive air system, for a fluid jetting device that jets a fluid in a fluid droplet path prevents the ingress of dust and debris to the fluid jetting device and further prevents the introduction of dust and debris into the fluid droplet path. 
     The air system is configured to reduce the potential for dust and debris interfering with the jetting pattern. The system further forces debris from an article that is to have the jetted fluid applied thereto. Such a system provides an envelope of the local print head environment and around the jetted fluid to prevent the ingress of outside dust and debris into the local environment. 
     Importantly, the system minimally, if at all, adversely interferes with the jetted fluid. The system includes at least one wall that defines a barrier and encloses the fluid jetting device. The barrier defines the local environment. Preferably, the barrier is defined by three or four walls around the print head. 
     The wall has a plurality of orifices formed therein that are configured to direct a stream of pressurized air therefrom in a direction that diverges from the fluid droplet path. That is, the fluid droplet path and the pressurized air stream direction do not converge so that the pressurized air flowing from the orifices does not interfere with the fluid moving through the droplet path. 
     The barrier or enclosure can be formed as includes three walls defining an upper wall and a pair of opposing side walls. In one embodiment, the walls each include a primary air branch that divides into secondary air branches that divide into tertiary air branches that in turn terminate at orifice branches. The air branches are configured so as to provide a substantially equal pressure drop from the primary air branch to each of the orifices. 
     To further assure a balanced air flow and pressure at the orifices, one or more restrictors can be positioned in the air branches to provide the substantially equal pressure drop. Diverters can also be positioned within the air branches to direct air into the branches. 
     Preferably, the walls are oriented at an angle to the fluid drop path so that air that is deflected from an object onto which the fluid is jetted, is deflected away from the fluid jetting device. 
     Alternately, the positive air system includes an air knife having a pressurized air reservoir. An air inlet provides air to the reservoir and a restricted pressurized air outlet provides an exit for the air. The air outlet is formed to direct a stream of pressurized air therefrom in a direction that diverges from the fluid droplet path. In this manner, the fluid droplet path and the pressurized air stream direction do not converge. The pressurized air flowing from the outlet prevents the ingress of dust and debris to the fluid jetting device and further prevents the introduction of dust and debris into the fluid droplet path, and wherein the pressurized air flowing from the orifices does not interfere with the fluid moving through the droplet path. 
     In this embodiment, the outlet can be formed as a plurality of orifices. Alternately, the outlet can be formed as an elongated orifice-like slot. The air knife can be formed having a body and including a cover and a spacer disposed between the cover and the body. In this arrangement, the spacer defines a slot forming the air outlet. 
     Alternately, the system includes an enclosure for the fluid jetting device that defines a local environment and an air flow path. An air supply supplies air into the local environment to maintain the enclosure at a pressure greater than the pressure of an environment outside of the local environment. The air supply is configured so as to not interfere with the droplets. A pressure relief device can be used for relieving air pressure from the enclosure when the pressure exceeds a predetermined value. 
     These and other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The benefits and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of a front view of one embodiment of a positive air system for an ink jet print head in accordance with the principles of the present invention, the system being shown with a box approaching the print head; 
         FIG. 2  is a perspective front view of the positive air system; 
         FIG. 3  is a cross-section of an exemplary air curtain taken along line  3 — 3  of  FIG. 1 ; 
         FIG. 4  is a cross-section of an alternate air curtain configuration; 
         FIG. 5  is a top view of the positive air system of  FIG. 1 ; 
         FIG. 6  is perspective view of an alternate embodiment of an air knife embodying the principles of the present invention; 
         FIG. 7  is a cross-section of the air knife of  FIG. 6  taken along line  7 — 7  of  FIG. 6 ; 
         FIG. 8  is a perspective view of a still another alternate embodiment of an air knife; 
         FIG. 9  is a perspective view of yet another alternate embodiment of an air knife; 
         FIG. 10  is a schematic illustration of the positive air system of  FIGS. 1-4  shown with an optional pre-cleaning air knife; 
         FIG. 11  is an air flow pattern diagram of the air system of  FIG. 10  as the box approaches the print head; 
         FIG. 12  is a rear perspective view of the air flow pattern diagram of  FIGS. 10-11  as the box passes in front of the pre-cleaning knife; 
         FIG. 13  is a top perspective view of the air flow pattern diagram of  FIGS. 10-12  as box passes in front of the print head; 
         FIG. 14  is a schematic illustration of an alternate embodiment of a positive air system that includes a positive air enclosure, embodying the principles of the present invention illustrated with a box as the box approaches the print head; 
         FIG. 15  is a rear perspective view of the air flow pattern diagram of  FIG. 14  as the box passes in front of the print head; and 
         FIG. 16  is a front perspective view of the air flow pattern diagram, similar to  FIG. 15 , as the box passes in front of the print head. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention is susceptible of embodiment in various forms, there is shown in the drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated. 
     It should be further understood that the title of this section of this specification, namely, “Detailed Description Of The Invention”, relates to a requirement of the United States Patent Office, and does not imply, nor should be inferred to limit the subject matter disclosed herein. 
     All patents referred to herein, are hereby incorporated herein by reference, whether or not specifically do so within the text of this disclosure. 
     In the present disclosure, the words “a” or “an” are to be taken to include both the singular and the plural. Conversely, any reference to plural items shall, where appropriate, include the singular. 
     Referring now to the figures and in particular to  FIGS. 1 and 2 , there is shown a print head, for example, an ink jet print head having a positive air system  12  in accordance with the principles of the present invention. The positive air system  12  reduces the potential for dust and debris interfering with the print head jetting pattern and reduces the potential for dust and debris fouling the print head  10 . The system  12  effectively envelopes the environment E around the jetted fluid to prevent the ingress of dust and debris to the local environment E, and minimally, if at all, interferes with the pattern of the jetted fluid. 
     In a very basic form, a printing system  14  includes a conveyor  16  along which boxes B or the like are conveyed past the print head  10 . The print head  10  jets a fluid, such as ink, onto the box B to, for example, provide a bar code, a description of the package contents, a mailing address, or the like. Those skilled in the art will recognize the various arrangements by which a print head is mounted near a conveyor for such. 
     The air system  12 , as shown in  FIGS. 1 and 2  includes air knives or air curtains  18 , to define an enclosure  20  around the print head  10 . As illustrated, three air knives  18  are positioned such that, along with the conveyor  16 , they envelope the print head  10 . Each air knife  18  is formed as a wall  19  having a plurality of orifices  22 , formed in a linear array  24 , through which air is exhausted or vented. As illustrated, one air knife  18  is positioned above the print head  10  (air knife  18   a ), with the array  24  generally parallel to the direction D of conveyance of the box B. A pair of opposing knives  18   b ,  18   c  are positioned on either side of the print head  10 , with their respective arrays  24  generally perpendicular to the direction D of conveyance of the box B. An air supply  26  supplies clean, debris-free air to the air knives. 
     Referring now to  FIG. 3 , there is shown a cross-sectional view of an exemplary air knife  18 . One of the novel features of the present positive air system  12  is the ability to maintain the “cleanliness” of the environment enveloping the print head; that is, the area between the print head and the boundaries defined by the air knives  18   a,b,c , e.g., the local environment E. The present positive air system  12 -controls this environment, i.e., maintains a positive pressure to reduce or eliminate the ingress of dust and debris, while at the same time, preventing interference with the fluid jetting patterns. 
     An air path  28  is formed in each knife  18  that branches from a main or common branch  30  to each of the orifices  22 . The path  28  is configured such that the pressure drop (or the ultimate pressure) at each orifice  22  is equal to the pressure at each other orifice  22 . In this manner, there are no unaccounted for, or undetermined, air flow patterns. Rather, by balancing the pressure drop, the air flow pattern is predictable so as to prevent interference with the fluid jet pattern. In a present air knife  18 , the primary branch  30  is divided into three secondary branches  32 . Each of the secondary branches  32  is further divided into three tertiary branches  34  which in turn are divided into paired orifice feed branches  36 . 
     Each of the orifice feed branches  36  is about the same length as each other orifice feed branch  36 . As such, the pressure drop across each of the orifice feed branches  36  is about equal as well. However, the secondary  32  and tertiary branches  34  are not of equal length; thus, the pressure drop could differ between branches (that is among the secondary branches  32  or among the tertiary branches  34 ). In order to assure that the pressure drop across each of the branches  32 ,  34  is about equal, a diverter  38  is positioned at about the branch  32  or  34  junctures. In this manner, the diverter directs or diverts air flow into the various branches  32  and  34  to effect an equal pressure drop (and thus outlet pressure) at each of the orifices  22 . 
     In addition to the diverters  38 , a pin  40  can be positioned at the entrance to each of the shortest of the secondary  32  and tertiary  34  branches . The pin  40  further assists in balancing the pressure drops through the various branches to effect a balanced pressure at the orifices  22 . 
     Optionally, a restrictor such as that indicated at  42 , can be positioned at about each of the orifices  22 . The restrictor  42  is configured so as to assist in effecting an equal pressure drop (e.g., equal pressure at the orifices), and to further limit the velocity and pressure of the air exiting the orifices. Unlike known positive pressure systems which use relatively high air pressures, the present system  12  uses air at a pressure of about 1 psig to about 5 psig. It has been found that an air pressure of about 1 psig is advantageous over known high pressure systems in that the air pressure is sufficiently low so that there is little to no adverse effect on the jetted fluid. That is, the air does not move the jetted fluid from the path that the fluid would other traverse toward the media (e.g., box B) onto which it is applied. 
     An alternate embodiment of an air path  128  for an air knife  18  is shown in FIG.  4 . In this embodiment, the air path  128  is formed different from that of the embodiment  28  in FIG.  3 . The path  128  includes a main or primary branch  130  that divides into three secondary branches  132 . Each of the three secondary branches  132  in turn divides into three tertiary branches  134  which in turn divide into three orifice feed branches  136 . Again, pins  140 , diverters  138  and restrictors  142  can be used (if desired) to facilitate the balancing or equalizing or air pressure at each of the orifices  122 . Additionally, a restriction  144  (as a decrease in diameter or a restrictor) can be formed at about the primary branch  130  to further facilitate pressure balancing. 
     As seen in  FIG. 4 , the orifices  122   a  at about the edge of the knife  118  can be angled outward. In this manner (because the knives  118  are angled outward and/or upward relative to the print head  10 , as best seen in FIGS.  10 - 13 ), any gaps in air flow that may otherwise occur at the “corners” where the upper and side knives meet, are “filled”. 
     Still other embodiments of the air knife or air curtain are shown in  FIGS. 6-9 . In these embodiments, rather than a plurality of pathways, a relatively large, contained chamber  220  provides a pressurized air reservoir  223 . Air is directed out of the,reservoir  223  through a plurality of small orifice-like openings  222  in the body of the chamber  220  (FIG.  6 ), or through an elongated, narrow orifice-like slot  228  in the chamber  220  or in a cover plate  226  ( FIG. 8 ) for the chamber  220 , overlying the reservoir  223 . 
     In still another embodiment  318  as seen in  FIG. 9 , a thin spacer plate  330  (about {fraction (1/1000)} inch or 1 mil) having a notched or etched portion  332  is positioned between the chamber body  320  and the cover plate  326 . The notch  332  is open to the reservoir  323  so that air exits the reservoir  323  from between the chamber body  320  and the cover plate  326  through the an elongated orifice-like slot  322  that is defined by the notch  332 . This arrangement provides a continuous restricted flow path or continuous restriction, and as such, provides for a controlled flow (and pressure) along the length of the slot  322 . 
     An exemplary cross-section of the air knife embodiments  218 ,  318  is illustrated in FIG.  7 . As can be seen, an entrance  234 ,  334  to the reservoir  223 ,  323 , formed in the chamber body  220 ,  320  is relatively small (thus defining a restriction) compared to the size of the reservoir  223 ,  323 . As such the pressure drop at any of the orifices  222  is about equal to the pressure drop at any of the other orifices  222  and, likewise, the pressure drop at any location along the elongated slot  228 ,  322  is about equal to the pressure drop at any other location along the slot  228 ,  322 . 
     Similar to the angled orifices  122   a  of the embodiment  118  illustrated in  FIG. 4 , the spacer plate  330  can have an angled edge (as indicated at  333 ) to direct air outwardly, at an angle, to account for the angled orientation of the knives  318 . This prevents “gaps” at the corners or junctures of the upper and side knives  318 . 
     In conjunction with the novel use of a low pressure system, as seen in  FIG. 10 , the present positive air system  12  uses angled curtains or knives  18  to facilitate directing the deflected air away (indicated by the arrow at  44  in  FIG. 8 ) from the print head  10 . That is, rather than the orifices  22 ,  122 ,  222  (or slots  228 ,  322 ) directing air perpendicular to the box surface S onto which the indicia is printed, the orifices  22 ,  122 ,  222  (or slots  228 ,  322 ) direct the air at an angle relative to the surface S. In this manner, the air that deflects off of the surface S is directed away from the print head  10 , rather than toward the print head  10 . It has been observed that this arrangement blows the dust and debris away from the local environment E to maintain the print head  10  and environment E contaminant free. This arrangement also prevents the formation of eddy currents within the local environment E (e.g., immediately around the print head  10 ), that could otherwise adversely impact the fluid droplet path. 
     Also as seen in  FIGS. 10-13 , the positive air system  12  can include a supplemental box cleaner knife  46  positioned upstream of the print head  10  and its associated knives/curtains  18 ,  118 ,  218 ,  318 . This supplemental knife  46  facilitates maintaining the local environment E contaminant-free by removing any dust or debris that may be present on the box B before the box B is presented at the print head  10 . 
     An alternate embodiment of the positive air system  50  is illustrated in  FIGS. 15-16 . In this embodiment, the print head  10  is disposed within an enclosure  52  that essentially forms a tunnel  54 . As such, the air flows through the tunnel  54 , including around the print head  10 , and out a forward end  56  of the tunnel, past the print head  10 . 
     To prevent over-pressurization of the tunnel  54 , as when the box B moves passed the tunnel front  56 , a flapper valve  58  is positioned in one of the enclosure walls  60  that provides communication between the tunnel  54  and the outside environment. The flapper valve  58  is closed during normal operation, thus isolating all but the tunnel front  56 . When a box B passes in front of the tunnel  54 , moving passed the print head  10 , the flapper valve  58  opens to relieve any pressure increase in the tunnel  54 . In this manner, the air that is supplied through the tunnel  54  does not adversely effect the operation of the print head  10  (i.e., effect the fluid droplet path). Again, air is supplied from a clean, debris-free air supply  62 . 
     From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. The disclosure is intended to cover by the appended claims all such modifications as fall within the scope of the claims.