Abstract:
Several components are used in combination to effect an apparatus for a spray system, which improves the transfer characteristics and efficiency of polymer applied to the surface of a printing plate. In accordance with this invention, the apparatus comprises: a nozzle for spraying a solid stream of polymer, a circumferential conduit surrounding the polymer spray that carries heated, high-pressure air, which heats, atomizes and improves the transfer efficiency of the polymer, a mixer that allows the mixing of the polymer&#39;s constituent reagents immediately prior to the polymer entering the spray nozzle, a fast shut-off valve that allows precision control of the spray nozzle&#39;s output, a shroud equipped with vacuum to remove and recycle “overspray”, and a cleaning arm, equipped with a vacuum, that may be activated to clean unused polymer from the components of the nozzle, and to prime the spray nozzle with fresh polymer.

Description:
FIELD OF THE INVENTION 
     The invention relates to pressurized spray systems. Specifically, the invention relates to a spray system used to apply a polymer to the surface of a printing plate. 
     BACKGROUND OF THE INVENTION 
     In general, the surface of a commercial printing plate is coated with reactive materials. The layer of reactive material applied to the plate is capable of being imaged (frequently the imaging process involves laser light) and is also capable of retaining the image on the plate. The image bearing plate may subsequently be used in a printing press to impart the image onto a printing surface. Recently, “direct-to-press” technology has permitted printing plates to be imaged directly on the printing press. The reactive materials used to coat printing plates are typically light or heat sensitive polymers, which are sprayed onto the printing plate through some sort of pressurized spray nozzle. Throughout this application, the word “polymer” is used to describe the reactive material sprayed on a printing plate. However, the word “polymer” should not be construed in a limiting sense, as the invention would apply to any material used to coat printing plates. In addition, the spray nozzle that is the subject of this invention may be used to temporarily spray other materials used in plate preparation, such as cleaning agents, stripping agents, water and other chemicals. For simplicity, use of the word “polymer” should also be understood to incorporate these other materials temporarily sprayed on a printing plate surface. 
     Many types of spray nozzles exist in the prior art and they disclose various techniques for spraying different kinds of materials. U.S. Pat. No. 5,360,165 discloses a spray paint nozzle with a conical shroud, used to contain the sprayed paint, and a “recirculation mechanism”, used to reclaim leftover paint that does not adhere to the target surface. U.S. Pat. No. 5,441,201 discloses a liquid spray device for use in agricultural applications that is equipped with a shroud and uses hot air to break up conical sheets of sprayed liquid into droplets. U.S. Pat. No. 5,285,967 teaches a high velocity thermal spray gun for spraying high temperature melted powdered plastics, which also involves a shroud and a mechanism for cooling the melted powder down prior to reaching the target. U.S. Pat. No. 4,218,019 involves an “air shroud”, which contains the sprayed liquid and directs it toward the target. Finally, U.S. Pat. No. 5,057,342 discloses an apparatus for improving the feathering of the output from an “airless” spray nozzle. While these prior art inventions teach techniques that may be generally applicable to spray nozzles, none of them address concerns particularly related to the application of polymer to the surface of printing plates. 
     In order to ensure the quality of the printed image the application of the polymers to a printing plate must be “complete” (i.e. a coating on the entire imaging area of the plate) and “uniform” (i.e. a consistently even layer in all imaging areas of the plate). An additional consideration that is important to the spraying of polymers onto printing plates is the actual transfer efficiency of the spraying process. As the polymers used on printing plates are expensive, it is obviously beneficial to maximize the amount of polymer transferred from the nozzle to the plate and minimize the amount of wastage. These criteria of completeness, uniformity and transfer efficiency are most easily achieved when the polymer is sprayed in an atomized mist, in a fashion similar to commercially available “spray-paint” canisters. Unfortunately, the molecules of polymers used on commercial printing plates tend to become entangled with one another, making the polymer difficult to atomize, substantially reducing the effectiveness of conventional low-pressure spraying techniques. Accordingly, a spraying apparatus is required to obtain a complete, uniform coating of relatively entangled polymer on a printing plate with a high transfer efficiency. 
     In addition to the above requirements, the spraying of polymers onto printing plates involves a number of additional complications. Typically, the polymers used on printing plates are comprised of two or more reagents that must be mixed prior to spraying. The mixing process initiates a chemical reaction, similar to that of epoxy resin, which causes the polymer to cure and harden. Consequently, the mixing must be done immediately prior to spraying to avoid premature curing. In addition, any excess polymer that is mixed but not sprayed is wasted because it cures and is no longer sprayable. As such, an apparatus is required to mix the polymer&#39;s constituent reagents immediately prior to spraying in a manner that will minimize the amount of polymer that is wasted by being mixed, but not sprayed. 
     Excess wastage is also a problem when applying polymer to a plate that is already mounted on the drum of a printing press. This process is common in today&#39;s “direct-to-press” technology. Since part of the drum surface (referred to in this application as the “plate-mounting gap” or “gap”) is used to mount the plate and does not require a coating of polymer, any polymer sprayed into the plate-mounting gap is wasted. As such, an apparatus is required to minimize the amount of polymer sprayed into the plate-mounting gap. 
     A final consideration is the need for cleaning of the spray nozzle apparatus. If the polymer collects in the nozzle mechanism, it may cure and impair the functionality of the device. Consequently, an apparatus is required to facilitate the efficient cleaning of the spray apparatus. 
     SUMMARY OF THE INVENTION 
     The invention herein disclosed concerns an apparatus for a spray system operative to spray a substantially liquid polymer onto the surface of a printing plate. A fluid nozzle receives the polymer from the internal features of the spray system and ejects the polymer in a substantially liquid state. Surrounding the fluid nozzle, there is a conduit, which carries heated, high pressure air. The air heats the polymer, prior to its ejection from the fluid nozzle. The conduit also ejects the heated air, in such a manner that the air physically interacts with, and atomizes, the substantially liquid polymer, creating a mist of polymeric matter. The heating of the polymer by the air in the conduit makes it easier to atomize the polymer. 
     The apparatus also comprises a solid shroud surrounding the fluid nozzle and extending toward the printing plate. The shroud is equipped with at least one aperture attached to a vacuum source, such that the shroud, aperture and vacuum source act in combination to remove excess polymer that does not adhere to the printing plate. A fast shut-off valve, which controls the ejection of the polymer from the fluid nozzle, is located proximate to the fluid nozzle. In this manner, the amount of wasted polymer due to non-required ejection is minimized. The apparatus may also include a mixer operative to thoroughly and homogeneously mix the polymer from a number of constituent reagents. As with the shut-off valve, the mixer is located proximate to the fluid nozzle, so as to minimize the amount of the polymer, which is mixed, but not ejected. 
     Finally, the apparatus may comprise a cleaning mechanism. The cleaning system itself consists of a plurality of switches, which arrest the flow of the constituent reagents (if required) and permit at least one cleaning fluid to flow through (and simultaneously clean) the mixer, fast shut-off valve and fluid nozzle. The cleaning system also comprises a cleaning arm equipped with a source of vacuum suction. The cleaning arm moves, by either translation or rotation, between an active position (directly external to the fluid nozzle) and a non-intrusive position (out of the way, so as not to interfere with the ejected polymer). During cleaning, the cleaning arm is in the active position, directly external to the fluid nozzle. In this manner, the cleaning arm collects cleaning fluid, left-over polymer and any other materials ejected from the fluid nozzle. 
     Advantageously, the aperture located in the shroud may be further operative in combination with the vacuum source, to remove excess polymer (i.e. polymer that did not adhere to the target surface) from a vicinity of the fluid nozzle. In this manner, the excess polymer may be recycled. 
     Preferably, the fast shut-off valve may be located within the actual fluid nozzle. 
     Preferably, the mixer may further comprise a substantially cylindrical and hollow mixing column, which receives the constituent reagents from external reservoirs. The mixer may also include a substantially cylindrical mixing shaft, concentrically located within the mixing column. Finally, the mixer may also comprise a motor, which rotates the mixing shaft within the mixing column, so as to thoroughly and homogeneously mix the constituent reagents in a region between the exterior surface of the mixing shaft and the interior surface of the mixing column. 
     Advantageously, the exterior surface of the mixing shaft may be patterned, so as to improve the mixing process and to provide a suction force, which draws the constituent reagents into the mixing column. 
     The movement of the cleaning arm between the active position (i.e. directly external to the fluid nozzle) and the non-intrusive position may be accomplished by an external electro-mechanical switch or even by the vacuum suction source within the cleaning arm. 
     Advantageously, the plurality of switches and the cleaning arm, which comprise the cleaning mechanism, may be independently operative. 
     Preferably, the cleaning system may be further operative to assist in priming the spray system after cleaning. During a priming operation, the plurality of switches in the cleaning system may be operative to arrest a flow of the cleaning fluid (if required) and to permit a flow of the constituent reagents and polymer instead. The constituent reagents flow through the mixer and the polymer flows through both the fast shut off valve and the fluid nozzle. The cleaning arm may then be positioned directly external to the fluid nozzle and may be used to collect the substantially liquid polymer, any left-over cleaning fluid and any other materials that are ejected from the fluid nozzle, until the spray system is sufficiently primed. 
     Further advantages of the invention will become apparent when considering the drawings in conjunction with the detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the entire apparatus as disclosed herein. 
     FIG. 2 is a close up cross-sectional view of the nozzle head and the shroud. 
     FIG. 3 depicts an implementation of the mixing process and apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows apparatus according to the present invention. The apparatus comprises a nozzle system  10  for applying polymer to a printing plate  33 . The printing plate  33  may be affixed to the surface of a cylindrical drum  34 , but the invention is also applicable when the plate  33  is in a flat orientation (not shown). 
     Typically, a polymer is made out of more than one reagent. Generally, any number of reagents may be used, but for the purposes of this disclosure, FIG. 1 displays only two reagents A and B, which are housed in reservoirs  11 A and  11 B respectively. The liquid reagents A and B are forced into conduits  12 A and  12 B respectively by pumps  14  and directed towards the manifold  20 . In general, the pumping of reagents A and B are suspensions or dispersions (rather than true solutions). In this scenario, the reagents A and B must be circulating within the system to avoid settling. The fluid displacement of reagents A and B may also be accomplished by gravity, pressure, vacuum, or other means. Once inside the manifold  20 , the reagents A and B encounter switches  21 A and  21 B, which are activated by solenoid actuators  18 A and  18 B. Solenoid actuators  18 A and  18 B are controlled by the signal  19 , originating from the control logic (not shown). In the state depicted in FIG. 1, the solenoid actuators  18 A and  18 B are both positioned so as to let the reagents A and B pass through switches  21 A and  21 B into conduits  36 A and  36 B. Switches  21 A and  21 B are important because they control the amount of reagents A and B that reach the mixer column  17 . Once the reagents A and B are mixed, they must be used or they will cure and be wasted. Consequently, switches  21 A and  21 B are important because they reduce wastage and increase the overall transfer efficiency of the system. 
     From conduits  36 A and  36 B, reagents A and B enter the mixer column  17 , where they are mixed by the rotation of the mixer shaft  16 . As motor  15  rotates the mixer shaft  16 , the reagents A and B are mixed while being simultaneously drawn up the mixer column  17 . Once the newly mixed polymer  38  reaches the top of the mixer column  17 , it exits through conduit  22  toward the nozzle head  24 . At the same time, heated air at high-pressure (not shown) is forced through conduit  28 . The phrases “high-pressure” and “low-pressure” are used frequently herein and should be interpreted in a relative (rather than absolute) context. The heated air in conduit  28  surrounds the newly mixed polymer  38  in conduit  22 , warming it and reducing its viscosity. The mixed polymer  38  encounters the quick shut-off valve  25  in the nozzle head  24 . The quick shut-off valve  25  is controlled by signal  30  (originating from the control logic (not shown)) and is used to cut off the flow of mixed polymer  38  into nozzle head  24 . Once past the quick shut-off valve  25 , the mixed polymer  38  is ejected from the nozzle tip  37  in substantially liquid form and directed towards the printing plate  33 . 
     Referring to FIG. 2, the nozzle head  24  is depicted in more detail. The mixed polymer  38  travels from the mixer (not shown) to the nozzle head  24  via conduit  22 . Simultaneously, heated air  42  at high-pressure is forced through conduit  28  into the nozzle head  24 . As mentioned earlier, in the nozzle head  24  the heated air  42  in conduit  28  is brought into proximity of the mixed material  38  in conduit  22 , heating the mixed polymer  38 . The heating of the mixed polymer  38  immediately prior to its ejection from the nozzle head  24  provides several advantages. Most notably, heating “loosens” the entangling of polymer molecules, making it easier to atomize the mixed polymer  38  after ejection. In addition, for some highly reactive polymers, heating the mixed polymer  38  immediately prior to ejection is required (particularly when the mixed polymer  38  is highly reactive), because the rate of the cross-linking (curing) reaction of the mixed polymer  38  is generally increased with the addition of heat. Consequently, if the mixed polymer  38  is heated too early, the curing reaction would take place prior to ejection from the nozzle head  24 , and any prematurely cured polymer would be wasted. 
     In the nozzle head  24 , the mixed polymer  38  encounters the quick shut-off valve  25 . The quick shut-off valve  25  and its control signal  30  are functionally important, because they can be configured, so as to control the flow of mixed polymer  38  through the nozzle tip  37  and substantially reduce the amount of mixed polymer  38  sprayed into the plate-mounting gap (not shown). As a matter of system design, it is important to locate the quick shut-off valve  25  as close as possible to the nozzle tip  37 . In this manner, when the shut-off valve  25  is activated, the amount of polymer  38  left “downstream” of the activated shut-off valve  25  is reduced. Minimizing the polymer  38  left downstream of the shut off valve  25  is important, because such material  38  may continue to be ejected from the nozzle tip  37  into the plate-mounting gap, creating waste. Thus, the location and control of the quick shut-off valve  25  increase the overall system transfer efficiency by reducing the wastage of mixed polymer  38 . 
     The high-pressure heated air  42 , which arrives at the nozzle head  24  via conduit  28  performs a number of secondary functional roles. After passing through the nozzle head  24 , the high-pressure, heated air  42  is ejected from the aperture  23  forming spray profile  41 . The heated air  42  is ejected at high pressure so that the velocity of the air stream at the aperture  23  is at relatively high speed. However, the overall flow rate of the air is small. While the heated air  42  is ejected from aperture  23 , the mixed polymer  38  is simultaneously ejected in substantially liquid form from nozzle tip  37 . The heated air stream  41  performs the function of atomizing the mixed polymer  38 , forming a fine mist of polymer droplets (not shown). As the air stream  41  interacts with mixed polymer  38 , its speed is substantially reduced, so that by the time the air  41  and the polymer droplets reach the printing plate  33 , the cloud of polymer droplets and the heated air stream  41  have a relatively low speed. The low speed of the air  41  and the polymer droplets provide excellent adhesion of the polymer droplets to the plate  33 , because the low speed reduces the amount of “bounceback” of the polymer droplets. In this manner, the ejected air stream  41  reduces the amount of oversprayed polymer (i.e. excess polymer resulting from “bounceback” or that otherwise does not adhere to the plate  33 )(not shown) and increases the overall system transfer efficiency. 
     The amount of heated high pressure air  42  ejected from aperture  23  into the air stream  41  has a lower limit determined by the need to adequately atomize the substantially liquid mixed polymer  38 . However, increasing the amount of air in the air stream  41  can not be done without limitation, because increases in air flow  41  cause an increase in the speed and turbulence imparted on the atomized mixed polymer droplets, and a corresponding increase in the amount of “bounceback” of the polymer droplets. That is, the higher the pressure of the air in the air stream  41 , the more overspray and the lower the overall system transfer efficiency. This phenomenon illustrates the advantage of pre-heating the mixed polymer  38  in the nozzle head  24 , because pre-heating makes it easier to atomize the substantially mixed polymer  38 , reducing the amount of high pressure air  42  required in the air stream  41 . Consequently, the invention depends on selecting the correct flow of heated high pressure air  42 , in the air stream  41 , so as to fully atomize the substantially liquid mixed polymer  38 , while simultaneously effecting a controlled transfer of the polymer droplets, generating less overspray and “bounceback” and maximizing the overall system transfer efficiency. 
     FIG. 2 also displays the shroud  27 , which encases the spray nozzle head  24  in a conical manner. The shroud  27  is equipped with a hole  32  leading to conduit  31 . Conduit  31  is attached to a source of negative pressure (i.e. a vacuum source) (not shown). The shroud  27  in combination with the vacuum, the hole  32 , and the conduit  31 , is useful to help remove overspray. 
     The excess overspray, or polymer (not shown) can be immediately reclaimed and possibly recycled. Although FIG. 2 depicts only one hole  32 , there may be a plurality of holes in the shroud  27 , each attached to a vacuum source and each functioning to remove oversprayed polymer that does not adhere to the target surface  33 . In this manner, the shroud  27  and aperture  32  help to reduce the wastage of polymer  38  and also prevents oversprayed polymer from accumulating, and possibly curing, in undesired areas. 
     Referring to FIG. 3, an implementation of the mixer  43  is depicted in accordance with the present invention. Typically, the mixer  43  will be located in the manifold (not shown in FIG. 3, see  20  in FIG.  1 ). The basic components of the mixer  43  are a motor  15 , a mixing shaft  16  and a mixing column  17 . Using pumps (not shown in FIG. 3) the two reagents A and B (not shown in FIG. 3) are introduced to the mixing column  17  via conduits  36 A and  36 B. Although the depiction in FIG. 3 shows only two reagents, there is no general limitation on the number of reagents and  3  or more may be common. As the pressurized reagents A and B enter the mixing column  17 , the motor  15  rotates the mixing shaft  16  (typically btw 1000 and 5000 RPM) in such a manner that the reagents A and B are thoroughly mixed as they travel up the mixing column  17  toward the exit conduit  22 . The rotational speed of the mixing shaft  16  is a function of the mixing column  17 , size, and the flow of reagents A and B. By the time that the reagents A and B reach conduit  22 , they have become mixed polymer  38 . Additionally, the mixing shaft  16  may be patterned with some features, such as spiral grooves that help to mix the reagents A and B or that help to pump the reagents through the mixer. 
     The mixer embodiment  43  described above has several advantageous features. In addition to the mixer  43  providing homogeneous mixtures, the relatively thin column  17  minimizes the amount of trapped material and the overall design of the mixer  43  facilitates easy cleaning. 
     Referring back to FIG. 1, the spray nozzle  10  may be configured in a cleaning mode. In such a state, logic signal  19  is used to trigger relays  18 A and  18 B, which activate switches  21 A and  21 B, causing them to block the flow of reagents A and B and facilitating the flow of cleaning fluid  13 , which is pumped (by pump  14 ) through conduit  35 . As with the reagents A and B, the pump  14  is not necessary and the fluid flow may be provided by any means, including gravity, pressure, or vacuum. In general, the cleaning fluid  13  may be some combination of water and/or other solvents. In addition, switches  21 A and  21 B may be implemented by any other means of diverting liquid and should not be limited to relay activated switches. 
     After flowing through the switches  21 A and  21 B, the cleaning fluid  13  is conducted to the mixer  43  via conduits  36 A and  36 B, ending up in the mixing column  17 . Once in the mixing column  17 , the cleaning fluid  13  is subjected to the same mixing action as reagents A and B in a typical spraying application, which facilitates complete coverage and thorough cleaning of the interior of mixer  43 . 
     After cleaning the mixer  43 , the cleaning fluid  13  exits the mixing column  17  through conduit  22  and travels towards the nozzle head  24 . The cleaning fluid  13  cleans conduit  22 , the interior of nozzle head  24  and nozzle tip  37 , prior to being ejected from the spraying device. 
     Referring back to FIG. 2, the invention provides for an additional cleaning mechanism comprising cleaning arm  26 A, which may be used independently or in conjunction with the cleaning fluid  13 . During spraying operation, cleaning arm  26 A is in the position indicated by solid lines, safely out of the way of the spray profile  40  of the mixed polymer  38 . However, in cleaning mode operation, cleaning arm  26 A is rotated to position  26 B indicated by dotted lines, so that collector  44  is positioned over the nozzle tip  37 . Vacuum (not shown) is applied to conduit  29  and creates negative pressure at the collector  44 . The vacuum action is used to suck remaining particles of wasted polymer and other materials from the nozzle head  24  and the nozzle tip  37  into collector  44 . If the cleaning arm  26 A is used in conjunction with cleaning fluid  13 , then the vacuum can be used to take up (into collector  44 ) all of the ejected cleaning fluid  13 , along with dissolved polymer and other contaminants from the mixer  43  and the manifold  20 . 
     The movement of the cleaning arm  26 A from the position indicated by solid lines to the position  26 B indicated by dotted lines and back may be implemented by an independent motor (not shown) or, alternatively, by the action of the vacuum in conduit  29 . When vacuum is turned on in conduit  29 , it exerts negative pressure on the base of cleaning arm  26 A, pulling pin  45  around the groove  46  and causing the cleaning arm  26 A to rotate to the position  26 B indicated by the dotted lines. The cleaning arm  26 A may be spring loaded so as to return to the position indicated by solid lines when the vacuum in conduit  29  is shut off. In general, the invention does not depend on the mechanism by which cleaning arm  26 A is activated. As a result, the invention should be understood to incorporate any known methods of causing the cleaning arm  26 A to move between position indicated by the solid lines and position  26 B indicated by the dotted lines. 
     In addition to performing a cleaning function, the cleaning mechanism comprising cleaning arm  26 A may be used to prime the nozzle after a cleaning operation and prior to the start of a new spraying operation. Priming may be necessary to purge the system of excess cleaning fluid  13 . Priming is accomplished by running the nozzle using the desired reagents A and B and operating the cleaning arm  26 A in the position  26 B indicated by dotted lines so that when the vacuum (not shown) is applied to conduit  29 , the mixture of residual cleaning solution  13  and mixed polymer  38  is collected by collector  44 . Priming the nozzle is performed until substantially all of the residual cleaning fluid  13  is purged from the system and the nozzle ejects substantially pure mixed polymer  38 . The cleaning arm  26 A can then be returned to its inactive position indicated by solid lines and the system is primed for spraying. 
     It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. Those skilled in the art will appreciate that various modifications can be made to the embodiments discussed above without departing from the spirit of the present invention.