Abstract:
A cleaning system for a continuous ink jet printer includes a first solvent supply conduit connected to a solvent source for conveying solvent through a supply opening and onto the front face of the print head. A second solvent supply conduit is connected to the solvent source for conveying solvent through a supply opening and onto a surface of the catcher. The solvent that is supplied to the print head and the catcher is removed under vacuum and returned to the ink supply system. The cleaning system may include an orifice unclogging mechanism that causes said solvent disposed on said front face to flow into said orifice in the reverse of the direction ink flows through said orifice for printing. The cleaning system may also include a piezoelectric element for generating a stress wave in the print head during cleaning. The piezoelectric element may comprise a piezoelectric oscillator that is also used during printing to creates perturbations in the ink flow at the nozzle so as to generate a stream of spaced drops from the nozzle.

Description:
RELATED APPLICATIONS 
   This application is a continuation-in-part of application Ser. No. 10/802,256, filed Mar. 17, 2004 and entitled “Ink Jet Print Head Cleaning System,” the entire disclosure of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   Embodiments of the present invention generally relate to a print head for an ink jet printer, and more particularly to an ink jet printer having a system for cleaning the nozzle and the catcher. 
   Conventional continuous ink jet printers supply electrically conductive ink under pressure to a drop generator, which has an orifice or orifices (nozzles) that are typically arranged in a linear array. The ink discharges from each orifice in the form of a filament, which subsequently breaks up into a droplet stream. Individual droplets in the stream are selectively charged in the region of the break off from the filament, and these charged drops are then deflected as desired by an electrostatic field. The deflected drops may proceed to a print receiving medium, whereas undeflected drops are caught in a gutter or catcher and recirculated. 
   After the printer is shut down for a period of time, ink around the orifices dries up, often partially blocking, and sometimes completely clogging, the outer openings to the orifices. Furthermore, during a long shut down period, such as an entire day or weekend, the dried ink may form a block within the orifice or passages attached to the orifice, depending on the type of ink. 
   Typically, print head cleaning systems and methods are limited to the nozzle, or drop generator. However, ink deposits and residue also accumulate around the catcher, for example. Ink droplets often settle on and within the catcher. As ink deposits and residue accumulate on these components, printing quality suffers due to the clogging of the components and conduits therebetween, or due to interference between built-up residue and ink droplets. That is, the recycling rate of ink and other fluids through these components decreases as the accumulation of deposits and residue increases. Often, the ink jet printer is completely shut down in order for an operator to manually clean these components, thereby precluding use of the printer. 
   Thus, a need exists for a system and method for more effectively cleaning various components of a print head of an ink jet printer. Overall, a need exists for an efficient system and method of cleaning a print head of an ink jet printer. 
   BRIEF SUMMARY OF THE INVENTION 
   According to an embodiment of the present invention, a cleaning system is provided for a continuous ink jet printer. The printer has an ink flow system wherein ink flows from a reservoir to a print head. The ink is ejected from the print head in a series of discrete droplets directed at a substrate upon which an image is to be formed by applying droplets to the surface of the substrate. Droplets which are not to be applied to the substrate are collected in a catcher and recycled via a return line to the ink flow system for reuse. The print head includes a front face and at least one orifice extending through the front face. The cleaning system a first solvent supply conduit connected to a solvent source for conveying solvent through a supply opening and onto the front face of the print head. A second solvent supply conduit is connected to the solvent source for conveying solvent through a supply opening and onto a surface of the catcher. 
   The cleaning system may include an orifice unclogging mechanism that causes said solvent disposed on said front face to flow into said orifice in the reverse of the direction ink flows through said orifice for printing. According to one embodiment, the printer further includes a main conduit for supplying ink to said orifice and the orifice unclogging mechanism includes a vacuum conduit connected to the main conduit so that negative pressure may be applied to suction solvent from the front face, through the orifice and into the vacuum conduit. A check valve may be disposed in the vacuum conduit, the check valve being adapted to open to allow solvent to be suctioned through said vacuum conduit in a first direction and to close to prevent backflow through said conduit in the opposite direction. The check valve is preferably made as rubber duck-bill valve, which has been found to prevent or minimize the mini spills that occur at start up and shut down. 
   The cleaning system may also include a piezoelectric element for generating a stress wave in the print head during cleaning. The piezoelectric element may comprise a piezoelectric oscillator that is also used during printing to creates perturbations in the ink flow at the nozzle so as to generate a stream of spaced drops from the nozzle. 
   Another embodiment relates to a method of cleaning a continuous ink jet printer of the type having an ink flow system in which ink is adapted to flow from a reservoir to a print head from which the ink is ejected in a series of discrete droplets directed at a substrate upon which an image is to be formed by applying droplets to the surface of the substrate and in which droplets which are not to be applied to the substrate are collected in a catcher and recycled via a return line to the ink flow system for reuse, the print head having front face and at least one orifice extending through the front face, the orifice defining a nozzle for ejecting the ink. The cleaning method comprises flowing solvent through a solvent supply conduit to a front face of the print head such that the solvent moves along the front face adjacent to the orifice, suctioning the solvent from the front face and into a drain conduit to remove said solvent from the front face of the print head, flowing solvent directly onto a surface of the catcher, and suctioning the solvent from the catcher through the return line. The method may also include the step of flowing the solvent disposed on the front face of the print head into the orifice in the reverse of the direction ink flows through the orifice for printing. The method may also include generating a stress wave in the print head during cleaning so as to loosen dried ink in the print head. 
   Another embodiment relates to a method of cleaning a continuous ink jet printer of the type having print head with a front face presenting an orifice for emitting a droplet stream toward a substrate during a printing cycle. The cleaning method comprising the steps of supplying solvent to a front face of the print head such that the solvent moves along said front face adjacent to said orifice; and generating a stress wave in the print head during the cleaning process so as to loosen dried ink in the print head. 
   Another embodiment relates to a cleaning system for a continuous ink jet printer having a print head including a front face and at least one orifice extending through the front face. The cleaning system comprises a conduit for supplying solvent to the front face of the print head, adjacent the orifice. A main ink conduit is provided for supplying ink to the orifice. A vacuum conduit is connected to the main conduit so that negative pressure may be applied to suction solvent from the front face, through the orifice and into the vacuum conduit. A check valve is disposed in the vacuum conduit. The check valve is adapted to open to allow solvent to be suctioned through the vacuum conduit in a first direction and to close to prevent backflow through the conduit in the opposite direction. 

   
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a simplified schematic side view of components of an ink jet printer of an embodiment of the present invention with the drop generator shown in the cross section. 
       FIG. 2  is a diagram of the system for circulating the solvent in the ink jet printer in accordance with an embodiment of the present invention. 
       FIG. 3  is a cross-sectional view of a drop generator in accordance with an embodiment of the present invention. 
   

   The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings, certain embodiments. It should be understood, however, that the present invention is not limited to the arrangements and instrumentalities shown in the attached drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a printer that incorporates a cleaning system according to an embodiment of the present invention. The printer includes print head  10  having a drop generator  12 , a charge electrode  14 , a ground plate  16 , a high voltage deflection plate  18 , and a catcher  20 . The charge electrode  14 , the ground plate  16 , the high voltage deflection plate  18 , and the catcher  20  are positioned between the drop generator  12  and a substrate  21 , which is remotely located from the print head window (not shown). During printing, the drop generator  12  receives ink from a main conduit  24  as shown and described in U.S. Pat. No. 6,575,556, entitled “Self-Cleaning Print Head for Ink Jet Printer,” which is hereby incorporated by reference in its entirety. A piezoelectric cylinder  26  is bonded around the main conduit  24  in order to impart vibrational energy of a selected frequency to the ink received by the drop generator  12 . A droplet stream is thus created and selectively charged by the charge electrode  14 . An electrostatic field formed between the deflection plate  18  and the ground plate  16  deflects the charged drops of ink over the catcher  20  and onto the substrate  21 . Uncharged drops that pass between the deflection plate  18  and ground plate  16  are not deflected and pass directly into the catcher  20 , which is vacuum assisted to recirculate the ink back into ink reservoir  30  via a return line  31 . 
   The drop generator  12  has an outer housing or body  32  with a front face  34 . The front face  34  may include a solvent-wettable, generally planar surface as described in the &#39;556 patent. The surface is solvent-wettable in order to spread out the solvent to maintain the solvent as a thin film when the viscosity of the solvent is low. The solvent-wettable material can be PEEK (polyetheretherketone), for example. For purposes of this application, a solvent-wettable surface is one on which a solvent tends to spread out, whereas a non-solvent wettable surface is one on which a solvent tends to bead up. 
   An orifice  36  extends through the front face  34  at an end of the main conduit  24  for emitting the ink stream. The drop generator  12  also has a solvent supply conduit  40  with one end terminating at a supply opening  42  on the front face  34  near the orifice  36 . The opposite end of the solvent supply conduit  40  is connected to a solvent supply system  44 . As described in the &#39;556 patent, a flow restrictor (not shown) with a narrow slit or hole may be positioned within the solvent supply conduit  40  for influencing the pressurized solvent to form a thin film at the supply opening  42  by reducing the pressure on the solvent as it flows from supply opening  42 . 
   On the opposite side of the orifice  36  from the position of the solvent supply opening  42 , a drain opening  48  communicates with a drain conduit  50  connected to a solvent return system  52 . Drain opening  48  may be larger than supply opening  42 . The drain conduit  50  under vacuum pressure (for example, approximately 10″ mercury). The solvent  54  flows out of the supply opening  40 , over orifice  36  and into drain opening  48 , is explained in the &#39;556 patent. 
   Referring to  FIG. 2 , the solvent supply system  44  includes a pump  60  that runs the cleaning solution or solvent from a solvent makeup container  62 , through a conduit  64  and to the supply conduit at the drop generator  12 . The conduit  64  is shown with an alternative flow restrictor  66  connected in the solvent supply system  44 . The alternative flow restrictor  66  can be used instead of the flow restrictor disposed within the solvent supply conduit  40  in the drop generator  12 . The flow restrictor  66  is provided to regulate the flow of solvent through adjustment of the solvent supply pressure. A valve  68 , such as a solenoid actuated valve, is interconnected between the conduit  64  and the supply conduit  40  for controlling the flow of solvent to the drop generator  12 . Similarly, a valve  70 , such as a solenoid activated valve, is interconnected between the conduit  64  and the catcher supply line  71  for controlling the flow of solvent from the solvent supply system  40  to the catcher  20 . Alternatively, a single valve could be used to regulate the flow of solvent to both the catcher  20  and drop generator  12 . 
   A valve  74  is provided in the solvent supply system  44  for providing compressed air  76  to the pump  60 . The pump  60  uses the compressed air  76  to force or push the solvent to the print head  12  and the catcher  20 . It will be appreciated, however, that other pumping systems that do not use compressed air could be used instead. 
   The solvent return system  52  has an ink pressure solenoid-activated valve  80  (hereafter, referred to merely as ink pressure solenoid  80 ) connected through conduit  82  to an ink pressure regulator  84 , which in turn is connected to an ink pressure tank  86  though conduit  88 . Ink pressure tank  86  is further connected to main conduit  24  through conduit  90 . Solenoid  80  also connects with a valve  92  through conduit  94 . In one direction, the valve  92  also connects to a conduit  96  that links to drain conduit  50  at the drop generator  12 . In another direction, the valve  92  connects to a conduit  98  that opens to the ink reservoir  30 . 
   Referring to  FIGS. 1 and 2 , when the ink jet printer is running, ink is pumped from the reservoir  30  by transfer pump  100 , pressurized in ink pressure tank  86  and then supplied to main conduit  24  via conduit  90  for printing. The ink is pressurized by energizing the ink pressure solenoid  80 , which allows compressed air into conduit  82 , ink pressure regulator  84 , conduit  88  and the ink pressure tank  86 . Compressed air in the conduit  94  closes air operated valve  92 , which closes off conduit  96  from the ink reservoir vacuum conduit  96 . 
   For the cleaning process (preferably before start-up, after shut down and/or during maintenance operations), the ink supplied to the main conduit  24  is shut off by de-energizing the ink pressure solenoid  80  to de-pressurize the ink pressure tank  86 , which turns off the ink stream. De-energizing solenoid  80  also allows valve  92  to open and connects conduit  50  to the ink reservoir  30  (under vacuum) through conduit  96 . This permits used solvent and residue ink from the front face  34  of the drop generator  12  to be placed in the ink reservoir  30 . Similarly, the solvent that is supplied to the catcher  20  during cleaning is suctioned through the return line  31  and into the reservoir  30 . As the total amount of solvent added to the ink system during cleaning is relatively small, ink composition control is substantially unaffected by the cleaning operation. 
   Shortly after ink pressure solenoid  80  is de-energized, valve  74  is energized. This allows compressed air  76  to flow through conduit  78  to air operated pump  60 . The valves  68 ,  70  are selectively opened to regulate the flow of solvent from the pump  60  to the drop generator  12  and the catcher  20 . The conduit  64  can include a check valve  102  to prevent reverse or back flow. From conduit  64 , the solvent supply system  44  supplies solvent under pressure through solvent supply conduit  40  in the drop generator  12  and onto front face  34 . On the front face  34 , the solvent spreads over an area adjacent orifice  36 . The solvent flow may be uniform or pulsating. The type of solvent flow will depend on its supply pressure mechanism. For example, different pump restrictions or pump control systems can provide either uniform or pulsed fluid pressures, thus providing either uniform or pulsating solvent flow. 
   While the flow of solvent dissolves residue, ink accumulations or any other particles on the front face  34  and in the orifice  36 , the solvent is sucked into drain opening  48  and follows drain conduit  50  back to the solvent return system  52 . As described in the &#39;556 patent, appropriate negative pressure or vacuum from drain conduit  50  sustains the solvent flow on the front face  14  in any print head spatial orientation, independent of gravity, and prevents solvent from dropping off the print head  12 . After a predetermined cleaning time, valve  74  is de-energized to stop the flow of compressed air  76  and turn off pump  60 , thereby stopping the flow of solvent. 
   Referring again to  FIG. 1 , the drop generator  12  may also provide with a vacuum conduit  110  that is connected at one end to the main conduit  24  just behind the orifice  36 . The other end of the vacuum conduit  110  is connected via conduit  112  to the ink reservoir  30  under vacuum. During the cleaning process, when conduit  110  is applying negative pressure or vacuum, part of the solvent flowing over the orifice  36  is drawn through the orifice  36  in the reverse of the direction of ink flow during printing. The solvent is then drawn into main conduit  24  and vacuum conduit  110 , and finally returned to the ink reservoir  30  via conduit  112 . This portion of solvent flow effectively cleans the interior of the orifice  36  as well as adjacent parts of the main conduit  24 . The remainder of the solvent on the front face  34  flows as described above into drain conduit  50 . Pulsating flow may be used to aid in dissolving residue in the interior of orifice  36 . 
   An elastomeric check valve  114  is provided in the conduit  110 . The valve  114  opens to allow the flow of solvent in a direction from the orifice  36  to the reservoir  30  and closes to prevent fluid flow in the reverse direction. The check valve is preferably in the form of a duck bill valve and may be made of an elastomeric material such as rubber. In addition to preventing back flow at the end of the cleaning process, the valve  114  also provides dampening to the ink flow during start up and shut down. The dampening provided by the valve  114  is beneficial for reducing ink splatter during start up and shut down. Specifically, at start up there is a quick increase in pressure, which causes a jittering flow effect. This can cause the ink to splatter during start up. The ink splatters settle on the parts of the print head, solidify, and accumulate over time. These accumulations of ink can obstruct or interfere with the ink jet. Similarly, ink splatter can occur during shut down because the ink pressure does not immediately drop to zero. As the ink jet looses pressure it can break down, resulting in ink splatters. The elastomeric duck bill valve dampens the ink flow during start up and shut down, thereby reducing the tendency for ink splatter to occur. 
   During the cleaning procedure described above the flow of solvent output from supply opening  42  is divided between the conduits  50 ,  110 . The ratio of flow through conduits  50 ,  110  depends on the amount of vacuum in those conduits and on the geometric dimensions of those conduits. For example, the relatively small diameter of orifice  36 , which may be on the order of 66 micron, causes a comparatively small amount of flow to be drawn into conduit  110 ; a majority of the solvent flows across the face  34 , around the orifice  36  and into drain opening  48 . As will be appreciated, the flow ratio can be adjusted by varying the amount of vacuum in one or both of the conduits  50 ,  110 . The ratio can be optimized by changing the vacuum amounts in one or in both of those lines. 
   According to certain aspects of one embodiment, the piezoelectric element  26  is operated during the cleaning process. The piezoelectric element  26  generates stress waves, which assist the cleaning process. The stress waves loosen particles, facilitating their removal by the makeup flow. The voltage and frequency applied to the piezoelectric element  26  can be the same as those used during printing. For example, 30–75 V and 66 KHz. Alternatively, a frequency sweep 30–90 KHz might be applied for more efficient cleaning. 
   Referring to  FIG. 3 , the design and location of the piezoelectric element in the nozzle, contribute to creating effective stress waves. According to one presently preferred embodiment, the ratio between the parameters of the piezoelectric elements  26  are L t :OD:ID=2.0:1.4:1. 
   Where:
         L t  is piezoelectric tube length   OD is tube outside diameter   ID is tube inside diameter       

   In order to generate desirable stress waves, the piezoelectric element  26 , as well as the feature it is bonded to, should have cylindrical forms. The piezoelectric element  26  is a ceramic tube plated with metal wherein, the outer portion has a negative charge and inner portion has a positive charge. Positive and negative lead wires  140 ,  142  are connected to the positively and negatively charged portions  144 ,  146  of the piezoelectric element  26 . It is difficult to attach the positive lead wire  142  to the positively charged inner portion without breaking cylindrical form of the piezo tube or the feature it is attached to. Therefore, the positive portion  144  is expanded so that it covers a small portion of the outside of the tube (designated as  144   a ) in order to provide a connection point for the positive lead wire  140 . This design allows both lead wires  140 ,  142  to be attached to the outside of the piezoelectric element  26 . Preferably the piezoelectric element  26  is constructed such that the negative portion of the outer diameter area remains at least 66% of the entire outer diameter area. 
   The distance from the piezoelectric element  26  to the orifice preferably equals less than 1.1 OD. Moreover, the conductive portion/end of the OD is preferably directed towards the orifice  36 . These parameters have been found to provide effective cleaning. 
   Clean start up is also provided by certain sequencing and timing. Specifically after ink is allowed into the drop generator  12  via the conduit  24 , the cavity  120  remains connected with the vacuum for a period of time necessary to fill the cavity  120  with ink. This ensures that no air is left inside the print head  12 . Eliminating air from the print head is beneficial because such air would otherwise be drawn into the ink flow during printing, thereby creating voids in the flow and interrupting normal printer operation. 
   The design of the drop generator  12  also contributes to clean printer start up. Specifically, conduit  24 , which delivers ink to the orifice  36 , is straight, which as been found to be effective in reducing ink splatters during start up and shut down. Bypass conduit  110  includes a first portion  118 , which connects to main conduit  24  at a right angle for ease of manufacture. According to one presently preferred embodiment, the conduits  24 ,  118  were configured as:
 
 L/d= 2.3
 
d=d c 
 
   Where:
         L is the distance from the orifice to the interconnection between conduits  24  and  110 ;   d is the diameter of the conduit  24 ; and   and d c  is the diameter of conduit  110 .       

   This ratio has been found to be effective for both ink jetting and cleaning. 
   As was discussed above, backflow from the cavity  120  is prevented by the check valve  114 . The elastomeric valve  114  accommodates pressure fluctuations, and prevents ink splatters during shut down and start up. Preventing even small splatters is important because such splatters settle on ground or deflection plates. Over time such ink splatter builds up and can obstruct ink jet and therefore interrupt normal printing process. 
   As is shown in  FIG. 3 , the body  32  of the drop generator may comprise mating first and second portions  150 ,  152 . The tubular piezoelectric element  26  is mounted over a tubular member  154  formed on the interior of the first portion  152 . The tubular member  154  defines the main conduit  24 . The check valve  114  mounted within the compartment  120  and is sandwiched between the first and second portions  150 ,  152 . 
   While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.