Abstract:
A system and method are provided for cleaning the printhead of a continuous ink jet printing system. A cleaning fluid is introduced and used to flush ink residues and debris from the interior of the drop generator, the exterior of the orifice plate, the charge plate face and the catcher face. This system and method removes dried ink residues and other debris and deposits by providing a cleaning fluid with a low surface tension to dissolve or flush away the unwanted material from the orifices. This is particularly advantageous in that the flushing and rinsing is accomplished without mechanical contact which could abrade or damage the orifices.

Description:
TECHNICAL FIELD 
     The present invention relates to continuous ink-jet printing and, more particularly, to the cleaning of printhead orifices and charging leads. 
     BACKGROUND ART 
     Continuous ink jet printheads utilize a series of orifices separated from charging leads by a small gap. Fluid is forced through the orifice while the printhead is in operation. Upon shutdown, the ink floods the leads and the area around the orifices. This fluid then dries, leaving behind non-volatile components in the form of solids or gels. Depending on the ink chemistry, this ink may polymerize as it dries, rendering it insoluble. Upon subsequent startups, the failure to remove or redissolve all of this material in the orifice and gap creates disturbances in the shape or direction of the emerging jet. Heavy deposits may block the orifice altogether. Deposits left on the charging leads may leave films which impair the proper charging of the drops as they form, causing insufficient deflection of the drop. 
     Current ink jet systems consist of a fluid module with a removable printhead. In the course of operation it may become necessary to move a printhead from one system to another. Ink residue remaining in the printhead from the previous system may contaminate the second system if the ink color or chemistry is incompatible. 
     This problem has been addressed in the prior art. For example, U.S. Pat. No. 5,706,039 distributes a cleaning fluid externally, in the plane of the orifices, not through them. This requires the use of a two layer construction, or forming internal passages within the orifice plate. The vacuum used to remove cleaning fluid in the vicinity of the orifice may also carry external debris into the orifices. U.S. Pat. Nos. 5,570,117 and 5,555,461 utilize wipers to remove ink from the orifices, with no additional cleaning fluid used. U.S. Pat. No. 5,557,307 uses a cleaning thread to wipe the orifices. Ink is adsorbed onto the thread, removing it before it dries. 
     Unfortunately, mechanical devices such as wipers and thread need replacement or maintenance from time to time and may serve to push particles into the orifices. It is seen, then, that there is a need for a system and/or method for cleaning a printhead which will avoid the problems associated with the prior art. 
     SUMMARY OF THE INVENTION 
     This need is met by the printhead flush and cleaning system and method according to the present invention. In accordance with the present invention, there is provided a means for cleaning a printhead which avoids the formation of deposits. The present invention removes dried deposits by providing a cleaning fluid with a low surface tension to dissolve or flush material away from the orifices, all without mechanical contact which could abrade or damage the orifices. 
     In accordance with one aspect of the present invention, a system and method are provided for cleaning the printhead of a continuous ink jet printing system. A cleaning fluid is introduced and used to flush ink residues and debris from the interior of the drop generator, the exterior of the orifice plate, the charge plate face and the catcher face. This system and method removes dried ink residues and other debris and deposits by providing a cleaning fluid with a low surface tension to dissolve or flush away the unwanted material from the orifices. 
     Other objects and advantage of the present invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a continuous ink jet printer fluid system, illustrating printhead interface controllers and printheads; and 
     FIG. 2 is a flow chart diagram illustrating a shutdown sequence, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, the fluid system may be configured with one or more printheads. A common cleaning system serves multiple printheads in the multi-headed configuration. Since the separate plumbing within each printhead interface controller (PIC) and printhead is identical, the following description will make reference only to a single printhead, without restricting the invention to a single printhead. 
     Referring to FIG. 1, a preferred embodiment of the invention comprises a cleaning fluid supply tank  1 , fed by an external source  2 . Fill valve  3  is solenoid actuated, controlled by a float switch  4 , maintaining the cleaning fluid level within the supply tank. The air above the supply tank is maintained at a partial vacuum of 10-18 in Hg, providing a pressure gradient for flow. 
     A pump  5 , with integral manifold  6 , moves the fluid to the printhead  7  via the PIC manifold  8 . The same pump supplies cleaner to multiple printheads in a multiple printhead system, splitting the flow within the pump manifold. Check valve  51  prevents reverse flow through the pump, as the supply tank  1  is under vacuum. A solenoid actuated purge valve  9  allows the cleaning fluid into the droplet generator  10 , through a filter  11 , for example, a 1.2 micron filter. With vacuum supplied to the drop generator through the open outlet valve  14 , the cleaning fluid flushes the ink residue from the interior of the drop generator. 
     Closing the outlet valve  14  causes the cleaning fluid to flow through the orifices  12 . The cleaning fluid then rinses the ink residues from the face of the charge plate and the catcher  13 , as the catcher is under vacuum, pulling the cleaning fluid with ink residue back to the fluid system. In this way the exterior of the drop generator and the face of the charge plate and catcher can be cleaned. Opening the ink filter purge valve  26  allows the cleaning fluid to flush the ink filter. In this way, problems associated with ink drying in the final filter can be eliminated. 
     In a preferred embodiment of the present invention, the cleaning fluid comprises a dyeless fluid having low surface tension. Since it is important not to contaminate clean ink with the waste mixture of cleaning fluid and residue, the waste is ported by a pair of 3-way waste valves  15   a  and  15   b , to a separate internal waste tank  16 . The waste is then pumped, as the tank fills, by waste pump  17  to external waste tank  18 . 
     After the interior of the drop generator and exterior of the orifices and the face of the charge plate and catcher are rinsed with cleaning fluid, air pump  19  is activated to dry the interior of the droplet generator. The air passes through filter  20 , such as a 70 micron filter, and a solenoid air valve  21 . The air leaves the drop generator through the open bar outlet valve, and is exhausted through vacuum pumps  22   a  and  22   b . To sense proper operation of the flushing system, pressure switch  24  and pressure transducer  25  are used to determine air and purge pressures. 
     A preferred embodiment of the shutdown sequence for the present invention comprises the steps illustrated in flow chart  30  of FIG.  2 . First, at step  32 , ink is evacuated from the droplet generator and catcher. The air valve is then opened and the air pump actuated, at step  34 , providing pressure to blow residual ink out of the air filter. This step conserves ink that would otherwise be diverted to waste as the drop generator is flushed. 
     Continuing with FIG. 2, cleaning of the interior of the droplet generator with cleaning fluid occurs at step  36 , with the bar outlet valve open. Closing of the bar outlet valve occurs at step  38 , diverting the cleaning fluid through the orifices and onto the charge plate leads and catcher face. Step  40  provides for a dwell time to allow deposits to dissolve, before repeating steps  36  and  38 . Alternatively, a longer flush cycle could be used to completely dissolve deposits. The use of a dwell time reduces the amount of flush fluid required for cleaning. After steps  36  and  38  have been repeated, as determined at decision block  42 , the flow chart proceeds to step  44  where the droplet generator interior is dried with air circulated from the air pump, through the air and bar outlet valves, and exhausted by the vacuum pumps. At step  46 , the catcher and external surfaces are dried with air drawn through the catcher by the vacuum pumps. 
     An additional enhancement to the cleaning process may be the use of the drop generator stimulation to provide additional energy to remove debris. This ultrasonic stimulation is provided by the piezoelectric crystals used in normal droplet generator operation. This may be used in any of the flushing states or in the dwell state. 
     Additionally, the cleaning states in combination with the waste valves may be used to clean the printhead ink filter and other printhead components for changing of ink colors or removing a printhead, wherein the mixed ink and flush fluid is diverted to waste. This is performed by opening the ink filter purge valve  26  while performing steps  32  through  44  of the shutdown sequence. Steps  32  and  34  remove the bulk of the ink from both filters. Cleaning fluid is diverted into both the ink and air filters, in states  36  and  38 , removing residual ink trapped in the filter pores. A low surface tension fluid aids in the wetting of the filter, allowing dilution of the ink and its removal. Both filters are then dried together. 
     There are times in which it is desirable to employ a partial cleaning cycle rather than the complete cycle described here. One example is a printhead shutdown/restart intended to clear a crooked jet or a print defect. In such an instance, it may be desirable to rinse the face of the charge plate. As the printhead will be restarted immediately after the clean cycle there is no need to dry out the printhead. In such an instance, the cleaning cycle might include only the steps  34  through  38 . After completion of step  38 , the printhead might be restarted in its normal sequence. 
     The implementation of the cleaning system may be incorporated into a fluid system as described above, or the components may be part of an additional stand alone module. An installation of more than one fluid system may share a common external cleaning fluid supply tank and waste tank. 
     Industrial Applicability and Advantages 
     The present invention is useful in the flushing and cleaning and shutdown of printheads in an ink jet printing system. The system of the present invention, which cleans the orifices and charge leads of a printhead, has the particular advantage of allowing printheads to be moved within and among systems, even if ink color and chemistry are incompatible. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that modifications and variations can be effected within the spirit and scope of the invention.