Abstract:
A shutdown procedure is provided for removing all solvent ink from the printhead of an ink jet printer system that uses volatile ink for printing. A shutdown method is provided for an inkjet printer that uses volatile inks for printing. Initially, a colorless flush fluid is provided which readily dissolves the ink. The flush fluid is crossflushed through the drop generator and caused to weep out of the orifices in the drop generator to dissolve and rinse away ink residues from the charge plate and the exterior of the orifice plate. The flush fluid is used to rinse off charging electrodes of the charge plate, the catcher face, and the catcher return line. Fluid crossflushed through the drop generator cleans the interior of the drop generator and cleans the crossflush valve.

Description:
TECHNICAL FIELD 
   The present invention relates to solvent ink printing systems and, more particularly, to a shutdown procedure for a continuous ink jet printhead operating with solvent ink. 
   BACKGROUND ART 
   Ink jet printing systems are known in which a printhead defines one or more rows of orifices which receive an electrically conductive recording fluid from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. Printers using such printheads accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and depositing at least some of the drops on a print receiving medium, while others of the drops strike a drop catcher device. 
   Over the years, a number of inkjet printers using binary array continuous inkjet printing have been developed, with continuing improvements in speed, reliability, and ease of use. These printers are used in a variety of print applications, often using aqueous inks. Using aqueous ink, these printers can print for hours and have demonstrated highly reliable shutdown and subsequent startups without operator intervention. In spite of advances in aqueous ink technology, solvent inks, such as ethanol or MEK based inks, are preferred for some applications. For example, in applications such as printing on metals or plastics, solvent inks are preferred over aqueous inks as a result of the solvent ink characteristics of being much faster drying and more permanent than aqueous inks. 
   The same characteristics that make solvent inks preferred for printing on metals and plastics, however, make solvent inks much harder to run in inkjet printers. Just as the inks dry quickly on the print media, they also dry quickly on the various components in an inkjet printhead and fluid system. In particular, these inks can dry quickly on the orifice plate and the charge plate in the printhead. On the orifice plate, the dried ink can plug the orifices through which the ink is to be jetted, adversely interfering with jet directionality. When dried on the charge plate, the dried ink can produce shorting conditions between charging electrodes. 
   As a result of these problems, prior art inkjet printers using solvent inks have required significant intervention by highly trained operators, for proper operation both when the printers are started and shutdown. There is a need for a printer for use with highly volatile solvent based inks which can be shut down in a manner that will allow subsequent reliable start up. 
   SUMMARY OF THE INVENTION 
   This need is met by the shutdown procedure according to the present invention, wherein all solvent ink residue and fluid is removed from the printhead. In accordance with the present invention, the catcher face, eyelid seal, catcher line, and bar-out line are all flushed. When compared to existing fluid systems, the purge shutdown procedure of the present invention includes the addition of a separate line to the printhead and an additional valve at the printhead to deliver replenisher fluid directly to the printhead to act as a cleaning fluid. 
   In accordance with one aspect of the present invention, a shutdown method is provided for an inkjet printer that uses volatile inks for printing. Initially, a colorless flush fluid is provided which readily dissolves the ink. The flush fluid is crossflushed through the drop generator and caused to weep out of the orifices in the drop generator to dissolve and rinse away ink residues from the charge plate and the exterior of the orifice plate. The flush fluid is used to rinse off charging electrodes of the charge plate, the catcher face, and the catcher return line. Fluid crossflushed through the drop generator cleans the interior of the drop generator and cleans the crossflush valve. 
   Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustration a fluid system with which the shutdown procedure according to the present invention can be applied; 
       FIG. 2  is a solvent ink shutdown state table; and 
       FIG. 3  is a block diagram illustration of an alternative embodiment of a fluid system to which the shutdown procedure of the present invention can be applied. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In accordance with the present invention, the shutdown procedure can be applied to a fluid system configured with one or more printheads. Since the separate inlets and outlets 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 use with a fluid system having only a single printhead. 
   The present invention allows a printing system to go from a printing state to a down state, and subsequently return to a reliable printing state. Prior to the introduction of the present invention, in printing systems using volatile ink, the catcher and bar-out lines in the printing system would become plugged with dried ink. When the present invention is applied to a volatile ink printing system, reliability for both the printhead and the fluid system can be maintained. 
   In accordance with the present invention, used flush fluid is directed into a waste fluid tank, and does not cause the ink concentration to be driven downward with each flush cycle. From the waste fluid tank, the used flush fluid can be drained into an external waste tank for disposal. Alternatively, the waste fluid can be used as a replenishment fluid. This embodiment is possible because the flush fluid itself comprises replenishment fluid. Although the used flush fluid will have some ink dissolved in it, the concentration of ink will be quite low. When used as a make up or replenishment fluid for evaporated ink solvent, the used flush fluid will reduce the ink concentration back toward normal but slightly less than if clean replenishment fluid is used. If excessive amounts of used flush fluid are produced, however, it may be necessary to dispose of some of the fluid, as the continual use of used flush fluid instead of clean replenishment fluid will eventually cause the ink concentration to creep upward. 
   Prior to initiating a shutdown, the printhead will be in a ready-to-print state. This state will be described with reference to the fluid system  10  schematic shown in FIG.  1 . In ready to print condition, ink pump  50  is pumping ink out of the ink tank  16 , through the filter  52  and up to the printhead  28  via umbilical  54 . The ink supply valve  64  is open allowing ink to flow into the drop generator  34 . The flush fluid valve  30  and air ingest valve  66  are closed at this time. The crossflush valve  32  is also closed, allowing the ink pressure at the drop generator to rise to the normal operating pressure. At the normal operating pressure, ink is jetted from the orifices of the orifice plate  40  which is attached to the drop generator  34 . Piezoelectric actuators (not shown) attached to the drop generator cause the drop generator to vibrate, inducing the ink streams to break off into uniformly sized drops. Charging electrode means are located adjacent to the region where the ink drops break off from the ink streams. Selectively applying a voltage to these charging electrodes allows one to selectively charge the ink drops. The charged drops can then be deflected into catcher means  44 , while uncharged drops continue undeflected and strike the print media. The printhead eyelid  62 , which is used to seal the printhead when not printing, is now open to allow the print drops to exit the printhead  28 . The ink striking the catcher  44  is returned to the ink tank  16  by way of the open catcher valve  46  and diverter valve  38 . The return of the fluid to the ink tank is facilitated by vacuum maintained in the ink tank. This vacuum is provided by vacuum pump means  14 , which also provides vacuum to flush fluid tanks  20  and waste tank  18 . The exhaust from the vacuum pump is directed to an exhaust port  22  on the exterior of the fluid system cabinet. This prevents a buildup of solvent vapors inside the fluid system cabinet. It also provides a convenient means to direct these vapors into fire-safe room exhaust means. Air pump  12  provides a positive pressure in the printhead, reducing the concentration of flammable vapors in the printhead. 
   The automatic shutdown sequence of the present invention will now be described with reference to  FIGS. 1 and 2 . In state  0  of the State Table  100  of  FIG. 2 , the shutdown sequence begins with sequentially closing the eyelid  62 , and turning off the charge voltage, stimulation drive signal, and ink pump  50  in FIG.  1 . The crossflush valve  32  is also opened. After a predetermined period of time, for example about fifteen seconds, the ink supply valve  64  is closed in state  1 . In state  2 , the crossflush valve  32  is closed. In state  3 , the vacuum servo control system adjusts the restrictor  68  to increase the vacuum in the ink tank  16 , flush fluid tank  20 , and waste tank  16 . In state  4 , he crossflush valve  32  is opened, to drain some more ink out of the drop generator  34 , for about 0.5 seconds. 
   The flush fluid pump  24  is turned on to pump from the Flush fluid tank  20 , through the filter  26  and up to the printhead  28  in state  5 . The Flush fluid valve  30  is also opened in state  5 , to allow the flush fluid to flow through the drop generator  34 . The flush fluid servos has a target pressure of 0.5 psi in this state. The initial flow of flush fluid into the drop generator  34  forces the ink in the drop generator and the outlet line  72  through the open crossflush valve  46  and diverter valve  38  into the ink tank  16 . In state  6 , the waste valve  36  opens allowing fluid to begin flowing also to the waste tank  18 . 
   The diverter valve  38  is closed in state  7 , so that all the used flush fluid is directed into the waste tank  18 . This crossflush state continues for a predetermined period of time, for example about thirty seconds. With a target pressure at the drop generator for the flush fluid of 0.5 psi, the flush fluid has been weeping out of the orifices in the orifice plate of attached to the drop generator  34 . This fluid weeping out of the orifices serves to redissolve ink that may have dried on the exterior of the orifice plate, the face of the charge plate, and catcher face. It also serves to redissolve ink on the eyelid. Failure to clean the ink off the eyelid could leave excessive ink at the eyelid seal that can dry, and possibly prevent the eyelid from opening at the next startup. This ink flows out of the catcher  44  to the waste tank  18  through the open catcher valve  46  and waste valve  36 , as a result of the vacuum on the waste tank  18 . 
   The crossflush valve  32  is closed, and the target pressure for the flush fluid pump is increased to 5 psi in state  8 . As the ink pressure is rising to the target pressure, the rapid flow of ink out of the orifices pulls any fluid out of the gap between the orifice plate and the charge plate. This state continues for a predetermined time, for example about 20 seconds. This high volume flow of ink out of the orifice plate in this state serves to flush out the catcher valve  46 . Failure to remove ink residues from valves such as the catcher valve  46  and crossflush valve  32  which at shutdown have the ink drained from them can result in the valves being stuck with dried ink at the next startup. 
   In state  9 , the crossflush valve is again opened. The target pressure for the flush fluid pump is set to 0.5 psi. This again serves to flush ink residues out of the drop generator and out of the crossflush valve  32 . The flush fluid weeping from the orifices again serves to rinse away ink residue from the charge plate and catcher. This state continues for about thirty seconds. In state  10 , the vacuum level in the waste tank is increased. The flow of fluid through the printhead remains the same. 
   In state  11 , the flush fluid valve  30  and the crossflush valve  32  are closed. The flush fluid pump is also turned off. With the catcher line open, this state causes air to be draw into the catcher line. This blows the remaining fluid out of the catcher line. This state lasts for a longer time period, for example about sixty seconds. 
   The crossflush valve  32 , waste valve  36 , and the air ingest valve  66  are opened and the catcher line closed in state  12 . During this state, air is drawn in through filter  74  and the open air ingest valve  66  as a result of the vacuum on the outlet line  72 . This serves to evacuate the flush fluid from the drop generator and outlet lines. The air and the entrained flush fluid are directed into the waste tank  16 . This state typically lasts about twice as long as the previous state, or about one hundred twenty seconds. 
   State  13  again involves a weeping crossflush of the drop generator  34 . The air ingest valve  66  is closed, the fluid valve  64  and the crossflush valve  32  are open. The flush fluid pump  24  is energized and servo controlled to maintain a 0.5 psi pressure at the drop generator. This state lasts for a shorter time period, for example about thirty seconds. 
   In state  14 , the flush fluid pump  24  is again turned off and the flush fluid valve closed. The crossflush valve  32  and the air ingest valve  66  are open to again evacuate the flush fluid from the drop generator and outlet lines. The catcher valve  46  is also open to concurrently evacuate the catcher line. This state lasts for a predetermined time period, for example about sixty seconds. 
   The catcher valve  46  is closed in state  15 , while the other valves do not change. By closing the catcher valve, the vacuum in the waste tank can increase to enhance the evacuation of liquid from the drop generator  34  and the outlet line  72  of the drop generator. This state continues for a longer time period, typically about one hundred twenty seconds. This evacuation of the drop generator continues through state  20 . During this time, the crossflush valve is closed momentarily in states  17  and  19 . The momentary closings of the outlet valve serve to pulse the airflow though the valve and the outlet line. 
   In state  20 , the air ingest valve  66 , and the crossflush valve  32  are closed and the catcher valve  46  opened. This facilitates further evacuation of the catcher and the catcher return line  76  for a predetermined time period of about sixty seconds. Finally, everything is turned off in state  21 , with all the valves de-energized, and the shutdown sequence is complete. The shutdown sequence described here produces a clean, dry printhead that can be started reliably even after extended periods of shutdown. As this shutdown sequence evacuates the printhead, the printhead can be moved or removed from the fluid system without the risk of flammable fluid leaking out of the printhead. 
   In the fluid system shown in  FIG. 1 , the ink level sensor, not shown, in the ink tank  16  is used to determine the level of ink in the tank. The ink concentration is monitored by a concentration sensor  80 . Concentration control means, not shown, monitors the output of both the ink level sensor and the concentration sensor  80  to determine when to added additional ink or replenishment fluid to the ink tank. Ink is added to the ink tank from ink supply  78  via ink fill valve  82 . Replenishment fluid is added to the ink tank from replenishment fluid supply  84  by way of replenishment fill valve  86 . 
   In a solvent based printer, the replenishment fluid can be effectively used as a flush fluid. Therefore, the flush fluid tank  18  can also be refilled from the same replenishment fluid supply  84  that is used to replenish the ink. Flush fluid fill valve  88  controls the filling of the replenishment fluid into the flush fluid tank  18 . 
   In accordance with the present invention, the used flush fluid is directed into a waste tank  18 . In aqueous printers, where the flush fluid is much different from the replenishment fluid, it is mandatory that the used flush fluid be directed into a waste tank as was done in U.S. Pat. No. 6,273,103, to prevent contaminating the ink with the flush fluid. In a solvent printer, where the flush fluid is identical to the replenishment fluid, on the other hand it has been common to direct the used flush fluid into the ink tank. Directing the used flush fluid into the ink tank has the undesirable effect of diluting the ink in the ink tank. This problem is overcome by the present invention where the used flush fluid is directed into a waste tank. 
   Repeated flushing shutdowns can over time result in the waste tank  18  becoming full. The fluid system of  FIG. 1  provides a pump  90  to pump the used flush fluid out of the waste tank through waste port  92  into an appropriate chemical waste container. 
     FIG. 3  is a schematic for an alternate embodiment fluid system. The shutdown sequence employed with this schematic is the same as that using the  FIG. 1  embodiment. Recognizing that the used flush fluid that went into the waste tank is just replenishment fluid with some ink mixed into it, this embodiment uses the fluid in the waste tank as replenishment fluid. When the concentration control system calls for supplying the ink tank  16  with replenishment fluid, the waste pump  90  can be turned on, and the waste transfer valve  94  energized to pump fluid from the waste tank  18  into the ink tank  16 . Replenishment valve  86  would be kept closed. The waste tank includes level sensing means that can identify a waste tank full condition and a waste tank empty condition. If a waste tank empty condition is sensed, waste pump  90  is turned off, and waste transfer valve de-energized, stopping any further transfer of fluid from the waste tank. If further replenishment fluid is needed to replenish the ink in the ink tank, replenishment valve  86  is opened to transfer fluid from the replenishment supply  84 . 
   If the level sensor in the waste tank  18  senses a waste tank full condition, the waste pump  90  is turned on with the waste transfer valve  94  de-energized to pump fluid out of the waste tank through waste port  92  into an appropriate chemical waste container. 
   By using the used flush fluid as a replenishment fluid, the embodiment shown in  FIG. 3  is able to minimize costs associated with the purchase of replenishment fluids and the disposal of chemical wastes. While the use of the used flush fluid as an ink replenishment fluid has been described in regard to flushing of the printhead during a shutdown sequence, it must be recognized that the used flush fluid might also have been produced during a startup sequence flushing of the printhead as well. 
   Having described the invention in detail and by reference to the preferred embodiment thereof, it will be apparent that other modifications and variations are possible without departing from the scope of the invention defined in the appended claims.