Patent Publication Number: US-8540338-B2

Title: Printhead maintenance station including station backflush

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned, U.S. patent application Ser. No. 13/074,405, entitled “MAINTAINING PRINTHEAD USING MAINTENANCE STATION WITH BACKFLUSH, filed concurrently herewith. 
     FIELD OF THE INVENTION 
     This invention relates generally to the field of digitally controlled printing systems and, in particular to maintenance stations for inkjet printheads. 
     BACKGROUND OF THE INVENTION 
     In an inkjet printer, a printhead includes a plurality of jetting modules, each jetting module having a nozzle face in the form of a long narrow rectangular plate with a nozzle array, through which a liquid (e.g., ink) is jetted. When the printhead is not in use, liquid in the nozzle array may dry or attract dust and other contaminants, which can lead to clogging, resulting in decreased print quality, or printhead failure. Typically, when the printhead is not in use, it is moved to a maintenance station that removes the liquid, and other contaminates, so as to minimize the likelihood of clogging or failure. 
     The maintenance station will generally include the following components, at a minimum: a capping unit, a valve, and a waste tank. The capping unit engages the printhead nozzle face, providing a seal around the nozzle array. The valve is then opened, and a negative pressure from the capping unit or positive pressure from the printhead is applied, causing liquid to flow from the nozzles, which flushes dried ink, dust or other contamination (i.e., waste liquid) from the nozzles. The waste liquid is then transported to the waste tank. 
     However, the maintenance station components, specifically, orifices at connection points, are subject to fouling from the contamination removed from the printhead. This fouling can be more prevalent when the liquid jetted from the printhead is an ink containing a magnetic pigment, which is used in magnetic ink character recognition (MICR). 
     As such, there is an ongoing need for a maintenance station, having a reduced risk of becoming contaminated, which effectively removes liquid, dust, and other contaminants from a printhead. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a maintenance station apparatus for a printhead including a nozzle face with the nozzle face including a nozzle array for jetting a liquid is provided. The maintenance station includes a capping unit including a drain and a waste liquid tank that receives a waste liquid from the capping unit. A valve is in fluid communication with the capping unit through a first fluid passage and in fluid communication with the waste liquid tank through a second fluid passage. The valve includes a first state that permits the waste liquid to flow from the capping unit through the valve to the waste liquid tank and a second state that prevents the waste liquid from flowing from the capping unit through the valve to the waste liquid tank. A cleaning liquid tank is in fluid communication with the valve through a third fluid passage and is configured to provide a cleaning liquid through the valve and into the first fluid passage when the valve is in the second state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the example embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1  is a schematic of a prior art maintenance station where a capping unit can engage a printhead to remove liquid and other contaminants; 
         FIG. 2  is a schematic view of an embodiment of the maintenance station that includes a cleaning liquid passage used to provide a cleaning liquid to the components within the maintenance station; 
         FIG. 3  is a schematic view of an embodiment the maintenance station where the printhead contains multiple jetting modules with a corresponding number of capping units, and a cleaning liquid passage and a liquid level control system; and 
         FIG. 4  is a block diagram showing the method of maintaining a printhead according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. Throughout the description, common reference numerals are used for common parts. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     Referring to  FIG. 1 , a schematic view of a prior art maintenance station  5  is shown. A printhead  10  is located in a parked position that is vertically above a capping unit  15 . The capping unit  15  is movable along an axis perpendicular to the printhead  10 , illustrated using arrow  16 , such that the capping unit  15  can engage the printhead  10 , and form a seal around the nozzle array located on the printhead nozzle face. 
     The capping unit  15  includes a drain  17  that is in fluid communication with a valve  25  via a first fluid passage  40 . A waste liquid tank  20  is in fluid communication with the valve  25  via a second fluid passage  45 . 
     Operationally, valve  25  includes a first state and a second state. When the valve  25  is in the first state, the waste liquid removed from the printhead  10  is permitted to flow through the valve  25  to the waste liquid tank. When the valve  25  is in the second state, no waste fluid is permitted to flow through the valve  25 . As shown in  FIG. 1 , valve  25  is in the second state. The valve  25  can be any valve providing the function describe above, for example, an electro-mechanically operated valve or an air-operated valve. An example of an electromechanically operated valve is a solenoid valve. 
     In the arrangement shown, there is a purge pump  30  that creates a differential pressure to remove the waste liquid from the capping unit  15  and into the waste liquid tank  20 . While the purge pump  30  is shown to be between the valve  25  and the waste liquid tank  20 , other example embodiments position purge pump  30  after the waste liquid tank  30 . The purge pump is used to create a negative pressure within the waste liquid tank  30 , thereby drawing the waste liquid into the waste liquid tank  30 . 
     Alternatively, the jetting modules of printhead  10  can be pressurized to force any liquid, dust, or other contamination from the printhead  10 , into the capping unit  15 . Then, either gravity or pressurization of the jetting modules can also be used to remove the waste liquid from the capping unit  15  to the waste liquid tank  20 . 
     The flow of the waste liquid in  FIG. 1  is only in one direction. As such, the materials contained within the waste fluid, such as pigments or highly viscous humectants, can collect or rise in concentration within the valve  25 . This can cause fouling of the valve  25 , causing the valve to seize, to have restricted flow, or to otherwise fail to function properly. Additionally, when the valve  25  used within the maintenance station  5  is actuated using a solenoid or other electromechanical actuator, the current through the solenoid or electromagnetic actuator can heat the valve accelerating the drying of ink in the valve. Additionally, the motive force used to energize or actuate this type of valve to change the valve  25  from the first state to the second state, and vice versa, involves an electromagnetic field. This electromagnetic field, when applied, can affect the magnetic pigment contained in MICR inks which can increase the risk of fouling the valve. 
     Referring to  FIG. 2 , an example embodiment of the present invention is shown. Maintenance station  5  includes a third fluid passage  50 . Valve  25  is in fluid communication with a cleaning liquid tank  35  containing a cleaning liquid. During a cleaning operation, the capping unit  15  engages the printhead  10  and the waste liquid is removed, exiting the capping unit via the drain  17 , through the first fluid passage  40 . The valve  25  is in a first state, permitting the waste liquid to flow into the second fluid passage  45  and to the waste liquid tank  20  via the pressure differential created by the purge pump  30 . 
     Cleaning liquid tank  35  is positioned such that the cleaning liquid level  36  is vertically higher than the valve  25 , but vertically lower than the capping unit  15 , in either an engaged or non-engaged position with printhead  10 . This positioning, along with a vent  37  to atmosphere in the cleaning liquid tank  35 , allows the cleaning fluid to flow from the cleaning liquid tank  35  through the third fluid passage  50  through the valve  25  and into the first fluid passage  40  when the valve  25  is in the second state but not to overflow the capping unit  15 . As the cleaning liquid flows through the valve  25 , materials contained within the waste liquid are either displaced or diluted, helping to ensure that the valve  25  is functioning properly. After the waste liquid is displaced from or diluted in the valve  25  and the valve  25  may be moved back to the first state, the now contaminated cleaning liquid within the first fluid passage  40  is deposited into the waste liquid tank  30  along with the waste liquid collected from the printhead  10 . 
     The cleaning operation can then be repeated, by moving the valve  25  to the second state, allowing the cleaning liquid to again flow through the valve  25  from the cleaning liquid tank  25 . The capping unit can then be stored in this condition (i.e., stored “wet”), or the valve  25  can be moved back to the first state and the cleaning liquid drained to the waste tank  20  (i.e., stored “dry”). 
     In general, the higher the viscosity or the higher the solids content of the liquid being jetted, the more likely the maintenance stations  5  of the prior art will foul. Inks containing pigment have a higher risk of fouling the valves of the prior art maintenance stations than to dye based inks. MICR inks, which contain magnetic pigments, are even more likely to cause fouling. While the invented maintenance station is useful for a wide range of inks and other jetting liquids, it is of particular value when used with jetting liquids containing pigments or other fine particles in suspension and even more valuable when MICR inks or other jetting liquids containing magnetic particles in suspension are used. 
     For pigment based jetting liquids, it is preferred that the cleaning liquid contain a redispersant that is effective to redisperse the pigment contained within the jetted liquid. For example, the FF5124 MICR cleaning fluid, produced by Eastman Kodak Company, is an effective cleaning liquid containing the redispersant for MICR inks. For non-pigment based jetting liquids, the cleaning liquid preferably contains solvents to redissolve the various components found in dried or partially dried ink or other jetting liquid residues. Typically the cleaning fluid does not contain any pigments or other colorants. 
     In another example embodiment, a metering pump is located within third fluid passage  50  or the cleaning liquid tank  35  and used to control the flow of the cleaning liquid. The metering pump forces a defined amount of the cleaning fluid through the valve  25  into the first fluid passage  40 , when the valve  25  is in the second state, such that the cleaning liquid does not overflow the capping unit  15 . In this embodiment, the cleaning liquid level  36  does not necessarily need to be vertically higher than the valve  25 . A peristaltic pump works effectively as the metering pump, although piston, gear or other types of positive displacement pumps are also effective. 
     Referring to  FIG. 3 , another example embodiment of the present invention is shown. Maintenance station  5  is operatively associated with a printhead  10  that includes multiple jet modules  12 , each module  12  having a nozzle face with a nozzle array. There is a plurality of capping units  15  that correspond with each of the jetting modules  12 . The capping units  15  are movable, individually or as a group, along an axis perpendicular to the printhead  10 , as illustrated by the arrow  16 , so that the capping units  15  engage the jetting modules  12  and form a seal around the nozzle array located on the nozzle face. 
     The capping units  15  are in fluid communication with the valve  25  through the first fluid passage  40 . The first fluid passage  40  includes second valves  55  that correspond to each of the capping units  15 . An upstream portion  54  of the first fluid passage  40  provides fluid communication between the drain of a capping unit  15  and the corresponding second valve  55 . A downstream portion  56  of the first fluid passage  40  provides fluid communication between the second valve  55  and the first valve  25 . The second portion can include a manifold  60  to enable multiple second valves  55  to be in fluid communication with the first valve  25 . Each capping unit  15  is in fluid communication with the corresponding second valve  55 , of which each second valve  55  is in fluid communication with the manifold  60 , which collects waste fluid from the capping units  15  and drains the waste fluid through the valve  25  and into the waste tank  20 . Each second valve  55  can be operated individually, enabling a specific jetting module  12  to be purged rather than purging all of the jetting modules  12  contain within the printhead  10 . 
     The cleaning liquid tank  35 , with a vent  37  to atmosphere, is positioned such that the cleaning liquid level  36  is vertically higher than the second valves  55 , but lower than the capping units  15 . However, each time the maintenance station  5  is operated, moving the valve  25  from the first state to the second state, the cleaning liquid level  36  will decrease, eventually to a minimum cleaning liquid level, a point at which gravity does not provide the flow necessary to dilute or displace contamination. 
     As such, the maintenance station  5  includes a liquid level control system  65 . The cleaning liquid level  36  is monitored by a sensor within the cleaning liquid tank  35 , which signals the cleaning liquid supply to provide additional cleaning fluid when the cleaning liquid level  36  becomes too low. For example, the cleaning liquid tank  35  can include a float switch that signals the cleaning liquid supply to replenish the cleaning liquid tank  35 , from an external source, when the cleaning liquid level is too low. Upon receiving the signal, the cleaning liquid supply replenishes the cleaning liquid tank  35  so that the cleaning liquid level  36  is vertically higher than the second valve  55 . 
     Referring to  FIG. 4 , printhead maintenance begins with step  410 . 
     In step  410 , a capping unit is provided and includes a drain. A waste liquid tank receives a waste liquid from the capping unit. Step  410  is followed by step  415 . 
     In step  415 , a valve is in fluid communication with the capping unit through a first fluid passage connected to the drain. The valve is also in fluid communication with the waste liquid tank through a second fluid passage. The valve includes a first state that permits the waste liquid to flow from the capping unit through the valve to the waste liquid tank. The valve includes a second state that prevents the waste liquid from flowing from the capping unit through the valve to the waste liquid tank. Step  415  is followed by step  420 . 
     In step  420 , a cleaning liquid tank is in fluid communication with the valve through a third fluid passage. The cleaning liquid tank is configured to provide a cleaning liquid through the valve and into the first fluid passage when the valve is in the second state. Step  420  is followed by step  425  and step  430 . 
     In step  430 , the capping unit engages the printhead prior to operating the valve. Step  430  is followed by step  425 . In step  425 , the valve is operated to cause the valve to move between the first state and the second state. Step  425  is followed by decision step  433 . 
     Decision step  433  decides whether the pump should be operated. If yes, the next step is step  435 . If no, the next step is step  425 . In step  435 , the pump is provided. Step  435  is followed by step  440 . In step  440 , the pump is used to cause waste liquid to flow from the capping unit through the valve and into the waste liquid tank, when the valve is in the first state. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
         
           
               5  Maintenance Station 
               10  Printhead 
               12  Jetting modules 
               15  Capping unit 
               16  Arrow 
               17  Drain 
               20  Waste liquid tank 
               25  Valve 
               27  Second valve 
               30  Purge pump 
               35  Cleaning liquid tank 
               36  Cleaning liquid level 
               37  Vent 
               40  First fluid passage 
               45  Second fluid passage 
               50  Third fluid passage 
               54  Upstream portion 
               55  Second valve 
               56  Downstream portion 
               60  Manifold 
               65  Liquid level control system 
               410  step 
               415  step 
               420  step 
               425  step 
               430  step 
               433  decision step 
               435  step 
               440  step