Abstract:
An ink jet device with an ink reservoir and a jetting assembly, with an ink supply line being arranged to allow liquid ink to flow through the valve into the ink reservoir in accordance with and supported by gravity acting on the flowing ink, and a purging device adapted to apply a purging pressure to an inner space of the ink reservoir, wherein a passive one-way valve is arranged below a nominal minimal fill level (L) of the ink reservoir for blocking a connection of the ink supply line to the ink reservoir when a pressure within the ink reservoir is high enough but not required to be higher than the purging pressure.

Description:
[0001]     This application claims the priority benefit of European Patent Application No. 05110682.1 filed Nov. 14, 2005 which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to an ink jet device comprising an ink reservoir and a jetting assembly having at least one printing nozzle.  
         [0003]     In an ink jet device, for example an ink jet printer with an ink reservoir and a jetting assembly, the ink reservoir is usually arranged together with the jetting assembly in a printhead which is reciprocated over a printing area. When ink is also stored outside the reciprocating printhead, the printhead usually has an ink supply line for supplying ink to the ink reservoir. For example, in an ink jet printer operating with hot melt ink, i. e. with ink that is solid at room temperature, the ink supply line may comprise an ink melting unit for supplying melted ink to the ink reservoir, and solid ink pellets may be supplied to an open end of the ink melting unit from outside the printhead from time to time. In order to ensure that contaminants are prevented from entering the ink reservoir, an ink filter may be provided at the ink supply line, and the melted ink may be required to flow through the ink filter into the ink reservoir.  
         [0004]     Depending on the configuration of the ink jet device, it may be necessary to provide a suction device for applying a negative pressure to an inner space of the ink reservoir to prevent ink from leaking out of the printing nozzles of the jetting assembly. In this case, the ink filter may function as a hydraulic lock, so that the negative pressure can be provided to the ink reservoir while at the same time ink may be supplied to the ink reservoir through the ink filter.  
         [0005]     In order to dispose of air bubbles or contaminants that might be present in the jetting assembly, it is desirable to purge the printing nozzles of the jetting assembly from time to time, thereby removing any air bubbles and contaminants. Therefore, a purging device may be provided that is adapted to apply a purging pressure to the inner space of the reservoir for pressing ink into the printing nozzles. However, in case of an ink supply line having an open end, the ink supply line has to be shut off from the ink reservoir to permit the purging device to build up the necessary purging pressure in the ink reservoir.  
         [0006]     The necessary purging pressure is usually higher than the absolute value of the negative pressure which is applied to the ink reservoir for preventing ink from leaking ink out of the printing nozzles. In the case of the ink supply line having an ink filter as described above, the ink filter may not function as a hydraulic lock under the comparatively high purging pressure. Thus, a break-through of the ink filter, i.e., an undesired flow of air or ink from the ink reservoir through the ink filter, may take place. In this case, the necessary purging pressure cannot be built up within the ink reservoir.  
         [0007]     From U.S. Pat. No. 4,641,154, an ink jet apparatus having a tilt valve that is positioned at an upper wall of the ink reservoir is known. Hot melt ink is melted in an ink melting unit and flows through the tilt valve into the ink reservoir due to gravity. When a purging pressure is to be built up by an ink jet priming system, the tilt valve is moved to a closed position by an actuator. Thus, the tilt valve has to be actively closed in order to enable the ink jet priming system to build up a purging pressure within the ink reservoir. The necessary actuator for closing the valve and the moving parts of the valve which are arranged at the ink supply line are disadvantageous. A further disadvantage results from the fact that the valve is opened by the gravity force of the melted ink as the opening of the valve may be suppressed by capillary forces, i.e., adhesion between the valve element, the liquid ink and the top wall of the reservoir. In such a case, the ink supply line will be blocked.  
         [0008]     From U.S. Pat. No. 6,048,057, a hot melt ink jet printhead is known having an ink reservoir with a first chamber and a second chamber. The second chamber comprises a first valve and a second valve. In the normal printing operation of the printhead, ink flows from the first chamber to a nozzle head of the printhead. During a purging operation, a channel between the first chamber and the second chamber is closed, and a channel between the nozzle head and the second chamber is opened. Then, ink flows from the first chamber through the nozzle head into the second chamber. Accordingly, any air bubbles present in the nozzle head are moved into the second chamber. During the normal printing operation, ink is allowed to flow from the second chamber to the first chamber. The valves are arranged at opposite arms of a lever to be alternately closed by a movement of the lever. The lever is activated by an actuating mechanism. A disadvantage is that always one of the valves is in a closed position, so that during printing the ink is not circulated in the chamber between the nozzle head and the second chamber. Moreover, the valves do not shut off the ink supply line from the ink reservoir.  
         [0009]     From U.S. Pat. No. 5,489,925, an ink jet printing system is known, wherein a liquid ink supply line extending from a remote liquid ink reservoir to an ink reservoir of a printhead is shut off by a passive check valve. The check valve is spring-biased toward its closed position. The ink reservoir of the printhead may be elevated above the remote reservoir, and ink is supplied through the check valve to the ink reservoir by a pump, thereby opening the check valve. A pressure control system controls the pressure of the ink in the ink reservoir of the printhead to permit purging of air bubbles and contaminants from orifices and passageways. A disadvantage of this ink jet printing system is that a pump is necessary to actively pump the ink through the check valve in the ink supply line.  
       SUMMARY OF THE INVENTION  
       [0010]     It is an object of the present invention to provide an ink jet device that allows a build up of purging pressure within an ink reservoir, and in which the ink supply line is reliably shut off during a purging operation and is reliably opened after the purging operation, without requiring a pump for supplying the ink.  
         [0011]     According to the present invention, this object is achieved by an ink jet device comprising an ink reservoir and a jetting assembly having at least one printing nozzle, the jetting assembly being connected to the ink reservoir for receiving liquid ink from the ink reservoir, the ink jet device further comprising an ink supply line for supplying ink to the ink reservoir, said ink supply line comprising a valve for blocking a connection of the ink supply line to the ink reservoir, said valve being arranged at a lower end of the ink supply line. The ink supply line and the ink reservoir are arranged to allow liquid ink to flow through the valve into the ink reservoir in accordance with and supported by gravity acting on the ink. The ink jet device further comprises a purging device adapted to apply a purging pressure to an inner space of the ink reservoir for pressing ink into the at least one printing nozzle, wherein the valve is arranged below a nominal minimal fill level of the ink reservoir. The valve is a passive one-way valve that is adapted to block the connection of the ink supply line to the ink reservoir when a pressure within the ink reservoir reaches a predetermined value. This predetermined pressure value may be a value between the set value of the pressure to be applied to the ink reservoir during printing and the purging pressure set value. Due to the valve, the ink supply line can be reliably shut off from the ink reservoir during the purging operation, so that the purging pressure can be built up within the ink reservoir. Moreover, because the valve is a passive valve that closes when the pressure within the ink reservoir is high enough, wherein said high enough pressure is not required to be higher than the purging pressure, the valve automatically closes when the purging pressure is built up. Therefore, an actuator for closing the valve can be dispensed with.  
         [0012]     Moreover, a reliable opening of the valve is achieved for the following reasons. Because the valve is arranged below a nominal minimal fill level of the ink reservoir, the valve is surrounded by the ink of the ink reservoir and by the ink that is present within the ink supply line. Therefore, there will be no capillary adhesion forces that could prevent the valve from opening. Therefore, the valve may be adapted to be opened due to or supported by gravity forces acting on ink which is present in the ink supply line. Furthermore, the valve preferably comprises a valve element which has a greater mass than the ink which it displaces. Thus, the opening of the valve is also supported by gravity acting on the valve element.  
         [0013]     Because the liquid ink flows through the valve into the ink reservoir supported by gravity, a pump may be dispensed with. This facilitates a simple construction of the ink jet device.  
         [0014]     In a preferred embodiment, the ink supply line further comprises an ink filter, the valve being arranged between the ink filter and the ink reservoir. In this case, the valve of the invention is extremely useful to prevent an ink filter break-through due to the purging pressure. Preferably, the only ink path into the ink reservoir is through the ink filter, and, following the ink filter, through the valve. Preferably, the ink filter is arranged above the ink reservoir. In one embodiment, the ink supply line further comprises a sub chamber which is arranged between the ink filter and the valve. Thereby, the gravity force acting on the ink which is present within the sub chamber contributes to opening the valve.  
         [0015]     In a preferred embodiment, the ink supply line further comprises an ink melting unit for supplying melted ink through the ink filter to the ink reservoir.  
         [0016]     Preferably, the ink jet device comprises a suction device which is adapted to apply a negative pressure to the inner space of the ink reservoir. Thereby, ink may be prevented from leaking from the printing nozzle. In case the ink filter is provided at the ink supply line, the ink filter preferably functions as a hydraulic lock, so that the negative pressure may be applied while still allowing liquid ink to be supplied to the ink reservoir through the filter.  
         [0017]     In one embodiment, the valve is adapted to be continuously open during a printing operation of the ink jet device and is adapted to be closed by the purging pressure being applied to said inner space of the ink reservoir.  
         [0018]     In a preferred embodiment, the valve comprises a spherical valve element, a cage and a valve seat for the valve element, the valve element being freely movable between the valve seat and the cage, the mass of the valve element being greater than the mass of the liquid ink which is displaced by the valve element. The valve is adapted to be closed when the valve element is pressed against the valve seat. Alternatively, the valve may be a duck bill valve.  
         [0019]     Preferably, the cage forms a diffuser for ink which flows through the valve into the ink reservoir. Thereby, a circulation of the liquid ink within the ink reservoir is facilitated.  
         [0020]     In a specially preferred embodiment, the ink supply line further comprises a vertical tube, the valve seat being arranged at an inner wall of said tube, the tube surrounding at least an upper half of the valve element when the valve element is pressed against the valve seat. Thereby, during the build up of the purging pressure within the ink reservoir, a momentary movement of ink towards the valve more reliably pushes the valve element against the valve seat. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     A preferred embodiment of the present invention will now be described in conjunction With the drawings in which:  
         [0022]      FIG. 1  is a schematic sectional view of an ink jet device with an ink supply line for hot-melt ink;  
         [0023]      FIG. 2  is a schematic cross-sectional view of the lower part of the ink jet device of  FIG. 1  taken along the line II-II; and  
         [0024]      FIG. 3  shows the ink jet device of  FIG. 1  during a purging operation. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     FIGS.  1  to  3  show an ink jet device with a jetting assembly  10  forming a lower part thereof. An array of printing nozzles  12  is arranged at the jetting assembly  10  to eject ink droplets in a vertical direction. In  FIG. 1 , one of the printing nozzles  12  is shown, and ink droplets  14  are indicated. The jetting assembly  10  is connected via a filter  16  to an inner space  18  of an ink reservoir  20 .  
         [0026]     The ink reservoir  20  is normally filled with liquid hot-melt ink  22  at least to a nominal minimal fill level L indicated with a chain-dotted line. An electric heating device  24  is arranged in a manner which is generally known in the art at the ink reservoir  20  to keep the hot-melt ink  22  in its liquid state. A fill level sensor  26  is arranged at a wall of the ink reservoir  20  to detect whether the actual ink fill level within the ink reservoir  20  sinks to nearly the nominal minimal fill level L, like it is shown in  FIGS. 1 and 2 . In this case, new liquid ink is supplied to the inner space  18  of the ink reservoir  20  via an ink supply line  28 . In  FIG. 3 , an actual fill level is shown that is higher than the actual fill level shown in  FIGS. 1 and 2 .  
         [0027]     Above the ink reservoir  20 , there is arranged an ink melting unit  30  of the ink supply line  28 . Solid ink pellets are supplied to the ink melting unit  30  when needed. At a wall of the ink melting unit  30 , there is arranged an electric heating device  32  for melting the ink pellets which are present in the ink melting unit  30 . The ink melting unit  30  forms a funnel which opens at its lower end into a first sub chamber  34  of the ink reservoir  20 . The first sub chamber  34  extents substantially over the cross sectional area of the ink reservoir  20 . It has a lower wall which is formed by an ink filter  36  which connects the first sub chamber  34  to a second sub chamber  38  which is arranged below the ink filter  36 . The upper part of the second sub chamber  38  forms a funnel which is connected to a vertical tube  40  that forms the lower part of the second sub chamber  38 . At the lower end of the vertical tube  40 , the ink supply line  28  is connected to the inner space  18  of the ink reservoir  20  via a valve  42  which will now be described in detail.  
         [0028]     At a lower end of the vertical tube  40 , the inner wall of the vertical tube  40  forms a valve seat  44  for a ball which forms a spherical valve element  46 . At the valve seat  44 , the vertical tube  40  forms a valve opening  48 . In a closed position of the valve, the spherical valve element  46  is pressed against the valve seat  44  by a pressure within the ink reservoir  20  and thereby closes the valve opening  48 . Thus, the connection of the ink supply line  28  to the ink reservoir  20  is blocked. This state is shown in  FIG. 3 .  
         [0029]     In  FIGS. 1 and 2 , an open position of the valve is shown wherein the valve element  46  is held by a cage  50  which is arranged at the lower end of the vertical tube  40 . The valve element  46  is free to move between the cage  50  and the valve seat  44 . Since the valve element  46  has a greater mass than the ink which it displaces, the valve element  46  will usually rest on the cage  50  in the open position of the valve  42 . Thereby, the influence of the valve  42  onto the ink flow into the ink reservoir  20  is minimized. However, the vertical tube  40  is arranged to surround at least an upper half of the spherical valve element  46  in the open position of the valve  42 . Thereby, when the pressure is built up within the ink reservoir  20 , a resultant movement of the ink towards the vertical tube  40  assures that the valve element  46  is pressed against the valve seat  44 . In the closed position of the valve  42 , the vertical tube  40  surrounds an even larger part of the valve element  46 .  
         [0030]     An ink flow from the ink supply line  28  through the open valve  42  into the ink reservoir is divided by the cage  50  into at least two branches. Thus, the cage  50  forms a diffuser for the ink flow. Accordingly, the ink enters into the inner space  18  of the ink reservoir  20  in at least two different directions. Thereby, a circulation of the ink  22  within the ink reservoir  20  is achieved. In the embodiment shown, the ink flows through openings between the cage  50  and the vertical tube  40  at two opposite sides of the valve  42 , namely, to the left and to the right of the vertical tube  40  in  FIG. 1 .  
         [0031]     At an upper area of the inner space  18  of the ink reservoir  20 , there is connected a ventilation conduit  52  to the inner space  18 . The ventilation conduit  52  is, for example, formed by a flexible hose. The ventilation conduit  52  connects the inner space  18  of the ink reservoir  20  via an air filter  54  to a combined suction and purging device  56 . During a normal printing operation of the ink jet device, the combined suction and purging device  56  applies a negative pressure via the ventilation conduit  52  to the inner space  18 . Thereby, liquid ink  22  is prevented from leaking through the printing nozzles  12  of the jetting assembly  10 . Under this negative pressure, the ink filter  36  of the ink supply line  28  functions as a hydraulic lock because capillary channels of the ink filter  36  are filled with ink. Thereby, the negative pressure can be maintained within the inner space  18  of the ink reservoir  20 . During the printing operation, the valve  42  is continuously open.  
         [0032]     When a purging operation of the ink jet device is to be performed, the combined suction and purging device  56  exerts a higher positive pressure to the inner space  18  of the ink reservoir  20 . This is called the purging pressure. When the purging pressure is applied to the ink reservoir  20 , the valve element  46  is pressed against the valve seat  44  and thus, the valve  42  is closed. Thereby, the connection of the ink supply line  28  to the ink reservoir  20  is locked. Thus, the ink filter  36  does not interfere with the building up of the purging pressure within the inner space  18  of the ink reservoir  20 . In particular, a break-through of the ink filter  36  is prevented.  
         [0033]     After the purging operation has been performed, the combined suction and purging device  56  may again apply a negative pressure to the inner space  18  of the ink reservoir  20 . Due to gravity acting on ink within the vertically tube  40  and due to the negative pressure within the ink reservoir  20 , the valve element  46  experiences a force that moves the valve element  46  downward. Because the valve element  46  is arranged below the nominal minimal fill level L, it is surrounded by liquid ink  22  in the closed position of the valve  42 . Thereby, capillary adhesion forces are prevented from occurring between the valve element  46  and the valve seat  44 . Therefore, after a purging operation the valve  42  reliably opens.  
         [0034]     Thus, the valve  42  functions as a passive one-way valve that reliably blocks the connection of the ink supply line  28  to the ink reservoir  20  when the purging pressure is applied to the ink reservoir and reliably opens again when the pressure within the ink reservoir is at its normal level during a printing operation.  
         [0035]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.