Abstract:
An ink jet printhead having a nozzle face in which at least one printhead nozzle is formed is cleaned by disposing a suction nozzle in front of the nozzle face, so that it forms a small gap therewith, and sucking ink out of the prinhead nozzle. The ink is caused to spread or flow in the gap in a direction parallel to the nozzle face so as to clean the nozzle face.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and an apparatus for cleaning an ink jet printhead having a nozzle face in which at least one printhead nozzle is formed. 
     A typical ink jet printhead comprises a plane nozzle face in which a linear array of individual nozzles are formed. The printhead further comprises means for supplying liquid ink to the nozzles and for generating pressure pulses in the ink liquid, so that ink droplets are expelled from the nozzles. Since the nozzles tend to become clogged with dried ink or foreign matter, it is necessary to clean the nozzles from time to time. Further, the nozzle face surrounding the nozzle orifices opening may become soiled with dust or the like, and this may deteriorate the process of droplet formation and/or influence the direction in which the ink droplets are jetted out. For this reason it is also necessary to clean the nozzle face at least in the vicinity of the nozzles. 
     U.S. Pat. No. 5,574,485 discloses a cleaning head which is disposed in front of the nozzle face and can be moved along the linear array of nozzles in a cleaning operation. This cleaning head has a suction nozzle facing the printhead with a small gap formed between the end of the suction nozzle and the nozzle face of the printhead. The suction nozzle is connected to a suction device such as a vacuum pump and can be aligned with the individual printhead nozzles. A cleaning liquid is fed to the cleaning head and pumped against the nozzle plate. This cleaning liquid which dissolves the ink is sucked into the suction nozzle in order to scavenge and clean the printhead nozzle. 
     The cleaning head further has an ultrasonic liquid wiper juxtaposed to the suction nozzle so that it also confronts the nozzle face of the printhead. The wiper is formed by a tubular ultrasonic transducer the front end of which also forms a small gap with the nozzle face. The tubular transducer defines a supply channel through which a cleaning liquid, e.g. a solvent, can be supplied into the gap. The cleaning liquid forms a liquid bridge between the end of the transducer and the nozzle face. This liquid bridge is stabilized in the gap by the surface tension of the liquid and moves together with the transducer when the cleaning head is scanned along the nozzle array, so that the nozzle face is wiped with cleaning liquid. In order to enhance the cleaning effect, the transducer is energized so that ultrasonic waves are created in the liquid bridge. 
     U.S. Pat. No. 5,412,411 discloses an ink jet printer in which the whole nozzle face of the printhead can be immersed in liquid ink contained in a tank, whereby the ink in the tank is used for capping and cleaning the nozzle face. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus with which an ink jet printhead can be cleaned easily and efficiently. 
     According to the present invention, a suction nozzle is disposed in front of the nozzle face, so that it forms a small gap therewith, and ink is sucked out of the nozzle printhead and is caused to spread or flow in said gap in a direction parallel to the nozzle face, wherein pressure waves are generated in the ink volume which forms a liquid bridge ( 54 ,  56 ) between the nozzle face ( 28 ) and the suction nozzle ( 38 ) by activating the printhead ( 10 ) whereby ink is ejected from the printhead nozzles ( 16 ) that open into the gap ( 44 ). 
     Thus, according to the present invention, the ink itself is used not only for scavenging the nozzles but also for cleaning the nozzle face. Since the ink serving as cleaning liquid is supplied directly via the printhead nozzles, it is not necessary to provide separate supply means for the cleaning liquid. In addition, the cleaning or scavenging of the nozzles and the cleaning of the nozzle face surrounding the nozzle orifices can be achieved very efficiently in a single operation. The spreading of the ink in the gap is highly promoted by activating only the printhead nozzles that are currently facing the suction nozzle, so that ink droplets are actively ejected into the gap and the ink can accumulate in the gap. 
     The invention is useful for cleaning all sorts of inkjet systems, whether water based, solvent based or hotmelt ink based. In the latter case cleaning is performed when the ink is in a molten state. 
     Several effects can be used either alone or in combination for causing the ink supplied via the printhead nozzles to spread in the gap. For example, such spreading can be caused by capillary action which occurs when the gap is made small enough and the wetting angle of the ink with the nozzle face of the printhead on the one hand and the end face of the suction nozzle on the other hand is sufficiently small. 
     Another way to cause a spreading of the ink is to make the end face surrounding the mouth of the suction nozzle sufficiently large, so that it covers a plurality of printhead nozzles. Then, when air is drawn into the suction nozzle, a low pressure zone is created in the gap between the suction nozzle and the nozzle face of the printhead, so that there is an extra force sucking the ink from a plurality of printhead nozzles at a time which causes the ink to flow through the gap towards the mouth of the suction nozzle. 
     The suction nozzle may be formed as or combined with a vibrator or ultrasonic transducer for generating pressure fluctuations in the ink contained in the gap, thereby enhancing the cleaning effect. 
     When the suction nozzle is scanned along the array of printhead nozzles, the ink volume contained in the gap will move together with the suction nozzle so that a stripe shaped portion of the nozzle face containing the nozzles is wiped with ink. When the suction nozzle is moved to an inoperative position, e.g. at one end of the nozzle array where no printhead nozzles are present, and the suction device is still kept operative for some time, the ink will be removed from the gap without remnants. 
     The method and apparatus described herein are suitable for solvent-based inks, but are also particularly useful for hot-melt ink which is solid at room temperature and is kept at a temperature above its melting point, e.g. at about 100° C., when the printer and/or the cleaning device is operating. The method can also be used to remove air from the printhead. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic perspective view of an ink jet printhead and a cleaning head with some parts of the printhead broken away for illustration purposes; 
     FIG. 2 is an enlarged partial cross-section illustrating a mode of operation of the cleaning head; and 
     FIG. 3 is a cross-sectional view corresponding to FIG. 2 but illustrating a modified mode of operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is shown in FIG. 1, an ink jet printhead  10  comprises a channel plate  12  which has a front face  14  formed with a linear array of equidistant nozzles  16 . A plurality of ink channels  18  are formed in the top surface of the channel plate  12 . These ink channels are arranged in parallel to one another and each channel is connected to a corresponding nozzle  16 . 
     A diaphragm  20  is bonded to the top surface of the channel plate  12  so as to cover the open faces of the ink channels  18  and the nozzles  16 . 
     An actuator member  22  is superimposed on the diaphragm  20  and forms a plurality of piezoelectric actuators  24  which are configured as parallel, substantially downwardly extending fingers, the lower end faces of which are bonded to the diaphragm  20 . Each actuator is opposed to one of the ink channels  18 . 
     An enclosure member  26  encapsulates the actuator member  22 . Front faces of the enclosure member  26  and the diaphragm  20  are flush with the front face  14  of the channel plate  12 , so that a continuous plane nozzle face  28  is formed. 
     As is generally known in the art and not shown here, a recording paper is movable past the nozzles  16  in a direction perpendicular to the linear array of nozzles, ink supply means are provided for supplying liquid ink to the ink channels  18 , and electronic control means  24   a  are provided for selectively energizing the actuators  24 , thereby causing the diaphragm  20  to flex and compress the ink volume contained in the ink channels  18 , so that ink droplets are expelled from the nozzles  16  and are deposited on the recording paper in accordance with an image to be printed. By way of example it may be assumed that the print head  10  is adapted to operate with hot-melt ink. Accordingly, heating means (not shown) are provided for keeping the temperature of the ink above its melting point. 
     An ink jet printer may comprise a plurality of printheads  10  arranged with staggered nozzles  16  in order to achieve a high image resolution. The printheads may extend over the whole width of the recording paper. 
     A carriage  30  incorporates a cleaning head  32  which confronts the nozzle face  28  and the nozzles  16  formed therein. The carriage  30  is movable in the direction indicated by arrows  34  in FIG. 1, so that the cleaning head  32  may scan the array of nozzles  16 . During a printing operation the carriage  30  is held in an inoperative position at one end of the printhead  10 , outside of the path of transport of the recording paper. 
     In the shown embodiment the carriage  30  is directly mounted to the printhead  10  and is guided by guide rails  36 . Thus, the position of the cleaning head  32  relative to the nozzle face  28  is defined with high accuracy. In an alternative embodiment, the carriage  30  may be mounted on guide means which are separate from the printhead  10 . It is also possible to move the carriage in a direction perpendicular on the direction of the arrows  34  to clean all the nozzles simultaneously. 
     FIG. 2 is a simplified cross-sectional view of the cleaning head  32  and a portion of the channel plate  12  adjacent to the nozzle face  28 , the section being taken in the plane defined by the nozzles  16 . 
     The cleaning head  32  comprises a suction nozzle  38  connected to a suction device  40 . The suction nozzle  38  has a comparatively extended end face  42  held in parallel with the nozzle face  28  so that a narrow gap  44  is formed between the nozzle face and the suction nozzle. The suction nozzle  38  defines a channel  46  one end of which is connected to the suction device  40  while the other end defines a mouth  48  in the end face  42 . The width of the mouth  48  is smaller than the distance between two adjacent nozzles  16 , whereas the end face  42  as a whole extends over a plurality of nozzles  16 . The channel  46  is surrounded by a tubular piezoelectric vibrator  50  or, alternatively, by two vibrators disposed on either side of the channel  46  in the direction of the array of nozzles. 
     Due to the static pressure of the ink in the ink supply system, the ink contained in the nozzles  16  is subject to a force which tends to press the ink out of the nozzles. This force, however, is counterbalanced by the surface tension of the ink, so that a bulging meniscus  52  is formed at the nozzle orifices. 
     When the suction device  40  is operated, at first, air is drawn in through the mouth  48  of the suction nozzle. Since the flow of air is restricted in the narrow gap  44 , the static and dynamic pressure of the air in the gap  44  is lowered, so that ink is sucked out of the nozzles  16 . More specifically, ink is sucked not only from the nozzle which is directly opposed to the mouth  48  but also from the other nozzles which open into the gap  44  defined by the end face  42  of the suction nozzle. Thus, the ink sucked from the nozzles which are offset from the mouth  48  is caused to flow through the gap towards the mouth  48 . As a result, an ink volume or a liquid bridge  54  is formed in the gap  44 . The ink flowing through the nozzles  16  and the gap  44  is effective not only in cleaning the nozzles  16  but also in cleaning the portions of the nozzle face  28  surrounding these nozzles. In case one of the nozzles shows a higher resistance for ink e.g. caused by clogging of the ink in one of the nozzles, the above system will not work effectively. According to the present invention in case of cleaning, the nozzles are activated to make sure that even clogged ink is forced out of the nozzles. 
     The nozzle face  28  is made of or coated with a material which is not wetted by the ink. Nevertheless, the wetting angle between the ink and the nozzle face is normally smaller than 90°, typically in the order of 70°, so that the ink is subject to capillary forces which tend to enlarge and stabilize the liquid bridge  54 . In the shown embodiment the end face  42  of the suction nozzle  38  consists of or is coated with a material which forms a smaller wetting angle with the ink, so that the capillary forces are increased. 
     When the suction pressure generated by the suction device  40  is low, the liquid bridge  54  may spread over the whole surface of the end face  42  of the suction nozzle. When the suction pressure is larger, it tends to contract the liquid bridge  54  until an equilibrium state is reached in which the flow of ink through the total of the nozzles  16  is equal to the flow of ink through the channel  46 . 
     The vibrator  50  communicates with the liquid bridge  54  and generates supersonic waves in the ink, thereby enhancing the cleaning effect. 
     As described above, according to the present invention the actuators  24  associated with the nozzles  16  from which the ink is sucked are energized. This will not only increase the flow of ink but will also produce pressure waves contributing to the cleaning effect, in particular within the nozzles. It is also possible to create an extra ink flow by increasing the overall pressure in the ink supply system. 
     When the carriage  30  is driven to move the suction nozzle  32  along the nozzle face  28 , the capillary forces and suction forces will cause the liquid bridge  54  to move together with the suction nozzle  38 , so that the liquid bridge  54  wipes the nozzle face  28  over the whole length of the nozzle array. 
     When the suction pressure is increased further, the capillary forces may no longer be sufficient to stabilize the liquid bridge  54 , and air may be drawn in especially from the zones above and below the linear array of nozzles, i.e. from above and below the plane of the drawing in FIG.  2 . In this mode of operation, which is illustrated in FIG. 3, the liquid bridge breaks down into a number of separate slugs  56  with bubbles  58  of air intervening therebetween. Since, in this case, the pressure gradient in the gap  44  is large, the ink will nevertheless be efficiently sucked out of all the nozzles  16  which open into this gap, and the slugs  56  will be rapidly accelerated towards the mouth  48  of the suction nozzle so that a high mechanical cleaning effect is achieved. 
     The ink flowing out through the channel  46  is separated from the air by known techniques and may be discarded or filtered and re-used. 
     While specific embodiments of the present invention have been described herein, it will occur to a person skilled in the art that various modifications can be made within the scope of the appended claims.