Abstract:
An ink jet device utilizing hot melt ink, said device including an ink reservoir, a heater arranged to heat ink contained in the ink reservoir, and a ventilation conduit which is connected to a ventilation opening in a top wall of the ink reservoir, said ventilation conduit containing a channel running upwards through a heated area. The ink jet device includes an ink melting unit for supplying melted ink to the ink reservoir, said ink melting unit being arranged to enable melted ink to flow into the ink reservoir from above, said channel of the ventilation conduit being in thermal contact with the ink melting unit.

Description:
This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 05109633.7 filed in Europe on Oct. 17, 2005, the entire contents of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to an ink jet device comprising an ink reservoir for hot melt ink, a heater arranged to heat ink contained in the ink reservoir, and a ventilation conduit which is connected at a lower end thereof to a ventilation opening in a top wall of the ink reservoir, said ventilation conduit comprising a channel, the maximum diameter of which is smaller than the length of the channel. For example, the channel consists of a tube. 
   Ink jet printers operating with hot melt ink, i.e., with ink that is solid at room temperature, comprise an ink reservoir which can be heated in order to maintain the ink in the liquid state in which it can be supplied to the printhead. To prevent ink from leaking through a printing nozzle of the printhead, it has been proposed to apply a suction to a space of the ink reservoir through a ventilation conduit. For example, a negative pressure of approximately 1 kPa (10 mbar) is maintained within the space of the ink reservoir. However, when the ventilation opening of the ink reservoir is splattered with ink, ink may enter the ventilation conduit and may solidify inside the ventilation conduit, thus clogging the ventilation conduit. Under these conditions, the negative pressure can no longer be maintained in the space of the ink reservoir. The ventilation conduit might also be clogged in a similar manner if the ink reservoir is overfilled with melted ink. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an ink jet device with an ink reservoir for melted hot melt ink which is provided with a more reliable ventilation system. 
   According to the present invention, this object is achieved by an ink jet device of the kind mentioned above wherein a channel runs upwards through a heated area. The channel is heated by the heated area to a temperature that is above a melting point of the ink. Thereby, any ink entering the ventilation conduit from the ink reservoir will be maintained in the melted state and will flow downwards back into the ink reservoir. Thus, clogging of the ventilation conduit is prevented. The smaller the diameter of the channel, the higher the ink might rise within the channel. Therefore, the invention is especially advantageous when the maximum diameter of the channel is smaller than half the length of the channel, and even more advantageous when the maximum diameter of the channel is smaller than one quarter of the length of the channel. 
   In one embodiment, the channel is, through a ventilation opening, permanently opened to the ink reservoir. In this case, no closure is provided at the ventilation opening which could prevent ink from entering the ventilation conduit. Therefore, the invention is particularly useful. However, the ventilation conduit may also have a non-return valve that closes in the case of a high enough pressure within the ink reservoir. 
   Preferably, the height of said channel at least corresponds to a capillary rise that melted ink filling the lower end of the ventilation conduit would experience. The capillary rise depends on the wetting properties of the channel walls and on the geometry of the channel. By adapting the height of the channel to the possible capillary rise, the optimum height of the channel in view of the necessary space and the desired clogging prevention effect is determined. 
   In the case where the ink jet device includes means for applying at least one of a negative pressure and a positive pressure through at least one of said ventilation conduit and a second conduit to a space of the ink reservoir, the height of said channel preferably at least corresponds to a rise that melted ink filling the lower end of the ventilation conduit would experience due to possible capillary forces and also said at least one of a negative pressure and a positive pressure. For example, in the case of the ink jet device comprising at least one printing nozzle and means for applying a negative pressure to a space of the ink reservoir through said ventilation conduit for preventing melted ink from leaking through the printing nozzle, the height of said channel preferably at least corresponds to the rise that melted ink filling the lower end of the ventilation conduit would experience due to the negative pressure and possible capillary forces. Therefore, although the negative pressure is applied by a suction through the ventilation conduit, melted ink filling the lower end of the ventilation conduit will not rise beyond the heated area of the channel. 
   In a preferred embodiment, at least a section of the ventilation conduit consists of a tube forming said channel and being open to the ventilation opening of the ink reservoir at the lower end of the ventilation conduit, said ventilation opening being arranged above a nominal fill level of the ink reservoir, the ventilation conduit extending substantially upright from said ventilation opening through said heated area. 
   According to a further development of the present invention, the ink jet device comprises an ink melting unit for supplying melted ink to the ink reservoir, said ink melting unit being arranged to enable melted ink to flow into the ink reservoir from above, said channel of the ventilation conduit being in thermal contact with the ink melting unit. For example, the ventilation conduit runs along or through the ink melting unit and is heated by the ink melting unit. Thus, the heat supplied by the ink melting unit is utilized for heating the channel of the ventilation conduit. This is advantageous, because the heated area through which the channel runs and the ink melting unit both have to be maintained at a temperature where the ink melts. Moreover, during a sleep mode of a printer, for example, it will be less probable that ink enters into the ventilation conduit, so that the ventilation conduit may be allowed to cool down together with the ink melt unit. 
   In one embodiment, the ink melting unit comprises a separate heater. That is, the heater is separate from the heater arranged to heat ink contained in the ink reservoir. Thus, the heating of the ink reservoir and of the ink melting unit is more efficient than it would be in the case of an integrated structure with only one heater for the ink reservoir and the ink melting unit. This is because there is more energy required to melt the ink in the ink melting unit than to maintain the melted ink in a melted state in the ink reservoir. 
   In another embodiment, the ventilation conduit is not guided through the melting unit but is separated therefrom such that no thermal contact exist between the ventilation conduit and the melting unit. In the latter case a separate heater may be provided to heat at least the lower part of the ventilation conduit which communicates with the ink reservoir. The lower part of the ventilation conduit is the part which may be exposed to intrusion of the melted ink from the ink reservoir. This intrusion may originate from uncontrolled movement of the melted ink and/or from overfilling of the ink reservoir and/or from suction, and/or from capillary forces. Separating the ventilation conduit from the melting unit creates a greater freedom in designing the print head. 
   In a preferred embodiment, a wall of the ink reservoir comprises an ink filter, the ink filter and the ink melting unit being arranged to enable melted ink to flow from the melting unit through the ink filter into the ink reservoir, the ventilation opening being arranged separately from the ink filter. By arranging the ventilation opening separately from the ink filter, the ink does not have to flow through the ventilation opening into the ink reservoir. Thus, a more reliable ventilation is achieved. 
   Preferably, a pressure sensor is arranged at the channel. The pressure sensor senses a pressure within the channel and can be utilized to monitor the negative pressure that is applied to the space of the ink reservoir. By arranging the pressure sensor at the channel, the sensor is at a position that is normally not filled with ink and, moreover, will be heated at least approximately to a determined temperature, thus enhancing the measuring accuracy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will now be described in conjunction with the following drawing, wherein 
       FIG. 1  shows a sectional view of an ink jet device with an ink reservoir and an ink melting unit. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The ink jet device comprises an ink reservoir  10  having walls  12  made of thermally conductive material. As is generally known in the art, an electric heater  14  is in contact with or integrated in the walls  12  of the ink reservoir so that hot melt ink  16  contained in the ink reservoir is maintained at a temperature of, for example, 120° C. and in any case at a temperature above its melting point, so that the ink is kept in a liquid state and is ready to be supplied to an ink jet printhead  18  which is arranged below the ink reservoir and which is in fluid connection with the ink reservoir  10  via a filter  20 . The ink jet printhead  18  comprises printing nozzles  22 , as generally known in the art. Some ink jets  24  are indicated as examples. As is also generally known in the art, the ink reservoir  10  and the printhead  18  may be mounted on a reciprocating carriage of a printer, so that the ink reservoir  10  is moved back and forth in the direction of the line of sight of the figure when the printer is operating. 
   A top wall  26  of a compartment of the ink reservoir  10  contains an ink filter  28 . A flat chamber  30  above the ink filter  28  is sealedly connected to a lower exit opening  32  of an ink melting unit  34  which is arranged above the ink reservoir  10 . Walls  36  of the ink melting unit  34  are made of a thermally conductive material and form a funnel. At the top of the ink melting unit  34 , there is arranged an inlet opening  40  for globular ink pellets  42  which consist of solidified hot melt ink. Ink pellets  42  are supplied into the interior of the ink melting unit  34  on demand. An electric heater  44  is in contact with or integrated in the walls  36  of the ink melting unit  34  so that an ink pellet  42  that is present in the ink melting unit  34  is melted and flows through the exit opening  32  and further through the ink filter  28  into the ink reservoir  10 . 
   The fill level of the ink reservoir  10  drops during printing and rises when a new ink pellet  42  is melted. Thus, the fill level of the ink reservoir  10  fluctuates around a nominal fill level which is below the ink filter  28  and thus below the wall  26 . 
   At a top wall  45  of the ink reservoir  10 , there is provided a ventilation opening  46  which is arranged separately from the ink filter  28 . The ventilation opening  46  connects a space  48  of the ink reservoir  10  above the melted ink  16  with a tube  50  forming a section of a ventilation conduit  52 . From the ventilation opening  46  at the lower end of the tube  50 , the tube  50  runs substantially upright through the ink melting unit  34 . In the area of the ink melting unit  34 , the tube  50  forms a channel  54  which is integrated in or is in contact with the walls  36  of the ink melting unit  34  over the whole height of the ink melting unit  34 . For example, the channel is integrated in a wall near the heater  44 . Thereby, the channel  54  is heated by the ink melting unit  34  to a temperature at which the ink is liquid. Alternatively, the channel  54  may run through the ink melting unit  34  at a distance from the walls  36 , thus being heated by the surrounding walls  36  and/or the melted ink. The maximum diameter of the channel  54  is, for example, smaller than 1/10 of the length of the channel  54 . 
   Through the ventilation conduit  52 , a suction is applied to the ink reservoir  10  by a suitable suction device  56 , thus maintaining a negative pressure of, for example, approximately 1 kPa (10 mbar) within the space  48  of the ink reservoir  10 . The suction device  56  may be implemented as known in the art. 
   If ink enters the ventilation conduit  52  at the ventilation opening  46  due to uncontrolled movement of the melted ink  16  or due to overfilling of ink reservoir  10 , this ink might be drawn up the tube  50  due to the suction, and, possibly also due to capillary forces depending on the wetting properties of the walls of the tube  50  and the geometry of the tube. A maximum rise level L may be, for example, approximately 90 mm above the printing nozzles  22 . In the described example, the channel  54  therefore extends to a height that is above the maximum rise level L. Thereby, ink entering into the ventilation conduit  52  will stay liquid, so that a clogging of the ventilation conduit  52  due to solidification of the ink is prevented. 
   In a modified embodiment, a pressure device  58  is connected to the space  48  of the ink reservoir  10  via a second conduit  60  and a second ventilation opening of the ink reservoir  10 . The second conduit  60  is configured similar to the ventilation conduit  52  with a tube forming a section of the ventilation conduit, said tube forming a second channel running through the ink melting unit  34  in parallel with the channel  54  and being in thermal contact with the ink melting unit  34 . In  FIG. 1 , the second channel lies behind the channel  54 . 
   The pressure device  58  is adapted to apply a pressure to the ink reservoir  10  for purging the printing nozzles  22 . While the pressure is applied, the ventilation conduit  52  is closed at the suction device  56 . However, ink entering the ventilation conduit  52  may be driven up the channel  54  due to the build-up of the pressure. Therefore, the height of the channel  54  is adapted to a maximum rise level L resulting from the pressure and possible capillary forces. The second channel of the second conduit  60  has the same height. Thus, ink entering into the ventilation conduit  52  or into the second conduit  60  will stay liquid, so that a clogging of the ventilation conduit  52  and the second conduit  60  due to solidification of the ink is prevented. 
   At the channel  54 , there is arranged a pressure sensor  62  for sensing the pressure within the channel  54 . The pressure sensor  62  is indicated with dashed lines. 
   In a modified embodiment only the pressure device  58  may be connected to the ink reservoir via a ventilation conduit as described above, while the suction device  56  may be connected to the ink reservoir  10  in a different manner as known in the art. For example, a non-return valve may be used. Alternatively, the pressure device may be connected via a non-return valve. 
   The embodiments as described above are only examples of an ink jet device according to the present invention and may be varied as is known in the art. For example, the heater  44  of the ink melting unit  34  may also be arranged at a position indicated by chain dotted lines at the right side of the ink melting unit  34  in the figure instead of being arranged at the wall near the channel  54 . Moreover, the pressure device  58  may be connected to the same ventilation conduit  52  as the suction device  56  or may be integrated in the suction device  56 . 
   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.