Patent Publication Number: US-6336700-B1

Title: Method and apparatus for recovering an ink discharging condition of an ink jet recording apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to a method and apparatus for recovering an ink discharging condition in an ink jet recording apparatus. In particular, this invention relates to removal of unwanted air bubbles within a die reservoir and/or an ink jet channel, and also relates to maintaining the temperature of the ink jet recording apparatus within acceptable limits necessary to perform an ink jet printing or discharging operation. 
     2. Description of Related Art 
     U.S. Pat. No. 5,479,196 discloses an ink jet recording apparatus for removing air bubbles from an inkjet channel. As described in conjunction with FIGS. 4A and 4B, undesirable air bubbles are removed by utilizing film boiling or the precursor to film boiling to generate vapor bubbles which then coalesce with the offending bubble. This procedure has the disadvantage of requiring an active heating element within the die reservoir to remove air bubbles therein, which is impractical if not impossible, and in practice, it is difficult to create a series of small bubbles that coalesce. In addition, U.S. Pat. No. 5,479,196 does not teach the use of a means for regulating the temperature of the ink jet recording apparatus within acceptable limits, especially following the removal of undesirable air bubbles. 
     SUMMARY OF THE INVENTION 
     Accordingly, one aspect of the present invention is to avoid the disadvantages and shortcomings of the related art. Another object of the invention is to provide a method and apparatus in which undesirable air bubbles are removed by heating the ink in the die reservoir to increase vapor pressure of the ink to cause mass transfer of water vapor molecules across an interface between an air bubble and the ink. Another aspect of the invention relates to removal of air bubbles by enlarging them until the bubbles fill the entire die reservoir space. Subsequently, a priming vacuum applied to the channels for a short duration can be used to effectively remove the void along with air molecules within it. 
     Another aspect of the present invention relates to maintaining the ink jet recording apparatus within an acceptable temperature range or below a certain temperature so as to improve performance and/or reduce down time. Temperature can be maintained, for example, by providing a heat sink to the die reservoir. The heat sink may be integrally formed with the die reservoir, formed as part of the maintenance station, or formed as an integral part of the carriage which traverses a recording medium during a printing operation. 
     According to a first embodiment of the present invention, there is provided a method for recovering an ink discharging condition in an ink jet recording apparatus having a die reservoir and at least one ink channel in communication with the die reservoir. The method comprises heating ink in the die reservoir to increase vapor pressure of the ink to cause mass transfer of water vapor molecules across an interface between an air bubble and the ink, and removing air bubbles entrapped within the die reservoir and the ink channel. 
     Another aspect of the invention relates to an ink jet recording apparatus comprising a die reservoir containing ink, at least one ink channel in communication with the die reservoir and a heating element operable to heat the ink in a recording mode in which a heat-induced ink jet bubble is created to jet the ink onto a recording medium, and to heat the ink in an ink discharging condition recovery mode in which vapor pressure of the ink is increased to cause mass transfer of water vapor molecules across the interface between the ink and an air bubble remaining after the recording mode. 
     According to yet another preferred embodiment of the present invention, an ink jet recording apparatus comprises a die reservoir containing ink, at least one ink channel in communication with the die reservoir, a heating element operable to heat the ink in a recording mode and an ink discharging condition recovery mode, and a heat sink provided to the die reservoir to cool the die reservoir below a predetermined threshold temperature. In other preferred embodiments, the heat sink may be fixedly attached to the carriage that supports the die reservoir in the ink channel, the heat sink may be fixedly attached to a maintenance portion of the ink jet recording apparatus, and/or the heat sink may be integrally formed with the die reservoir and the ink channel. 
     These and other aspects and embodiments of the present invention will be described with reference to the following detailed description of preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will be described in more detail with reference to the following drawings, wherein: 
     FIG. 1 is a schematic drawing illustrating the general architecture of an ink jet printing apparatus according one preferred embodiment of the present invention; 
     FIG. 2 is block diagram illustrating an ink jet print head according to one preferred embodiment of the present invention; 
     FIGS. 3A-3D illustrate a sequential process of enlarging air bubbles and applying a pressure differential across the ink channel to promote a substantially bubble free ink flow; 
     FIG. 4 is a chart illustrating the calculated final system temperature after heating the die and the substrate and then bringing the heat sink into contact with the print element; 
     FIG. 5 illustrates a second embodiment of the present invention wherein the heat sink is provided in or near a maintenance station of an ink jet recording apparatus; and 
     FIG. 6 illustrates a third embodiment of the present invention wherein the heat sink is connected directly to the carriage. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a first preferred embodiment of the present invention. A recording head  100  is carried by a carriage  109  that is slidably mounted on one or more carriage rails  113 . The recording head  100  is supplied with ink from an ink tank  111  through an ink supply tube (not shown) and discharges droplets of the ink in a predetermined timing in accordance with recording data signals. The discharged ink droplets are projected towards and received on a recording medium  112  that is being conveyed by a conveyor (not shown), whereby a desired image is formed by the ink droplets of the recording medium  112  by virtue of relative movement between the recording head  100  and the recording medium  112 . 
     A discharge recovery device  110  or maintenance station  110  for recovering the safe discharging condition of the recording head  100  from discharge failure is provided, for example, in the vicinity of a home position of the recording head  100 . The recovery device  110  may have, for example, a cap  340  (FIG. 5) capable of covering the surface of the recording head where a plurality of discharge openings open, and a pump for establishing a vacuum in the space closed by the cap  340  so as to suck the ink from the discharge openings. The cap  340  also serves to protect the discharge openings from drying and deposition of contaminants when the recording head  100  is not operating. 
     FIG. 2 is a simple block diagram showing one example of the recording head  100  according to one preferred embodiment of the invention. The recording head  100  includes a typical print manifold  114 , a substrate  116  and a larger thermal mass or heat sink  118 . The print manifold  114  is supplied with ink from the ink tank  111  of FIG.  1  and is positioned to deliver the ink to a die  200  (FIGS.  3 A- 3 D), e.g., a silicon die. The die  200  includes a die reservoir  204  and at least one ink channel  201  (shown in FIGS. 3A-3D) that are connected to the bottom of the recording head  100  so as to jet ink in the direction of arrow A. 
     The die  200  is mounted on the substrate  116 . The substrate  116  is made, for example, of a thermally conductive material such as copper and/or zinc. The substrate  116  is of low enough thermal mass to allow the temperature of the die to reach a predetermined elevated temperature, which is typically less than the boiling point of the ink, within a predetermined amount of time. The heat sink  118  is provided to rapidly cool the die and substrate  116 . The heat sink  118  can be made from metal, for example. 
     During operation, the recording head  100  is operable in a recording mode and an ink discharging condition recovery or maintenance mode. In the recording mode, a heat induced ink jet bubble is created to jet the ink onto a recording medium, as is conventionally known in the art. For example, the ink may be superheated in order to produce ink jet bubbles that can be jetted onto the recording medium  112 . 
     In the ink discharging condition recovery or maintenance mode, the recording head  100  is preferably moved to the discharge recovery device  110  (FIG. 1) when the recording head  100  moves to the home position. In this position, the process as shown in FIGS. 3A-3D can be carried out. In particular, FIG. 3A shows the die reservoir  204  and a plurality of ink channels  201  in communication with the die reservoir  204 . Each ink jet channel  201  includes a heating element  203  that heats the ink in both the ink jet recording mode and the ink discharging condition recovery mode. 
     Following an ink jet operation, undesirable air bubbles  317  can form within the ink channels  201 . These undesirable air bubbles can cause improper ink jet conditions, as is known in the art. In order to remove these air bubbles, the heating elements  203  heat the ink in the die reservoir  204  and/or the ink jet channels  201  to increase vapor pressure of the ink to cause mass transfer of water vapor molecules across an interface between an air bubble  317  and the ink. This causes the bubble bubbles  317  to enlarge until the enlarged bubble or bubbles  317  fill the entire space of the ink jet channels  201  and/or the die reservoir  204 , as shown in FIG.  3 B. Enlargement of the existing undesirable air bubbles  317  can be accomplished by heating the ink to a temperature range of between about 85° C. and the boiling point of the ink, preferably 5-10° C. below the boiling point of the ink, for a duration of time between about  10  seconds and about  30  seconds. 
     After the air bubbles enlarge to fill the entire volume of the ink jet channels  201  and/or the die reservoir  204 , a conventional priming operation is applied to create a pressure differential across the ink jet channels  201  and the die reservoir  204 , as shown in FIG.  3 C. After the priming operation is performed, the inkjet channels  201  and the die reservoir  204  are filled with substantially bubble-free ink, as shown in FIG.  3 D. 
     To enhance the ink discharging condition recovery process, it may be possible to decrease the surface tension of the ink by heating it by about 10° C. to help remove air bubbles from the die reservoir  204  and the ink channels  201 . The decrease in surface tension causes the bubble size to increase, and the viscosity of the ink channels  201  decreases significantly which allows for larger priming flows, which in turn helps sweep the bubble forward in the die reservoir  204  causing it to be deformed into the back of the channels  201  and subsequently removed. In addition, it is noted that the vapor pressure of the ink is less than atmospheric pressure during the process. 
     The pressure differential across the die reservoir  204  and the ink channels  201  to induce flow of substantial bubble-free ink into the at least one channel can be applied for a duration of at least about 200 ms, and could last up to about two seconds. The pressure differential is approximately −350 mm Hg±100 mm Hg. However, the priming operation can be carried out using any appropriate pressure differential. 
     FIG. 4 shows a graph entitled “Final temperature of total system (die, substrate, heat sink) as a function of heat sink initial temperature and thermal mass contained in the die and substrate (die and substrate at 110° C.); thermal mass of typical Xerox print head equals 2.35 cal/K).” For example, if the die  200  and the substrate  116  account for 10% of the total die/substrate/heat sink system and the die  200  and substrate  116  are heated to 110° C., when the die and the substrate  116  are attached to the heat sink  118 , which is cooler, the temperature of the die and substrate  116  will decrease significantly while the heat sink  118  warms slightly. For example, if the heat sink  118  temperature is 20° C. before recombining with the hot die and substrate  116 , the final equilibrium temperature would be 29° C., as shown in FIG.  4 . Of course, if the heat sink  118  is initially hotter due to printing, for example 50° C., then the right side of the graph shows that the final temperature is at around 55° C., which is also shown in FIG.  4 . However, if the fraction of thermal mass represented by the die and the substrate  116  is higher, the final temperature upon attachment with the heat sink  118  will be higher. This is represented by the shift of temperatures to the left as one moves vertically on the graph. 
     In the ink discharging condition recovery mode, one or more heating elements  203  operates in order to raise the temperature of the ink to a predetermined threshold temperature. The predetermined threshold temperature of the entire system should be between about 60° C. and 65° C. in order to allow an ink jet recording operation to be carried out properly. For example, the threshold temperature for a unit having 128 jets and a 300 dpi pitch can be reached within 10-30 seconds by providing 10 watts of power, which will heat the ink but does not jet it. 
     FIG. 2 shows an embodiment in which the heat sink  118  is provided as an integral part of the die reservoir  204  and the substrate  116 . However, the heat sink may also be formed as part of the printing apparatus, which decreases the unit manufacturing cost because only one heat sink  118  will be provided for each printing apparatus, rather than providing a heat sink  118  for each ink jet cartridge which is sold as replacement cartridge. For example, FIG. 5 shows an embodiment in which a heat sink  118 ′ is provided in the vicinity of the maintenance station or a discharge recovery device  110 , for example, near the priming cap  340  of the maintenance station  110  (FIG. 1) such that the ink jet recording head  120  can be cooled during the maintenance process. This embodiment has the additional advantage that the motor for driving the carriage  109  can be made smaller because the weight of the heat sink  118 ′ is not added to the load imposed upon the motor. FIG. 5 also shows that the heat sink  118 ′ may include a very thin, compliant, thermally conductive layer  121 . 
     In another embodiment shown in FIG. 6, a heat sink  118 ″ is provided directly on the carriage  109  such that cooling of the ink jet print head can be accomplished not only during the maintenance operation, but during the printing operation. In FIG. 6, the heat sink  118 ″ can be coupled to the substrate  116 , using, for example, magnets. In both the embodiments of FIGS. 5 and 6, it is important that the interface between the heat sink  118  and the portion of the ink jet head  100  being cooled have a proper interface such that proper thermal transfer can occur. 
     While the embodiments disclosed herein are preferred, it will be appreciated from these teachings that various alternatives, modifications, variations or improvements therein may be made by those skilled in the art, which are within the spirit and scope of the present disclosure.