Patent Publication Number: US-6336699-B1

Title: Self-cleaning wet wipe method and apparatus for cleaning orifices in an AIP type printhead

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to acoustic ink printing and, more particularly to, a method and apparatus which allows for cleaning and maintaining AIP printheads which implement unique orifice plates, and where a wetted wiping element is cleaned to allow for re-use of the wiping element. 
     It has been shown that acoustic ink printers which have printheads with emitters, including acoustically illuminated spherical or Fresnel focusing lenses can print precisely positioned picture elements (pixels) at resolutions that are sufficient for high quality printing of complex images. Significant effort has gone into developing acoustic ink printing, see for example, U.S. Pat. Nos. 4,308,547; 4,751,530; 4,697,195; 4,751,530; 4,751,534; 5,028,937; and 5,041,849, all of which are among many commonly assigned to the present assignee. 
     For performing acoustic printing, each of the emitters of the printhead launches a converging acoustic beam into a pool of ink, with the angular convergence of the beam being selected so that it comes to focus at or near the free surface (i.e., the liquid/air interface) of the pool. Moreover, controls are provided for modulating the radiation pressure which each beam exerts against the free surface of the ink. That permits the radiation pressure from each beam to make brief, controlled excursions to a sufficiently high pressure level to overcome the restraining force of surface tension, whereby individual droplets of ink are emitted from the free surface of the ink on command, with sufficient velocity to deposit them on a nearby recording medium. 
     A main attraction of acoustic ink printing is the ability to control droplet size based on the frequency of the signal provided, rather than providing on the size of the nozzle emitting the droplet. For example, an AIP printer may emit droplets magnitude in size smaller than the AIP openings. On the other hand, conventional ink jet printing requires a minimization of the nozzle itself to obtain small droplets. 
     While this is a benefit of AIP type printing, the size of the droplet ejectors used in acoustic ink printing are nevertheless very small. In consideration of this, maintaining the droplet ejectors in a clean state is an extremely important aspect of proper operation. Not only can dirt particles and dust (particularly paper dust) clog the ejector ports, but ejected ink droplets which do not adhere to the recording medium or have such low velocity that they return back to the orifice plate, and can build up enough to disrupt the printing process. Additionally, whereas many conventional ink jet printers require the replacement of the printheads after a somewhat short period of time, AIP printheads can have an indefinite life span. As part of extending this useful life, maintaining the printheads clean is an important aspect. 
     Existing examples of printhead cleaning are substantially directed to cleaning of printheads configured to use nozzles, whereas acoustic printheads are nozzleless. For nozzle type printheads, a wiper blade is a common device used for cleaning. 
     However, an ink jet printhead configuration is significantly different from the printhead of an acoustic ink printer. Therefore, attempting to use a wiper blade cleaning device or other cleaning method or apparatus designed for nozzle type printheads will not achieve desired results. For example, use of a wiper blade cleaning device with acoustic ink printheads may result in clogging of the printhead rather than accomplishing the desired cleaning. 
     It has also been suggested that a non-wiping technique for improving the cleanliness of exposed surfaces of droplet ejectors for a fixed printhead could be used. However, while such a technique has benefits, it is less desirable for moving printheads and also involves significant engineering considerations and is more specifically designed to a fixed printhead situation. 
     U.S. patent application Ser. No, 09/340,741 entitled METHOD AND APPARATUS FOR CLEANING/MAINTAINING OF AN AIP TYPE PRINTHEAD, filed Jun. 28, 1999 and assigned to the same assignee, describes an apparatus and method of cleaning AIP type printheads. However, the described device only allows a single use of a portion of a cleaning element, such that the cleaning element becomes exhausted. 
     It has been determined desirable to find a method and apparatus of cleaning/maintaining acoustic ink printheads which have unique orifice plate design in which the ink menisci are maintained at an entrance edge of the orifice plate, defined by a very thin lip. It is also desired that such a method and apparatus be able to clean a cleaning element of the apparatus such that the cleaning element may be re-used. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a method and apparatus for providing in combination or individually a flooding, dry and wet wiping of acoustic ink printheads for maintaining the cleanliness of the exposed surfaces of the printhead. A flooding procedure initially attempts to use the ink of the printhead in the cleaning process. Following the flooding operation ink on the outside surface of the orifice plate is removed by use of wiping over it with a compliant wiper blade. Next, ink inside the orifice bore is removed using a self-cleaning wet wiping station. The wiping station of the present invention consists of a wiping element designed with an absorbent, hydrophilic, compliant material, a washing fluid which wetted the wiping element, and a squeegee which removes excess fluid from the wiping element prior to the cleaning process. The washing fluid and squeegee being further used to clean the wiping element following cleaning of the printhead. 
     During a first step of the wiping station operation, the wetted wiping element is pushed over the orifices while the printhead and wiping element are moved in opposite directions. The dirty ink, because of a higher pressure inside the printhead, is unable to reenter the printhead and is absorbed by the wiping element. In a following step, pressure inside the printhead is decreased to enable the menisci to retreat inside the lip. When the orifice is again wiped with the wiping element, the remaining ink is removed from the bore of each orifice as well as from the exit surface of the submerged lip, due to absorption into the absorbent material. Subsequent to the second step, the wiping element is passed through wash fluid and squeegeed a number of times until the dirty ink is removed from the wiping element. This procedure cleans the wiping element so that on a subsequent cleaning of the printhead, the wiping element can be reused. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangement of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a representative illustration of an acoustic ink printing element to which the present invention may be applied; 
     FIG. 2 is an orifice plate which is maintained by operation of the present invention; 
     FIG. 3 depicts a capping element used as part of the apparatus and method of the present invention; 
     FIG. 4 illustrates a printhead array aligned with but not engaged with the capping element of FIG. 3; 
     FIG. 5 illustrates the capping element and printhead array in a sealed capped arrangement; 
     FIG. 6 illustrates a first embodiment of the AIP printhead wiping station according of the present invention; 
     FIG. 7 depicts an ink-jet printhead prior to cleaning; 
     FIG. 8 depicts a first step of the printing process according to the present invention; 
     FIG. 9 depicts a second step of the cleaning procedure of the present invention; 
     FIG. 10 depicts a second embodiment of an AIP printhead wiping station according to the teachings of the present invention; 
     FIGS. 11-13 depict interactions during operation between the squeegee roller, roller and cleaning fluid according to the embodiment of FIG. 6; and 
     FIGS. 14-17 illustrate the interactions between the squeegee, belt and cleaning fluid of the embodiment of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 provides a view of an exemplary acoustic ink printing ejector  10  to which the present invention is directed. Of course, other configurations may also have the present invention applied thereto. Additionally, while a single ejector is illustrated, an acoustic ink printhead will consist of a number of the ejectors arranged in an array configuration, and the present invention is intended to work with such an array. 
     As shown, ejector  10  includes a glass layer  12  having an electrode  14  disposed thereon. A piezoelectric layer  16 , preferably formed of zinc oxide, is positioned on the electrode layer  14  and an electrode  18  is disposed on the piezoelectric layer  16 . Electrode layer  14  and electrode  18  are connected through a surface wiring pattern representatively shown by lines  20  and  22  to a radio frequency (RF) power source  24  which generates power that is transferred to the electrodes  14  and  18 . On a side opposite the electrode layer  14 , a lens  26 , such as a concentric Fresnel lens or other appropriate lens, is formed. Spaced from the lens  26  is a liquid level control plate (also called orifice plate)  28 , having an orifice  30  formed therein. Ink  32  is retained between the orifice plate  28  and the glass layer  12 . The orifice  30  is aligned with the lens  26  to facilitate emission of a droplet  34  from ink surface  36 . Ink surface  36  is, of course, exposed by the orifice  30 . 
     The lens  26 , the electrode layer  14 , the piezoelectric layer  16  and the electrode  18  are formed in the glass layer  12  through photolithographic techniques. The orifice plate  28  is subsequently positioned to be spaced from the glass layer  12 . The ink  32  is fed into the space between the orifice plate  28  and the glass layer  12  from an ink supply (not shown but such supply is well known in the art). 
     Turning attention to FIG. 2, the orifice plate  28  shown is illustrated in more detail, wherein a submerged menisci  38  is maintained at an entrance edge of orifice plate  28  defined by a thin orifice lip  40 . During the course of printing with the submerged menisci  38 , the inside walls  42  of orifice bore  44  of each orifice  30 , as well as the exit surface  46  of the orifice lip  40  can get dirty. As previously noted, due to the unique configuration of the orifice plate  28 , existing wiper blade cleaning and other cleaning technology is not sufficient to ensure that an acoustic ink printhead will be sufficiently cleaned so as to assure operational reliability. 
     In seeking a manner of appropriately cleaning acoustic ink printheads such as those having an orifice plate  28  depicted in FIG. 2, applicants have enlisted the physical component of a capping structure such as that depicted in commonly assigned U.S. patent application Ser. No. 09/340,938, AA Method And Apparatus For Filling And Capping An Acoustic Ink Printhead (filed Jun. 28, 1999), hereby incorporated by reference. This application discloses a capping structure  50  for rapidly filling an acoustic ink jet printhead, such as shown in FIG.  3 . The capping structure  50  includes a plunger  52 , a base  54 , and springs  56  attached to a cap portion  58 . The cap  58  includes a gasket seal  60 , a valve  62 , a drain nozzle  64  and wiper blades  66 . During a filling operation, the gasket seal  60  is pressed against an orifice plate such as  28 , but having an array of orifices  30 . This traps a small volume of air around the orifices  30 . During the fill/refill when ink enters the printhead the trapped air-cushion prevents the ink from exiting the orifices. The ink preferentially fills the printhead and exits the outlet path with no ink being spilled outside of the orifice hole. More details regarding the functioning of the capping structure for the fill/refill operations are disclosed in the co-pending U.S. patent application Ser. No. 09/340,938. 
     In the present invention, capping station  50  is used in a first step of cleaning an acoustic ink printhead, such as comprised of a plurality of ejectors  10  previously described. As shown in FIG. 4, capping structure  50  is moved into alignment with printhead array  70 , having a plurality of orifices  30 , in a manner known within the art. Next, and as shown in FIG. 5, capping structure  50  is engaged with printhead  70  such as to form a seal. For the cleaning operation of the present invention, once the dirty printhead is capped, the ink pressure in the printhead is increased significantly to allow ink to escape through the orifices and completely fill a small space or gap  72  inside capped structure  50 . It is to be appreciated that increasing ink pressure within the printhead is a known technique and accomplishable by one of skill in the art and understanding of acoustic ink printing. 
     Once the pressure has been increased to move the ink through the orifice structures, the orifices may be allowed to soak for a predetermined time period in order to attempt to dissolve dried ink and loosen dust debris. After a predetermined time period, vent valve  62  is opened to drain the ink through drain nozzle  64  which had been forced by pressure out of the ink printhead. Once the ink has been pushed out through the orifices, the ink pressure inside the printhead is lowered to an intermediate higher level. This pressure prevents the ink still remaining inside the bore  44  of each orifice  30  (see FIG. 2) from reentering the printhead  70 . Following this operation, the outside surface of the orifice plate may be cleaned off by wiping with the wiper blade  66 . One embodiment of the wiper blade as a part of the cap chamber is disclosed in the aforementioned co-pending U.S. patent application Ser. No. 09/340,938. 
     Once the effort to clean the printhead  70  by flooding has been completed, additional cleaning is undertaken through the use of the wiping station  80  as shown in FIG.  6 . It is to be appreciated that wiping station  80  of FIG. 6 may be part of the capping station or may be located at a separate area of the acoustic ink printer mechanism. 
     AIP printhead wiping station  80  is designed to allow automatic self-cleaning to a cleaning element of the wiping station  80 . The cleaning element in the present embodiment is a highly absorptive, hydrophilic and compliant material such as foam or sponge configured as part of a roller assembly  82 . The sponge/foam roller  82  works in combination with washing fluid  84 , and squeegee  86  to efficiently clean acoustic ink printheads  70 . Drive gear  88  is representative of an entire gear system which acts to motivate roller  82  and squeegee  86 . However, for the sake of convenience specific gearing is not shown, although it would be obvious to one of ordinary skill in the art to provide such a gearing arrangement. While a single squeegee  86  is illustrated in this figure, it is to be appreciated that multiple squeegees may be used in accordance with the teachings of this invention. Further, in place of a roller design, squeegee  86  may be configured in the form of a squeegee blade or other known design which would appropriately remove excess fluid. 
     Turning more specifically to the function of wiping station  80 , attention is drawn to FIG. 7 which illustrates a printhead  70  with undesirable dried ink/debris  90  on its surface, whose existence may cause misdirectionality due to interference with the meniscus  36 . The meniscus  36  is shown to be held within orifice plate  28  of printhead  70 . As can be seen by FIG. 7, ejected ink droplets  92  do not emit from the center  94  of meniscus  36 , resulting in undesirable marking. Therefore, the cleaning of the present invention removes the dried ink/debris in order to improve the directionality of ink droplets  92 . 
     As previously discussed, the present invention may be used in conjunction or alone with the flooding operation of capping structure  50 . In either case, when activated, AIP wiping station  80  is moved into engagement with printhead  70 . Particularly, the AIP printhead wiping station  80  provides a two-step process to remove the dried ink/debris  90  shown in FIG.  7 . In the first step, the ink flow rate of the printhead which normally operates, in one embodiment at 150 ml a minute, is increased to a higher rate, for example in this embodiment 190 ml a minute. As shown in FIG. 8, this increased pressure acts to flatten the meniscus  36  pushing ink out of printhead  70 . The roller  82  is engaged over orifice  30 , while printhead  70  is moving in a first direction  100 , in this embodiment at a printhead wipe speed for high flow operation of 0.50 inches per second (ips). At the same time, roller  82  is moving in an opposite direction  102  at approximately a speed of 0.25 ips. The force with which the roller  82  is pressed against the orifice plate is approximately between 230 and 300 gmf. 
     The action of wiping station  82  is two-fold. The first function is to dissolve dried ink/debris  90  from the orifices as well as the front surface of the printhead  70 . The other function is to transport the dissolved ink and contaminants away from the orifices and the front face of the printhead  70 . This is achieved by a varying combination of wet wiping; ink flow rates in the printhead, and translating the printhead at an appropriate speed during the wet wipe cycle, in a direction opposite wiping station  80 . A unique aspect of the wet wipe scheme of the present invention is that the meniscus unlike other ink jet technologies is on the back side of the orifice plate  28  which requires the wet wipe to extend into the structure to remove contaminants and excess ink from the orifices. 
     During the process in FIG. 8, the highly absorptive, hydrophilic and compliant material, i.e. the foam or sponge in the form of a roller  82 , is pre-moistened in washing fluid container  84  of FIG.  6 . Roller  82  is then dragged and squeezed over the orifice structure of the printhead to clean and remove the dried ink and debris off the orifices. The foam or sponge containing the debris and dirty ink is then immersed in the wash fluid  84  to remove the contamination and is next squeezed/pinched between the squeegee  86  to remove excess fluid in preparation of readying foam/sponge roller  82  for further cleaning/contact with the orifice structure of printhead  70 . 
     Thus, in this first step, the foam/sponge roller  82  is cleaned by passing through washing fluid container  84  and then being squeezed or pinched by squeegee  86 , to remove excess washing fluid retained from the previous washing/cleaning zone. Increasing the pressure within printhead  70  causes the ink to come out of the printhead  70  and is absorbed by foam/sponge roller  82 . 
     Following this initial high cleaning action, the ink pressure within printhead  70  is decreased, as shown in FIG.  9 . In this step of the embodiment the low ink pressure is approximately 75 ml per minute which results in retracting the meniscus  36  within printhead  70 . During this second wiping step, a force is applied by roller  82 , e.g. 230-300 gmf range, and minimal amounts of ink will exit the printhead  70 . This step is useful in removing any left over ink as well as assists in drying of printhead  70 . 
     It is noted that during this second step, the movement between printhead  70  and roller  82  is maintained differentially  100 ,  102 . However, the printhead wipe speed in this low-flow situation is 0.1250 ips and the speed of roller  82  is maintained at 0.25 ips. 
     Once the second step has been completed, wiping station  80  is disengaged from printhead  70 . At this time, wiping station  80  may continue to rotate roller  82  through washing fluid container  84  and past engaged squeegee  86  for several additional rotations. The rotations are continued in order to ensure a complete cleaning of the roller  82 . As will be discussed in greater detail below, once roller  82  is satisfactorily cleaned, squeegee  86  may be disengaged from contact with roller  82 , and roller  82  may be removed from washing fluid container  84 . The disengagement and movement of parts may be accomplished by known mechanical configurations. 
     It is noted that for proper operation, it is desirable that pressure within the printhead  70  relative to the pressure applied by roller  82  is such that ink will move out of printhead  70  and washing fluid will not pass into printhead  70 . Specifically, it is desirable that washing fluid does not enter the printhead and thereby dilute the ink. It is to be noted that in a preferred embodiment the area of cleaning would be approximately 5 mm for a particular orifice and a complete orifice plate is anticipated at being approximately 32 mm in length. 
     In one embodiment, it would be desirable to ensure that the diameter of the roller is sufficient so that an area of the roller only cleans the surface of the printhead once during a specific cleaning operation. This design will ensure repeated washing of roller  82  prior to again being used to clean printhead  70 . This ensures that roller  82  will be clean each time it engages with a surface of the printhead  70 . 
     As an aspect of the present invention is to provide a compact cleaning device, it is desirable to minimize the size of the roller  82 . However, when roller  82  is made too small of a diameter, there may be insufficient distance between the washing fluid and squeegee  86  to remove a sufficient amount of fluid from the roller prior to engaging printhead  70 . In this instance, a further embodiment of operation includes moistening of roller  82  in washing fluid container  84 , and thereafter disengaging roller  82  and washing fluid container  84 . The next step rotates roller  82  through squeegee  86  a predetermined number of times in order to provide sufficient removal of liquid. Thereafter, the moistened but non-saturated roller  82  is moved into contact with the printhead  70  for cleaning. 
     Turning to FIG. 10, illustrated is a second embodiment of a wiping station  120 . This embodiment is substantially similar to the embodiment of wiping station  80  of FIG.  6 . However, herein roller  82  is replaced with a belt mechanism  122  wherein a belt made of a highly absorptive, hydrophilic and compliant material such as a foam or sponge  124  is arranged around rollers  126  and  128 . In this embodiment, foam/sponge belt  124  engages the washing fluid container  84  when in the area of roller  128 . The printhead cleaning operation described previously in connection with the wiping station  80  of FIG. 6 is equally applicable to that of the present shown embodiment of FIG.  10 . Further, belt  124  is also cleaned by the wiping station  120  by a procedure discussed in connection with the cleaning of roller  82  of FIG.  6 . 
     In use of either roller  82  or belt assembly  122 , with the speeds discussed in the previous embodiment, it is anticipated that one pass of the wiping station  80  across the surface of printhead  70  is from 2-5 seconds. It has also been determined by the inventors that it is desirable to clean a printhead  70  at least once a day in a printing system. Since there will be two passes for a cleaning process, the entire cleaning process would result in engagement of the roller  82  or belt  122  with printhead  70 , for approximately 4-10 seconds a day. 
     It is also noted that when selecting the proper operational parameters the highly absorptive hydrophilic and compliant material  82  or  124  needs to have a proper absorption rate. If material has too little absorbency it will not hold sufficient washing fluid and will not be able to pull sufficient ink out of the printhead for proper cleaning. On the other hand, an overly absorbent material will inhibit the thorough cleaning of the roller or belt for additional cleaning operations. While the absorption rates will vary dependent upon specific parameters, including ink flow and velocity between the printhead and the roller or belt, with regard to one embodiment, an appropriate absorption rate for an anticipated embodiment is within the range of 50-250 seconds hydrophilicity (also called wet-out, a standard commercial foam specification, measuring absorption time of a known volume of water), and more preferably between 100-110. 
     With attention to relative speed of the printhead and roller or belt during the cleaning process, it is noted that a slower speed improves the cleaning process, but also increases the amount of ink removed from the system. Therefore the present invention has applied optimal characteristics for desired cleaning with minimal ink loss. Using the parameters discussed above, a relatively small amount of ink is removed from the printhead during each printing process. During testing of the present invention, the inventors have found that less than ¼ml of ink is used during each cleaning process, i.e. including both a first and second pass. 
     Cleaning station  80  may be used to clean more than a single printhead  70 , and may also be employed to clean printheads of different colors. This capability exists due to the fact that the cleaning process ensures that cleaning fluid does not enter into a printhead  70  being cleaned. Since the washing fluid does not enter the printhead  70 , there is an assurance that the ink in the printhead will not be diluted with the cleaning fluid or other colored ink. Thus, as long as the printheads are using inks which are compatible when mixed together within the washing fluid, the present invention may be used to clean a variety of printheads including those employing different colors. 
     The inventors have also determined that a washing fluid may be used in the cleaning process which has up to 15% of its volume as ink. 
     Turning to FIGS. 11,  12  and  13 , shown in side view are roller  82  and squeegee  86 . Initially, roller  82  is depicted in an engaged position during printhead cleaning, wherein squeegee  86  removes excess moisture from roller  82 . Upon completion of the cleaning process, roller  82  moved out of engagement with squeegee  86 . This procedure exists so as not to maintain squeegee  86  and roller  82  in permanent contact. As previously noted, the amount of time where actual cleaning occurs within the lifetime of the wiping station  80  is minimal, i.e. 10 seconds a day. Therefore, disengagement between the squeegee  86  and roller  82  is desired so squeegee  86  does not place a permanent indentation in roller  82 . FIG. 12 illustrates a further embodiment of this concept, but wherein roller  82  is in a fixed position and squeegee  86  is moved out of engagement. FIG. 13 illustrates a concept wherein the washing fluid container  84  is moved out of engagement with the roller  82 . It is to be appreciated that with regard to FIG. 11, since roller  82  is movable the present invention may be designed to have the roller  82  removed from washing fluid container  84 . 
     With attention to FIGS. 14 and 15, similar concepts as previously discussed, but in connection with belt assembly  122  are illustrated. In FIG. 14, entire belt assembly  122  is moved out of engagement with squeegee  86  while squeegee  86  is fixed. In FIG. 15, first roller  126  is moved so as to take belt  124  out of engagement with squeegee  86 . FIG. 16 illustrates a concept where the belt assembly  122  is in a fixed position and it is squeegee roller  186  which is motivated into and out of engagement. Lastly, FIG. 17 shows an arrangement where washing fluid container  84  is moved out of engagement with belt assembly  122 . The movement of the above elements into and out of contact with each other may be accomplished using a variety of known gears and levers, such as but not limited to a knife lever. Also, while the description has discussed ink as the fluid being emitted and cleaned, it is understood that such printheads or emitters may be used in conjunction with other fluids, and the present invention may also be used with such fluids. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.