Abstract:
A method and device are provided in an acoustic ink printhead for masking temperature-induced artifacts by shifting the artifacts to a high spatial frequency beyond the visual acuity of humans. A cooling device is provided in the printhead for reducing the temperature of ink in a high speed acoustic ink printhead that ejects ink drops from an array of ink drop ejectors as the printhead moves along a longitudinal path. The cooling device includes first and second heat sinks formed on the printhead to develop first and second temperature gradients in the ink held within the printhead. The first and second temperature gradients are oppositely oriented along the face of the printhead so that the first and second sets of ink drops ejected onto a paper sheet adjacent the printhead produce rows of printed spots having a substantially uniform average spot size in a direction transverse the longitudinal path of the moving printhead. To that end, the cooling device masks the visual effects of thermally-induced ink volume by high frequency interlacing the ink volume variations with the printed pixel information. The printhead is cooled by at least one pair of counter flowing thermally conductive fluids, preferably at least a one of diethylene glycol, triethylene glycol, tetraethylene glycol, and glycerol.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to acoustic ink printing and, more particularly, to methods and means for improving the visual quality of images printed from a high-speed acoustic ink printhead by shifting temperature induced visual artifacts to a high spatial frequency beyond the visual acuity of humans. Portions of the ink contained within the printhead are selectively cooled using counter-flowing heat absorbing fluids and images are generated so that adjacent pixels on a page are produced from interlaced opposite transverse hot-to-cold and cold-to-hot ink well portions of the printhead. 
     2. Description of Related Art 
     In acoustic ink printing, an array of ejectors forming a printhead is covered by pools of liquid ink. Each ejector selectively directs a beam of sound energy against a free surface of the liquid ink. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. When the radiation pressure is sufficiently high, individual droplets of ink are ejected from the liquid surface to impact upon a target medium, such as a sheet of paper, to complete the printing process. 
     Typically, the ejectors are arranged in a linear array that is aligned perpendicular to the movement of the recording medium which receives the ejected ink droplets. Alternatively, the ejectors may be arranged in an array of rows and columns, with the rows stretching across the width of the recording medium and the columns of ejectors arranged approximately perpendicular along the movement of the printhead relative to the recording medium. Often, the columns of ejectors are not arranged exactly perpendicular to the ejector rows, but at oblique angles with the rows. In other words, the ejector rows of the array are staggered. 
     Each ejector for an acoustic ink printer must be supplied with ink and a good ink supply system should maintain a constant flow of ink to the ejectors. A flowing ink supply system cools the ink and stabilizes the ink temperature more easily. Additionally, the flowing ink supply system keeps the ink free of various contaminants, such as paper dust which might settle upon the free surfaces of the ink, by sweeping the contaminants away. The constantly flowing ink also maintains a fresh ink supply to the free surfaces. Without the constant flow of ink, the differing evaporation rates of the constituents within inks that contain volatile components may adversely affect the uniformity of the ink composition associated with each ejector and, therefore, would also affect the uniformity of performance of the ejectors. 
     Ideally, each ejector when activated ejects an ink droplet identical in size to the droplets of all the other ejectors in the array. Thus, each ejector should operate the same under ideal conditions. 
     As can be appreciated, the specialized inks used in acoustic printers are sensitive to temperature. As a general rule, ink drop volume increases with temperature, so temperature non-uniformities can lead to unintended variations in the ink density on the receiver medium. The effect of thermally-induced ink volume variations across the face of standard acoustic ink printheads such as may be caused by the constant flow of ink through the printhead is visible to the naked eye. This results in an overall poor quality image on the transfer medium. 
     As a general rule of thumb, the total drop diameter non-uniformity should be held to within a target value of 5%. That target value is an approximate upper limit to achieve sufficient uniform optical density. However, a temperature difference of 1.3° C. across only four centimeters of the acoustic ink printhead can account for as much as 1.6% of the total drop diameter non-uniformity. 
     The present invention solves or substantially mitigates the problem of total drop diameter non-uniformity due to heat generation and thermal effects that occur in acoustic ink printheads. 
     SUMMARY OF THE INVENTION 
     The present invention uses at least two counter-flowing heat absorbing fluid flows to selectively cool portions of the ink in acoustic printheads so that the visual effects caused by thermally-induced ink volume variations are shifted to a high spatial frequency on the receiver medium and are thereby masked to the human eye. 
     In accordance with one aspect of the invention, a device is provided in an acoustic ink printhead for interlacing temperature induced ink drop volume artifacts to a pixel level frequency above the visual acuity of humans, preferably above about 300 dots per inch. The device includes first and second heat sinks on the printhead adapted to develop, respectively a first temperature gradients in first and second ink drops ejected from the printhead. The first temperature gradient is preferably oriented in a first direction transverse to the longitudinal path of the moving printhead. The second temperature gradient is preferably oriented in a second direction opposite the first direction of the first gradient and transverse to the longitudinal path of printhead motion. The printhead draws ink for adjacent pixels marked on the recording medium from ink wells associated with the first and second gradients in an alternating fashion so that temperature induced ink volume variation artifacts are carried or shifted to the pixel level frequency, preferably above 300 dpi. Ink droplet delivery alternates in a spatial direction transverse to the longitudinal path L across the face of the printhead between oversized drops produced from ink adjacent a first one of the heat sinks and undersized drops produced from ink adjacent a second one of the heat sinks. 
     In accordance with one aspect of the present invention, therefore, an apparatus is provided for cooling ink in an acoustic printhead having a plurality of rows of ink ejectors arranged on the printhead for ejecting a plurality of rows of ink drops as the printhead translates adjacent an ink drop receiving medium in alternate linear first and second translation directions. The apparatus includes a first tank containing a volume of a first thermally conductive fluid and a second tank containing a volume of a second thermally conductive fluid. A first inlet and a first outlet port on the first tank enable a transverse flow of the first thermally conductive fluid through the printhead. Similarly, a second inlet port and a second outlet port on the second tank enable a transverse flow of the second thermally conductive fluid through the printhead. The first tank has a first surface adapted to conduct thermal energy from a first portion of the ink in the printhead and the second tank includes a second surface adapted to conduct thermal energy from a second portion of the ink in the printhead. The first inlet and outlet ports are arranged on the first tank to establish a flow of the first thermally conductive fluid in a first direction transverse of the translation direction of the printhead. The second inlet and outlet ports are arranged on the second tank to establish a second flow of the second thermally conductive fluid in a second direction opposite the first direction and transverse the translation direction of the printhead. 
     In accordance with still another aspect of the invention, the first and second tanks are adapted to establish substantially equal and opposite thermal gradients in the ink contained within the printhead. The first tank forms a first thermal gradient in the first direction transverse the translation direction of the printhead. The second tank forms a second thermal gradient in the second direction opposite the first direction and transverse the translation direction of the printhead. Preferably, the first and second thermal gradients have substantially identical characteristics. In that way, the visual effects on the receiver medium caused by ink drop size variation occurring across lead rows of the printhead are effectively cancelled or masked by the substantially equal and opposite thermal effects that are created in the trailing ejector rows. 
     In accordance with yet another aspect of the invention, the first and second thermally conductive fluids are a one or more of diethelyne glycol triethylene glycol, tetraethylene glycol, or glycerol. In another aspect, the first and second thermally conductive fluids are ink pools flowing within the printhead. 
     In accordance with yet another aspect of the invention, a rigid divider member is disposed on the printhead between the first and second tanks to form a wall therebetween for separating the first flow of the first thermally conductive fluid from the second flow of the second flow of the second thermally conductive fluid and to provide a mechanical rigid support member in the thermal printhead for stiffening the printhead in a direction transverse the translation direction of the printhead. 
     In accordance with yet another aspect of the invention, the first and second tanks in the printhead containing the first and second volumes of thermally conductive fluid effectively form first and second heat sinks in the printhead that are adapted to develop, respectively, first and second temperature gradients in the ink held within the printhead. The first heat sink is disposed on the printhead at a first location adjacent a first set of rows of ejectors among several linear series of ejectors to develop the first temperature gradient in a first set of ink drops ejected from the first row of ejectors. The second heat sink formed by the second flow of the second thermally conductive fluid is disposed on the printhead at a second location adjacent a second set of rows of ejectors to develop a second temperature gradient in a second set of ink drops ejected from the second row of ejectors. 
     In accordance with yet a further aspect of the invention, the first and second heat sinks are adapted to respectively generate the first and second gradients at respective first and second levels and within respective first and second ranges so that thermal growth differences between the first and second sets of ink drops when delivered onto the transfer medium are substantially mutually visually offset. Also, the total drop diameter non-uniformity is held to well within the target value of 5% across the printhead. 
     One benefit of the invention is that the offsetting thermal gradients created within the printhead effectively mask the visual effect of thermally-induced ink drop volume variations. 
     It is yet another benefit of the invention that the wall formed between the chambers holding the first and second thermally conductive fluids provides a mechanical stiffening to the printhead to improve the mechanical integrity thereof. The addition of the physical separator between ejector rows in the form described substantially reduces the deflection in the printhead to within approximately two microns. 
     Still yet other advantages and benefits of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take physical form in certain parts and arrangements of parts, the preferred embodiments of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof; and wherein: 
     FIG. 1 is an isometric view showing a high speed acoustic ink printhead using the method and means of the subject invention for masking the effects of thermally-induced ink volume variations; 
     FIG. 2 is a chart illustrating ink temperature and temperature induced ink drop volume versus ejector location along the face of the printhead in a direction transverse to the printing direction; 
     FIG. 3 is a simplified schematic illustration of the underside of an acoustic ink printhead showing the preferred ejector array configuration according to the subject invention; and, 
     FIG. 4 is an isometric view of an alternative embodiment of the invention illustrating a countercurrent monolayer cooling fluid flow wherein the ink flowing within the printhead is used as the cooling fluid. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein the showings are for the purposes of illustrating the preferred embodiments of the invention only and not for purposes of limiting same, FIGS. 1 and 3 show the overall arrangement wherein a high-speed acoustic ink printhead  10  is provided with a cooling apparatus  20  formed in accordance with the invention. The actual details of the printhead  10  form no particular part of the invention but are illustrative of a relatively conventional acoustic ink printhead structure. In general, the acoustic printhead ejects ink drops  12  from an ejector array  14  as the printhead  10  moves along a longitudinal path L. As noted above, the ejector array  14  includes a set of individual ejectors that are each adapted to selectively eject a single drop as the printhead moves to produce a single pixel at appropriate locations on the printed page. The ejectors are arranged in rows and columns, preferably eight (8) rows and one hundred twenty eight (128) columns, with the rows stretching across the width of the recording medium and the columns of ejectors arranged at slightly oblique angles with the rows so that the ejector rows are staggered. Cool ink  16  is supplied to the printhead  10  and unused heated ink  18  is returned to the ink supply. As can be seen from FIG. 1, the subject cooling apparatus  20  is formed integrally with the printhead  10 . 
     In general, as shown with reference to FIGS. 1 and 2, the cooling apparatus  20  in accordance with the invention includes a first heat sink  22  formed on the printhead. The first heat sink generates a first temperature gradient  24  in the ink held within the printhead. The cooling apparatus  20  further includes a second heat sink  28  formed on the printhead  10 . The second heat sink develops a second temperature gradient  30  in the ink within the printhead. As shown in the figures, the first temperature gradient  24  is oriented in a first direction A substantially transverse to the longitudinal path L traversed by the printhead  10 . Similarly, the second temperature gradient  30  is oriented in a second direction B opposite the first direction A and substantially transverse to the longitudinal path L. 
     Preferably, with reference to FIGS. 1 and 3, the first heat sink  22  is disposed in the printhead adjacent a first set of four (4) side-by-side rows of ejectors (rows  14   a - 14   d ) of the ejector array  14 . Similarly, the second heat sink  28  is disposed in the printhead adjacent a second set of four (4) side-by-side rows (rows  14   e - 14   h ) of ejectors of the ejector array  14 . In use, the first row  14   a  prints a series of pixels including the first, ninth, seventeenth, etc. pixels onto the first pixel row on the recording medium. The fifth row  14   e  prints the pixel series containing the second, tenth, eighteenth, etc. pixels onto the first pixel row on the recording medium. Likewise, the second, third, and fourth ejector rows  14   b ,  14   c ,  14   d  print a series of every eighth pixel beginning with the third, fifth, and seventh pixels, respectively, onto the first pixel row on the recording medium. Lastly, the sixth, seventh, and eighth rows  14   f ,  14   g ,  14   h  print a series of every eighth pixel beginning with the fourth, sixth, and eighth pixels, respectively, onto the first pixel row on the recording medium. A similar sequence pattern is repeated thereafter for each and every pixel row on the recording medium. 
     As shown best in FIG. 2, the first and second temperature gradients  24 ,  30  each induce corresponding first and second ink drop volume gradients  26 ,  32 , respectively. As also shown in that Figure, the first and second ink drop volume gradients  26 ,  32  are effectively mutually offsetting to substantially produce, in a large scale sense, a uniform average ink drop volume across the printhead. More particularly, in accordance with the present invention, the first and second heat sinks  22 ,  28  are adapted to respectively generate the first and second temperature gradients  24 ,  30  at respective first and second levels as shown and within respective first and second ranges so that thermal growth differences between the first and second sets of ink drops produced at the ejectors  14  substantially mutually offset  25  on a large scale average. Preferably, the first and second levels and the first and second ranges are selected so that the ink drops ejected onto a paper sheet adjacent the printhead  10  produce rows of printed spots having a substantially uniform average spot size in both the first and second directions A, B transverse the longitudinal path L of the printhead. 
     It is a primary advantage of the invention that adjacent pixels marked on the recording medium are drawn from ink wells associated with the heat sinks  22 ,  28  in an alternating fashion so that the temperature induced ink volume variation artifacts are carried or “shifted” to the pixel frequency, preferably 600 dpi. Ink droplet delivery alternates in a spatial direction transverse the longitudinal path L across the face of the printhead between oversized drops produced from ink adjacent a first one of the heat sinks  22 ,  28  and undersized drops produced from ink adjacent a second one of the heat sinks  22 ,  28 . It is a further major advantage of the invention that this spatial frequency is sufficiently high that the human eye cannot perceive the ink volume variation artifacts. Essentially, the human eye performs a low pass filter operation on the ink droplets deposited onto the recording medium. The phenomenon is described in U.S. Pat. No. 4,920,501 assigned to the assignee of the instant application and incorporated herein by reference. The counter-directed thermal gradients  24 , 30  in the printhead effectively cool the ink and produce a substantially uniform ink drop size  27  on gross average. At the pixel size level, the thermally induced ink volume variation artifacts are imperceptible to the human eye because of the high spatial frequency interlacing of the temperature induced large and small pixel dots. 
     With reference once again to FIG. 1, in their preferred form, first and second heat sinks  22 ,  28  generally comprise first and second fluid tanks  40 ,  42  that are each adapted to hold a thermally conductive fluid, preferably a one or more of diethelyne glycol, triethylene glycol, tetraethylene glycol or glycerol. The first tank  40  contains a volume of a first thermally conductive fluid  44  and has a first surface  46  that is adapted to conduct thermal energy from a first portion  48  of the ink to the first heat conductive fluid  44 . Similarly, the second tank  42  contains a volume of a second thermally conductive fluid  50  and includes a second surface  52  that is adapted to conduct thermal energy from a second portion  54  of the print ink into the second conductive fluid. In that way, the first and second tanks  40 ,  42  function as heat sinks to remove thermal energy from the ink contained within the printhead. Preferably, the thermal energy is removed from the ink in a manner as illustrated in FIG.  2 . 
     With continued particular reference to FIG. 1, the first tank  40  on the printhead  10  includes a first inlet port  60  and a first outlet port  62 . The first inlet port  60  adapts the first tank  40  to receive the first thermally conductive fluid  44  from an operatively associated source (not shown) of the first fluid. Likewise, the first outlet port  62  adapts the first tank  40  to deliver the first thermally conductive fluid  44  to an operatively associated sink (not shown) of the first fluid. Of particular significance to the subject invention, the first inlet and outlet ports  60 ,  62  are arranged on the first tank  40  to establish a flow  56  of the first thermally conductive fluid  44  in a first direction A transverse the translation direction of the printhead along the longitudinal path L. 
     Similarly, the second tank  42  includes second inlet and outlet ports  64 ,  66  each respectively adapting the second tank to receive and deliver the second thermally conductive fluid  50  from operatively associated source and sink of the fluid. Of particular significance to the invention, the second inlet and outlet ports  64 ,  66  are arranged on the second tank  42  to establish a flow  58  of the second thermally conductive fluid  50  in a second direction B opposite the first direction A and transverse the translation direction of the printhead  10  along the longitudinal path L. Preferably, the first and second thermally conductive fluids are diethylene glycol. 
     As shown best in FIG.  2  and described briefly above, the first and second tanks  40 ,  42  are adapted to establish a first thermal gradient  24  oriented in a first direction A (FIG. 1) and a second temperature gradient  30  oriented in a second direction B (FIG. 1) opposite the first direction. 
     With continued reference again to FIG. 1, a rigid mechanical divider member  70  is disposed on the printhead  10  for reducing deflection in the plurality of rows of ink ejectors  14 . Further, and in accordance with the subject invention, the divider member  70  forms a wall between the first and second tanks  40 ,  42  for separating the first flow  56  of the first thermally conductive fluids  44  from the second flow  58  of the second thermally conductive fluid  50 . In addition to the above, the divider member  70  further provides a significant benefit to the printhead in the present invention by reducing deflection and heat or stress induced warping that commonly occurs in high speed acoustic ink printheads. Basically, the divider member  70  increases the rigidity of the printhead and reduces deflection thereof due to thermal growth and other factors. 
     Turning lastly now to FIG. 4, a second preferred embodiment of the invention is illustrated including a countercurrent monolayer cooling fluid flow wherein the working printhead ink is used as the cooling fluid. More particularly, as shown in overall arrangement, a high-speed acoustic ink printhead  80  is provided with a cooling apparatus  100  formed in accordance with the second preferred embodiment of the invention. In general, the acoustic printhead ejects ink drops  12 ′ from an ejector array  14 ′ as the printhead  80  moves along a longitudinal path L. As noted above in connection with the first preferred embodiment of the invention, the ejector array  14 ′ includes a set of individual ejectors that are each adapted to selectively eject a single drop as the printhead moves to produce a single pixel at appropriate locations on the printed page. The ejectors are arranged in rows and columns, preferably eight (8) rows and one hundred twenty eight (128) columns, with the rows stretching across the width of the recording medium and the columns of ejectors being arranged at slightly oblique angles relative to the rows so that the ejector rows are staggered. A first supply of cool ink  102  is supplied to the printhead from an operatively associated ink source. Similarly, a second supply of cool ink  104  is supplied to the printhead  80  from another associated ink source. As can be appreciated from the figure, the first and second supplies of cool ink flow through the head in a countercurrent fashion to generate the opposed thermal gradients described above in connection with the first preferred embodiment. 
     In general, as shown with reference to FIGS. 2 and 4, the cooling apparatus  100  in accordance with the second embodiment of the invention includes a first heat sink  106  formed on the printhead by the first supply of cool ink  102 . The first heat sink generates a first temperature gradient  24  in the ink drops  12 ′ ejected from the printhead. The cooling apparatus  100  further includes a second heat sink  108  generated by the second supply of cool ink  104  and adapted to develop a second temperature gradient  30  in the ink droplets  12 ′ ejected from the printhead. As shown in the Figures, the first temperature gradient  24  is oriented in a first direction A substantially transverse to the longtudinal path L traversed by the printhead  80 . Similarly, the second temperature gradient  30  is oriented in a second direction B opposite the first direction A and substantially transverse to the longitudinal path L. 
     Preferably, with reference to FIGS. 3 and 4, the first heat sink  106  is arranged in the printhead adjacent a first set of four (4) side-by-side rows of ejectors (rows  14   a - 14   d ) of the ejector array  14 ′. Similarly, the second heat sink  108  is disposed in the printhead adjacent a second set of four (4) side-by-side rows (rows  14   e - 14   h ) of ejectors of the ejector array  14 ′. 
     In their preferred form, the first and second heat sinks  106 ,  108  generally comprise first and second fluid tanks  110 ,  112  that are each adapted to hold and suitably direct the first and second supplies of cool ink  102 ,  104 , respectively. Preferably, the first tank  110  contains a volume of ink that is used by a first set of ejectors  114  in the printhead. Similarly, the second set of ejectors  116  contain a volume of ink that is used by a second set of ejectors  116  in the printhead. The first and second supplies of cool ink  102 ,  104  flow in a manner substantially as shown through the printhead in a direction substantially transverse to the direction of travel L of the printhead. As shown best in FIG.  2  and described above, the first supply of cool ink  102  flowing through the first fluid tank  110  establishes a first thermal gradient  24  oriented in a first direction A (FIG. 4) while the second supply of cool ink  104  flowing through the second fluid tank  112  establishes a second temperature gradient  30  oriented in a second direction B (FIG. 4) opposite the first direction. 
     Lastly in connection with FIG. 4, a rigid mechanical divider member  120  is disposed in the printhead  80  for reducing deflection in the plurality of rows of ejectors  14 . Further, and in accordance with the second embodiment of the subject invention, the divider member  120  forms a wall between the first and second fluid tanks  110 ,  112  for separating the first and second supplies of cool ink  102 ,  104 , respectively. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding this specification. As an example, although only a single pair of counter-flowing cooling fluids are shown. Multiple pairs of counter-moving fluid flows can be used to generate multiple mutually offsetting thermal and ink volume gradients as well. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.