Abstract:
In a computer controlled, drop-by-drop, inkjet printer, either thermal ink-jet or piezoelectric, an apparatus for dampening the vibration caused by expelling the drops of ink. The apparatus includes an inlet and an outlet flow conduit connected to the chamber from which the drops are expelled and means for sweeping the vibration out of the chamber and into one of the flow conduits. In operation, the apparatus first expels a drop of liquid from the chamber and thereby creates a region of vibration in the liquid remaining in the chamber. The flow of liquid through the chamber flushes the region of vibration out of the chamber and into the outlet flow conduit, thereby hydraulically dampening the vibration.

Description:
FIELD OF INVENTION 
     The present invention generally relates to computer controlled printers that expel ink drop-by-drop to form images and, more particularly, to methods and apparatus for improving the operation of such printers. 
     BACKGROUND OF THE INVENTION 
     Computer controlled printers and in particular ink-jet and piezoelectric printers have been commercially available since at least the late 1980&#39;s. Their general construction is also well known, being the subject of numerous patents world-wide. An example of this technology can be found in U.S. Pat. No. 5,455,613 entitled “Thin Film Resistor Printhead Architecture for Thermal Ink-Jet Pens” by Canfield et al. issued on Oct. 3, 1995. 
     In a computer controlled printer, the ink is expelled drop-by-drop in a controlled manner. In a thermal ink-jet printer a firing resistor is electrically pulsed which in turn generates a drive bubble. The drive bubble expands in the firing chamber and expels a drop of ink from the chamber. In a piezoelectric printer a piezoelectric transducer is electrically pulsed which in turn expels a drop of ink from the chamber. In both, a region of vibration in the ink in the chamber is formed by the process of expelling the drop of ink. In addition, in both, the ink in the chamber bulges out of the orifice and a generally convex meniscus across the orifice results. The meniscus is not uniformly curved; the meniscus is actually oblate and also sloshes back and forth under the influence of the vibration of the ink in the chamber. The meniscus responds to a surface tension phenomenon. The ink in the chamber and the meniscus act much like a classical mass-spring-dashpot system. 
     Referring to FIG. 1, reference numeral  12  generally indicates a drop  14  of ink being expelled from an orifice plate  16  on the wall of a chamber  17 . Reference numeral  18  indicates the generally convex meniscus resulting after the expulsion of the drop. 
     Before expelling the next drop of ink, the chamber should be refilled. Refilling the chamber with ink as fast as possible is a very desirable design goal. However, if ink flows into the chamber too fast, the ink will flow out of the orifice and leak into the printer. On the other hand, refilling too slowly will cause the printer to operate unnecessarily slowly and the media throughput of the printer will be adversely affected. 
     In addition, before expelling the next drop from the chamber, both the vibration in the chamber must be damped out as much as possible and the meniscus flattened, or the trajectory of the next drop will be adversely affected. Specifically, if the next drop is prematurely expelled, the drop will not travel along its designed path and the quality of the resulting image will be degraded. 
     The effects of less than optimum damping and refilling are best shown in the graph, FIG. 2, which illustrates how the weight of the drops expelled from an ink-jet print head vary as the frequency of a firing resistor is changed. The geometry of the chamber and the chemical properties of the ink remain unchanged in FIGS. 2 and 3. The optimum firing frequency for the resistor is indicated by reference numeral  20 . The chamber overshoots and is not being damped sufficiently in the area indicated by reference numeral  21 . 
     Heretofore, to properly damp the vibration in the chamber and to achieve optimum refilling times, five hydraulic resistance variables have been optimized either through computer modeling or trial and error or both. The two parameters for ink are viscosity and surface tension, and the three geometric parameters of the print head are the length, width, and height of the ink inlet channel to the chamber. 
     FIG. 3 illustrates a fully damped, prior art chamber in which the problem of being under damped, i.e., overshooting, was eliminated. Reference numeral  22  indicates the optimum firing frequency for this chamber. Typically to achieve this prior damping solution, the length of the inlet channel to the chamber was lengthened and the width and the height of the channel were decreased. However, although overshooting was eliminated, the optimum firing frequency  22  was reduced as compared to the optimum firing frequency  20  in FIG.  2 . The net effect was that the printer ran slower and the output of media per minute was reduced. 
     It will be apparent from the foregoing that although there are well known ways of dampening the vibration in the ink in printers, there is still a need for an approach that allows the printer to operate as fast as possible while tolerating the maximum hydraulic under-damping that achieves acceptable print quality. 
     SUMMARY OF THE INVENTION 
     Briefly and in general terms, an apparatus according to the invention includes a means for expelling a liquid from a chamber drop-by-drop in a controlled manner, two flow conduits connected to the chamber, and means for sweeping the vibration, produced by the expulsion of a drop, out of the chamber and into one of the flow conduits. 
     In operation according to the invention, the apparatus expels a drop of liquid from the chamber and thereby creates a region of vibration in the liquid remaining in the chamber. The flow of liquid through the chamber sweeps the region of vibration out of the chamber, thereby hydraulically dampening the vibration. 
     The principal advantage of the invention is that by dampening the vibration in the chamber in the manner described, a printer can be operated at higher speeds and thus have a greater throughput of printed media, i.e., produce more printed pages per minute. 
     Further, the traditional mass-dashpot-spring damping system for the chamber is replaced by a new form of hydraulic compliance. Now instead of a “ringing” in the chamber that must be damped out and a convex meniscus forming at the orifice of the chamber that must be controlled, the flowing liquid entrains the vibration and its flow flushes the region of vibration out of the chamber via a second flow conduit. 
     The smaller hydraulic resistance of the chamber compared to the larger hydraulic resistances of the two flow conduits form a venturi that lowers the pressure in the chamber compared to the pressures in the two flow conduits. This lower pressure in the chamber decreases the curvature of the meniscus and lessens the likelihood of liquid flowing out of the orifice plate and into the printer. 
     The flow of liquid through the chamber results in several other benefits. The overall reliability of the firing resistor in a thermal inkjet print cartridge improves because after firing, the drive bubble is swept away from the resistor before collapsing and cavitation damage to the resistor is reduced. The flow also sweeps any entrapped air bubbles out of the chamber and out of the liquid flow path and onto other regions more suited to warehouse them without affecting the operation of the print head, thereby removing another source of drop trajectory instability. Also, the flow of ink through the print head carries off the heat generated in the print head by expelling drops. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammic perspective view of a drop of ink being expelled from a printer. 
     FIG. 2 is a graph of the weight of drops expelled from a undamped thermal ink-jet printer as the frequency of firing varies. 
     FIG. 3 is a graph of the weight of drops expelled from a vibration damped thermal ink-jet printer as the frequency of firing varies. 
     FIG. 4 is a side elevational view, in section and partially cut away, of a thermal ink-jet print head according to one embodiment of the present invention. 
     FIG. 5 is a top plan view, in section and partially cut away, of the thermal ink-jet print head of FIG. 4 taken along line  5 — 5  thereof. 
     FIG. 6 is a side elevational view, in section and partially cut away, of a thermal ink-jet print head according to a second embodiment of the present invention. 
     FIG. 7 is a side elevational view, in section and partially cut away, of a thermal ink-jet print head according to a third embodiment of the present invention. 
     FIG. 8 is a side elevational view, in section and partially cut away, of a thermal ink-jet print head according to a fourth embodiment of the present invention. 
     FIG. 9 is a side elevational view, in section and partially cut away, of a thermal ink-jet print head according to a fifth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the drawings for the purposes of illustration, the invention is embodied in a computer controlled printer that expels liquid ink drop-by-drop from a chamber. The printer described below as the preferred embodiment is a thermal ink-jet printer, but a piezoelectric printer is contemplated to be within the scope of the invention as well. 
     Referring to FIGS. 4 and 5, reference numeral  24  generally indicates a thermal ink-jet print head. The print head is mounded on a print cartridge body  26  that is made of injection molded plastic and is of conventional construction. Rigidly attached to the cartridge body  26  is an orifice plate  28  from which ink is expelled drop-by-drop through an orifice  56  in a controlled manner by the printer. A silicon substrate  32 , FIG. 4, and a barrier layer  30  are rigidly affixed to the orifice plate  28 . The orifice plate  28  and the print cartridge body  26  form a containment for ink  34  that flows into the print head  24  from an ink reservoir  35 . 
     Referring to FIG. 5, reference numeral  38  generally indicates a chamber  38  from which the drops of ink are expelled by the printer. The chamber is formed by the top surface, as illustrated in FIGS. 4 and 5, of the silicon substrate  32 , the side walls  40  of the barrier layer  30 , and the bottom surface of the orifice plate  28 , including the orifice  56 . The chamber has a hydraulic resistance to the flow of ink of R 3 . Located on the silicon substrate  32  and below the orifice  56  is a firing resistor  41 . When the firing resistor is electrically pulsed by the printer, a drive bubble (not shown) is formed in the chamber and the bubble expands, expelling the drop of ink from the print head. FIG. 1 generally illustrates the process in which a drop of ink  14  is expelled from an orifice plate  16 . The general construction and operation of a thermal ink-jet print head firing chamber is disclosed in detail in U.S. Pat. No. 5,455,613 cited above. 
     Referring to FIG. 5, hydraulically connected to the chamber  38  is an inlet flow conduit  44  or inlet channel for the ink  34 . The inlet channel is formed by the top surface, as illustrated in FIGS. 4 and 5, of the silicon substrate  32 , the inlet side walls  46  of the barrier layer  30 , and the bottom surface of the orifice plate  28 . The inlet channel has a hydraulic resistance to the flow of ink of R 1 . Likewise, hydraulically connected to the chamber  38  is an outlet flow conduit  48  or outlet channel for the ink  34 . The outlet channel is formed by the top surface, as illustrated in FIGS. 4 and 5, of the silicon substrate  32 , the outlet side walls  50  of the barrier layer  30 , and the bottom surface of the orifice plate  28 . The outlet channel has a hydraulic resistance to the flow of ink of R 2 . Both R 1  and R 2  are larger than R 3 , the hydraulic resistance of the chamber  38 . 
     Referring to FIG. 4, reference numerals  53 ,  53 ′ indicate two pumps for inducing the flow of ink through the inlet and outlet flow conduits  44 ,  48  and through the chamber  38 . Although FIG. 4 illustrates a centrifugal pump, any pump for inducing the flow of ink through the chamber  38  is contemplated including a peristaltic pump, a vane pump, a fan type pump and a positive displacement pump. In FIG. 4 the pumps  53 ,  53 ′ are opposed so that the flow of ink from each is initially directed outwardly within the print head  24 . As illustrated, the flow of pump  53  around the silicon substrate  32  is counter-clockwise, and the flow of pump  53 ′ is clockwise. 
     In operation, the two pumps  53 ,  53 ′, FIG. 4 run at steady state and the ink  34  continuously recirculates in the print head  24 . The ink flows upward, counter-clockwise from pump  53  and clockwise from pump  53 ′. Referring to FIG. 5, the ink  34  flows into the inlet channel  44 , through the chamber  38 , across the firing resistor  41 , thereafter into the outlet channel  48 , and down the feed slot  54  located between the two portions of the substrate  32 . The flow of ink through each chamber  38  and across each firing resistor  41  in FIGS. 4 and 5 is continuous and at steady state. 
     A hydraulic venturi is formed in the print head  24  because the hydraulic resistances R 1  and R 2  to the flow of ink in the inlet and outlet chambers  44 ,  48  are larger than the hydraulic resistance R 3  of the chamber  38 . 
     When the firing resistor  41 , FIGS. 4 and 5, is electrically pulsed by the printer, the resistor heats and generates a drive bubble that forces the drop of ink  14 , FIG. 1 out of the orifice  56  of the orifice plate  16 . The drive bubble thereafter collapses in the chamber  38 . This process of generating a drive bubble and having it subsequently collapse generates an area of vibration in the ink in the chamber. This area of vibration is swept across the resistor  41 , out of the chamber  38 , and into the outlet flow channel  48  by the flow of ink described above. In effect, the area of vibration is entrained by the ink and flushed out of the chamber by the flowing ink. The process of generating a drive bubble and expelling a drop of ink occurs quickly compared to the rate of flow of the ink across the firing resistor so that the trajectory of the drop is not affected by the flow of ink. 
     The net effect of the flow of ink through the chamber  38  is that the chamber does not “ring” as much, the vibration of the meniscus is reduced, the ink is hydraulically damped optimumly, and the drive bubble does not collapse on the firing resistor  41 . Most importantly, the flow of ink through the chamber  38  shortens the time spent for ink to refill the chamber and shortens the time between drop ejection. 
     As the drops  14 , FIG. 1 of ink are expelled from the orifice  56 , the ink in the print head  24  is replenished from the ink reservoir  35 , FIG.  4 . 
     The flow of ink across the silicon substrate and through the chambers can be in either direction. Referring to FIG. 6, reference numeral  59  generally indicates a print head with circulating ink flow that is opposite in direction to the ink flow illustrated in FIG.  4 . In particular, a single pump  61  is directed upward into the feed slot  54  so that the flow of ink around the portion  62  of the substrate  32  is clockwise as illustrated in FIG.  6  and counter-clockwise around the portion  63  of the substrate. The positions of the inlet flow channels  44  and the outlet flow channels  48  on the substrate are, of course, reversed from those illustrated in FIG. 4 due to the reversed direction of flow. In all other respects, the construction and operation of the print head  59  is the same as described and illustrated in connection with the print head  24 , FIG.  4 . In like manner the single pump  61  can be any of the types described above. 
     In general, the inlet flow channels  44 , FIGS. 4 and 6, and the outlet flow channels  48 , FIGS. 4 and 6, have approximately the same hydraulic resistance R 1  and R 2 , respectively. This feature allows the ink to flow in either direction through the firing chambers  38 , i.e., there is no preferred direction of flow across the firing resistors  41 . 
     Further, the hydraulic resistance in the entire system must be sufficiently low so that the pump(s) and the resulting pressure in the firing chambers  38  do not force ink out of the orifices  56  by overcoming the surface tension of the meniscuses  18 . 
     It should be appreciated that although the flow channels are illustrated and described above as being in-line, i.e., co-axial, they can be axially displaced with respect to each other as long as they have approximately the same hydraulic resistance. In like manner the number of inlet and outlet flow channels can be increased as long as each combination has approximately the same hydraulic resistance. 
     The ink can be flowed across the firing resistors and through the firing chambers in various modes of flow. Referring to FIG. 7, reference numeral  66  generally indicates a print head in which the ink flow is controlled by piezoelectric transducers, in particular transducers  68 ,  70 ,  71 , and  72 . These transducers are of conventional construction and act in addition to any transducers that expel the drops of ink from the chambers such as the transducers in a conventional piezoelectric driven, non-thermal, ink-jet printer. The transducers  68 ,  70 ,  71 , and  72  are electrically connected to a sequencer and driver circuit  74  of conventional construction. The transducers  68 ,  70  on the portion  76  of the silicon substrate  32  are driven in co-operation by the circuit  74  as are the transducers  71 ,  72  on the portion  77 . In FIG. 7 the flow of ink passes through a first ink conduit or channel  79  and a second ink channel  80  in different modes and in different directions as described below. In all other respects the construction and operation of the print head  66  is as described above. 
     In operation, the print head  66 , FIG. 7 flows the ink through the firing chambers  38  driven by the piezoelectric transducers  68 ,  70 ,  71 ,  72  which in turn are electrically actuated by the sequencer and driver circuit  74 . In one mode of operation the ink flows across the firing resistors  41  continuously in steady state as described in connection with FIGS. 4 and 6. In another mode of operation the ink flows through the chambers  38  in a varying manner. As examples of such variation, the ink can flow in sinusoidal manner, either solely in one direction or back and forth, i.e., first in one direction and then in the other. In another mode of varying the flow, the ink is pulsed through the chambers in various abrupt patterns by the transducers. The ink can also flow in and out of the chambers with full, partial, or no recirculation around the portions  76 ,  77  of the substrate  32 , i.e., clockwise and/or counter-clockwise flow. In all cases, however, the ink that is expelled from the print head is made up from the ink reservoir  35 . 
     In all of the various operating modes in which the speed and direction of ink flow changes, the rate of change of such changes is substantially less than the speed at which the print head is being pulsed and drops of ink are being expelled. In effect, the ink within the firing chamber at the time drops are expelled is flowing at a speed such that the region of vibration is flushed out of the chamber, but the changes in the speed and direction of the ink neither affect the process of expelling the ink drops nor affect the trajectory of the drops. 
     Although FIG. 7 illustrates a print head  66  with four transducers,  68 ,  70 ,  71 , and  72 , any number can be used to produce the desired flow and similarly these transducers can be placed anywhere in the flow path of the ink. 
     The print head is also serviced by the flow of ink passing through the firing chamber. Particles of matter, gummy ink, and bubbles of air that have temporarily become lodged in the firing chamber are entrained in the flow and are flushed out of the chamber and onto regions of the print head where they will not affect its operation. These obstructions can also be removed by reversing the flow, pulsing the flow, and otherwise varying the flow through the chamber. 
     The flow of ink through the firing chambers can also be varied in accordance with changes in the operating status of the printer within which the print head is functioning. Referring to FIG. 8, reference numeral  83  generally indicates a print head incorporating this feature. The flow of ink through the firing chamber  38  is produced by a pump  84  that varies either in speed or output or both. The operation of the pump is varied by a pump control circuit  86  of conventional construction. The pump control circuit receives signals from the printer  87  in which the print head  83  operates. These signals indicate the operating status of either the printer  87  or the print head  83  or both and include, but are not limited to, either the temperature of the print head, the rate at which drops of ink are being expelled from the print head, or the speed at which the printer is operating. In all other respects, the construction and operation of this print head is the same as the print heads illustrated in FIGS. 4,  6 , and  7  and described above. 
     The flow of ink through the firing chamber of a print head can be generated without the use of either electrical or mechanical energy. Referring to FIG. 9, reference numeral  90  generally indicates a print head with a flow of ink through its firing chambers  38  produced by natural circulation. Warmer ink, generally located in the upper regions of the print head, is transported in a conduit  92  to a heat exchanger  91  of conventional construction. The ink is cooled in the heat exchanger by conventional means. The cooled ink is transported back to the print head in a conduit  93  to a cooler region of the print head so that a flow of ink through the firing chambers is established and maintained by thermal convection. 
     Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangement of parts so described and illustrated. The invention is limited only by the claims.