Abstract:
A liquid dispenser includes a liquid supply channel, a liquid supply adapted to feed a stream of liquid through the supply channel, a liquid return channel adapted to receive liquid from the supply channel, a liquid dispensing outlet opening, and a diverter member selectively movable into the supply channel to divert droplets to the dispensing outlet opening. The liquid flows from the liquid supply channel to the liquid return channel by Coanda effect when not diverted. The motion of the diverter member is substantially orthogonal to and opposes the direction of liquid flow, so that energy associated with moving the diverter member imparts no energy to the diverted droplets. The energy associated with moving the diverter member is less than 100 nJ per pL droplet volume. In some embodiments, the energy associated with moving the diverter member is less than 10 nJ per pL droplet volume.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of fluid dispensers and particularly, but not exclusively, to an on-demand dispenser of very small quantities of liquid. The invention is particularly useful in digitally controlled ink jet printing devices wherein droplets of ink are ejected from nozzles in a printhead toward a print medium. 
     BACKGROUND OF THE INVENTION 
     Traditionally, color ink jet printing is accomplished by one of two technologies, referred to as drop-on-demand and continuous stream printing. Both technologies require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce up to several million perceived color combinations. In drop-on-demand ink jet printing, such as shown in U.S. Pat. No. 6,065,825, ink droplets are generated for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print medium and strikes the print medium. The energy to propel such droplets from the ejector comes from the pressurization activator associated with that ejector. The formation of printed images is achieved by controlling the individual formation of ink droplets at each ejector as the medium is moved relative to the printhead. 
     Conventional drop-on-demand ink jet printers utilize a pressurization actuator to produce the ink jet droplet from the nozzles of a printhead. Typically, one of two types of actuators is used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink. This causes a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a pulse of mechanical movement stress in the material, thereby causing an ink droplet to be expelled by a pumping action. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate. 
     The volume of ink ejected by such nozzles is determined by the quantity of fluid ejected at each actuation of the drive mechanism, the velocity with which the fluid is ejected, and the rate of ejection. For a given geometry of the chamber, the pressure at which the fluid is supplied to the chamber and the operational characteristics of the drive mechanism determine all of those parameters. By increasing the supply pressure and the displacement of the drive mechanism in the forward stroke, either independently or as combined parameters, the ejection quality can be increased. However, if the supply pressure is to be increased substantially above the pressure at the outlet of the jet (which in printheads is generally atmospheric pressure), the fluid column cannot be contained in the chamber during the off periods of the dispenser i.e. during periods when no fluid is to be ejected from that particular jet. Fluid will therefore drip out of the jet during those periods. Hence, the most influential parameter in achieving high-quality drop-on-demand in these known dispensers is the maximum obtainable displacement of the drive mechanism, which is clearly limited. 
     The second technology, commonly referred to as continuous stream or continuous ink jet printing, uses a pressurized ink source for producing a continuous stream of ink droplets from each ejector. Typically, the pressurized ink is in fluidic contact with all the ejectors through a common manifold. The energy to propel droplets from the ejectors comes from the pressurization means pressurizing the manifold, which is typically a pump located remotely from the printhead. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink-capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism. 
     Other methods of continuous ink jet printing employ air flow in the vicinity of ink streams for various purposes. For example, U.S. Pat. No. 3,596,275 issued to Sweet in 1978 discloses the use of both collinear and perpendicular air flow to the droplet flow path to remove the effect of the wake turbulence on the path of succeeding droplets. This work was expanded upon in U.S. Pat. Nos. 3,972,051 to Lundquist et al., 4,097,872 to Giordano et al. and 4,297,712 to Lammers et al. in regards to the design of aspirators for use in droplet wake minimization. U.S. Pat. Nos. 4,106,032, to Miura and 4,728,969 to Le et al. employ a coaxial air flow to assist jetting from a drop-on-demand type head. 
     While this method does not rely on electrostatic means to affect the trajectory of droplets, it does rely on the precise control of the break off points of the filaments and the placement of the air flow intermediate to these break off points. Such a system is difficult to control and to manufacture. Furthermore, the physical separation or amount of discrimination between the two droplet paths is small further adding to the difficulty of control and manufacture. As such, these printheads suffer from a lack of precise control of the placement of drops on the print medium, which can produce visible image artifacts. 
     One problem associated with ink jet printers in general and such printers employing gas or air flows in particular is the drying of the ink. Ink drying in the vicinity of the printhead nozzles can lead to spurious droplet trajectories and nozzle clogging. In some cases, the evaporation of volatile ink solvents from the droplets as they fly through the air can increase the viscosity of the ink captured by the gutter, thereby causing difficulties during the ink recycling operation when the recycled ink is passed through a filter. This last problem becomes particularly difficult if the loss of solvent in the ink is large enough to cause the pigments in the ink to coagulate. Yet another problem associated with the guttering of inks is that the gutter is provided with a negative pressure, and is thereby subject to sucking wind, dirt, and frothy mist into the ink to be recycled. 
     European Patent Application No. EP-A-0436509 describes a fluid dispenser comprising a main chamber to which fluid is fed under pressure and a pair of outlet channels. A dispensing outlet channel leads to a dispensing outlet, whilst a recirculation outlet channel conducts the fluid back into the fluid supply. In use, the fluid normally veers towards the recirculation outlet channel leading back to the fluid supply. When a drop of fluid is to be dispensed, a driver device is momentarily energized so that the fluid flow switches over to the dispensing outlet channel. As soon as the required quantity of fluid has been dispensed, the flow is switched back to the recirculation channel by energization of a second driver device, so that the fluid again circulates back to the fluid supply. A disadvantage of the fluid dispenser is that two driver devices are required at each nozzle. Another disadvantage is that each nozzle requires a large footprint on the printhead to accommodate the pair of driver devices. 
     WO 95/10415 discloses a fluid dispenser comprising a supply channel; fluid supply means for feeding said main fluid to the supply channel under pressure; a first fluid path along which the main fluid is fed from the supply channel; a second fluid path including a fluid dispensing outlet; a control channel containing control fluid and having a control outlet adjacent the first fluid path, and means for changing pressure in said control fluid such that a wave front is formed in the main fluid and a droplet of said main fluid is dispensed from the fluid dispensing outlet. The main fluid flow follows the first fluid path due to Coanda effect except when diverted by change of pressure of the control fluid. While this fluid dispenser overcomes the need for two driver devices in European Patent Application No. EP-A-0436509, droplets to be dispensed are unsupported as they depart from the main fluid flow to exit the fluid dispensing outlet. As such, these printheads suffer from a lack of precise control of the placement of drops on the print medium, which can produce visible image artifacts. U.S. Pat. No. 4,345,259 is quite similar to WO 95/10415, and is cited here for the sake of completeness. 
     SUMMARY OF THE INVENTION 
     According to a feature of the present invention, a liquid dispenser includes a liquid supply channel, a liquid supply adapted to feed a stream of liquid through the supply channel, a liquid return channel adapted to receive liquid from the supply channel, a liquid return channel adapted to receive liquid from the supply channel, a liquid dispensing outlet opening, and a diverter member selectively movable into the supply channel to divert droplets to the dispensing outlet opening. 
     According to another feature of the present invention, the liquid flows from the liquid supply channel to the liquid return channel by Coanda effect when not diverted. 
     According to still another feature of the present invention, the motion of the diverter member is substantially orthogonal to and opposes the direction of liquid flow, so that energy associated with moving the diverter member imparts no energy to the diverted droplets. 
     According to yet another feature of the present invention, the energy associated with moving the diverter member is less than 100 nJ per pL droplet volume. In some embodiments of the present invention, the energy associated with moving the diverter member is less than 10 nJ per pL droplet volume. 
     According to yet another feature of the present invention, the amount and duration of motion of the diverter member is selectively adjustable to control diverted droplet volume. 
     According to yet another feature of the present invention, the liquid dispenser has a response frequency greater than 400 kHz. The diverter member may be a thermal bimorph transducer, a piezoelectric transducer, an electrostatic transducer, a magnetic transducer, or other suitable member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of a dispenser made in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a schematic plan view of the dispenser of  FIG. 1  in its “active” mode; 
         FIGS. 3-5  are detail views of a portion of the dispenser of  FIG. 1  showing three preferred embodiments of the present invention; 
         FIG. 6  is a schematic plan view of a dispenser made in accordance with another preferred embodiment of the present invention; 
         FIG. 7  is a schematic plan view of the dispenser of  FIG. 6  in its “active” mode; 
         FIGS. 8 and 9  are detail views of a portion of the printhead of  FIG. 1  showing two preferred embodiments of the present invention; 
         FIG. 10  is a schematic plan view of a dispenser made in accordance with still another preferred embodiment of the present invention; 
         FIG. 11  is a schematic plan view of the dispenser of  FIG. 9  in its “active” mode; 
         FIG. 12  is a detailed view of a portion of a dispenser made in accordance with yet another preferred embodiment of the present invention; 
         FIG. 13  is a schematic plan view of a dispenser made in accordance with still another preferred embodiment of the present invention; 
         FIG. 14  is a schematic plan view of the dispenser of  FIG. 13  in its “active” mode; 
         FIG. 15  is a schematic plan view of a dispenser made in accordance with still another preferred embodiment of the present invention; and 
         FIGS. 16-18  are detail views of a portion of the printhead of  FIG. 6  showing and alternative embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     With reference to  FIG. 1 , a dispenser  10  according to a preferred embodiment of the present invention is formed from a semiconductor material (silicon, etc.) using known semiconductor fabrication techniques (CMOS circuit fabrication techniques, micro-electro mechanical structure (MEMS) fabrication techniques, etc.). However, it is specifically contemplated and therefore within the scope of this disclosure that dispenser  10  may be formed from any materials using any fabrication techniques conventionally known in the art. 
     A supply channel  12 , which extends from a supply chamber  14 , carries a liquid pressurized by a pump  16  to be dispensed, on demand, from an outlet opening  18 . The liquid may be, for example, a printing ink. The liquid flows through ejector channel  17 ; and, when no drops are being ejected, flows entirely below outlet opening  18  at a velocity substantially equal to the velocity of the drops to be ejected from outlet opening  18  when fluid is being dispensed, as described below. The energy to sustain this flow is provided by pump  16  at all times. 
     A diverter member  20  is selectively movable from a passive position illustrated in  FIG. 1  to an active position as shown in  FIG. 2  by a controller  22 . When diverter member  20  is in its passive,  FIG. 1  position, liquid flowing through supply channel  12  is normally held by the Coanda effect in contact with a wall region  24 , so that it passes into a return channel  26 , along which it can be returned to the supply chamber. When controller  22  moves diverter member  20  to its  FIG. 2  active position, a portion of liquid flowing below outlet opening  18  flows along a ramp wall surface of the diverter member and emerges from the outlet opening due to the momentum of the liquid. Intermittent pulsing movement of diverter member  20  will shave-off liquid to deliver individual droplets  28  from the outlet opening  18 . 
     It will therefore be apparent that each time diverter member  20  is momentarily moved to its active position, a droplet of the liquid is dispensed from the opening  18 . The device can therefore be used in ink jet printing, and a number of the devices can be assembled side-by-side to form a printhead for dot matrix printing. This permits the dispensing of very closely spaced fluid droplets. 
     Specifically, the lag time between activation of diverter member  20  and separation of the liquid drop from diverter member  20  is very small, approximately equal to the ratio of the length of the diverter member divided by the velocity of the liquid in ejector channel  17 . Preferably, the diverter member is no longer than, say, ten microns and the fluid velocity is in the range of from five to thirty meters per second. Accordingly, the time between activation of diverter member  20  and separation of the liquid drop from the diverter member is less than two microseconds. This corresponds to a response frequency, which is defined as the inverse of the lag time, of greater than 400 kHz. The energy to propel such droplets derives from pump  16 , typically located remotely from the dispenser. Thus the dispenser and the printer so enabled are of the continuous inkjet type and the response time characterizing the lag between activation and drop ejection is very fast. 
     The dispenser may advantageously be micromachined from a block of material or fabricated by electroforming, electroplating, chemical etching or molding. Assembling separately-fabricated modules may alternatively form it. The dispenser may be used for depositing droplets for printing or for imaging applications, as well as other nonprinting applications where there is a requirement for dispensing precise volumes of fluids. 
     The dispenser of the present invention has a number of advantages over known devices. The velocity of emission of the droplet will directly depend on the supply pressure and not on control pressure, and the dispenser can thereby yield drop velocities in excess of twenty meters per second, which are much higher than those achievable with previous piezo-electric and thermal systems. The droplet size is controlled by the shape and position of the diverter member and the velocity of the liquid, and not by the dimensions of a nozzle. A dispenser in accordance with the invention may operate with a velocity and throw distance that exceeds those of previous devices. This enables deposits to be effected on surfaces which are further from the dispenser, which is required for industrial printing applications, such as printing on cans, boxes, containers, and the like. 
     The present invention provides a monostable fluid control device, which requires only a single ejector channel  17  without an associated control channel. Actuation can be effected by any means capable of imparting movement of the diverter member into the fluid stream and advantageously such means may be an actuator such as thermal bimorphs as illustrated in  FIG. 3  as  20   a , piezoelectric transducers as illustrated in  FIG. 4  as  20   b , or electrostatic or magnetic transducers as illustrated in  FIG. 5  as  20   c  with magnetic coil  21 . 
     The transducer may be located in the ejector channel or could be arranged outside it. For example, referring to  FIGS. 6 and 7 , the walls of ejector channel  17  include a flexible portion that forms a diverter member  30 . The diverter member may be deflected from a passive position illustrated in  FIG. 6  to its active position of  FIG. 7  by a piezoelectric transducer shown in  FIG. 8  or by a piezoelectric transducer  30   b  shown in  FIG. 9 . 
     As can be seen from  FIG. 6 , diverter member  30  moves mechanically in a direction substantially orthogonal to the fluid flow or moves in a direction opposing fluid flow. Thus, the energy to launch the drops does not come from the diverter member itself, but comes instead from flow energy supplied by pump  16 . This contrasts with the source of energy imparted to drops disclosed by the pressure increase mechanism of WO 95/10415 and U.S. Pat. No. 4,345,259 wherein energy is imparted to the ejected drops, as can be appreciated by one skilled in fluid mechanics. Thus the energy needed to activate the diverter member according to the present invention can be very small relative to the energy used by the afore mentioned prior art devices. In particular, for thermal bimorphs, the calculated energy to move the tip of the bimorph from its own equilibrium position to a position ten microns into the channel of  FIG. 2  is typically less than 100 nJ for a motion that releases drops of at least one pL volume. Thus, the ejection energy required per pL volume, a common measure of ejector efficiency, is typically less than 100 nJ/pL. Piezo actuators can be more efficient than thermal actuators because they require no energy input to hold their actuated positions, as is well known in the art of inkjet ejectors, and thus the ejection energy required per pL volume for piezo actuators, such as those of  FIG. 4 , is calculated to be less than 10 nJ/pL. These energies are additionally low in cases for which the actuators remain in their actuated position for a substantial time. 
     Referring to  FIGS. 10 and 11 , the walls of ejector channel  17  include a flexible portion that forms a diverter member  32 . Diverter member  32  is similar to diverter member  30  of  FIGS. 6 and 7 , except that it is located on the inner wall of ejector channel  17  rather than on its outer wall. Diverter member  32  may be deflected from a passive position illustrated in  FIG. 10  to its active position of  FIG. 11  by a thermal bimorph, piezoelectric, electrostatic or magnetic transducer. 
     In  FIG. 12 , the wall of ejector channel  17  to which diverter member  32  is attached has been formed with a tapered edge as illustrated to enhance the ejection of droplets  28 . 
     Referring to  FIGS. 13 and 14 , the walls of ejector channel  17  include a flexible portion that forms a diverter member  34 . Diverter member  34  is similar to diverter member  32  of  FIGS. 10 and 11 , except that it is located on the lower inner wall of ejector channel  17  rather than on the its upper inner wall. Diverter member  34  may be deflected from a passive position illustrated in  FIG. 13  to its active position of  FIG. 14  by a thermal bimorph, piezoelectric, electrostatic or magnetic transducer. 
     In  FIG. 15 , a dispenser is shown using two diverter members  36  and  38  simultaneously. Both diverter members are actuated to move into ejector channel  17 , thereby producing a height difference in the liquid flowing in the channel resulting in ejection of drops  28 . The drops thus ejected are larger than drops that would have been ejected from either diverter member alone, as each diverter member increases the liquid height difference. As can be appreciated by one knowledgeable in the art of inkjet ejectors, the timing of activation of the two diverter members can be adjusted slightly to improve drop formation and control, so that the two diverter members are actuated at approximately, but not exactly, equal times. It will also be appreciated that diverter members  36  and  38  can be independently operated without the other to provide a degree of gray scale capability for the printhead. 
       FIGS. 16-18  are detail views of a portion of the printhead of  FIG. 6  showing and alternative embodiment wherein a degree of gray scale can be attained by adjusting the amount and duration of motion of diverter member  30 . In  FIG. 16 , a small drop is produced by restricted motion and duration of deflection of the diverter member. In  FIG. 17 , a large drop is produced by increased motion of the diverter member for a shorter duration. In  FIG. 18 , a mid-sized drop is attained by restricted motion and longer duration of deflection of the diverter member. 
     While the foregoing description includes many details and specificities, it is to be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. 
     PARTS LIST 
     
         
           10 . dispenser 
           12 . supply channel 
           14 . supply chamber 
           16 . pump 
           17 . ejector channel 
           18 . outlet opening 
           20 . diverter member 
           20   a . thermal bimorph dispenser 
           20   b . piezoelectric transducer dispenser 
           20   c . electrostatic or magnetic transducer 
           21 . magnetic coil 
           22 . controller 
           24 . wall region 
           26 . return channel 
           28 . droplets 
           30 . diverter member 
           32 . diverter member 
           34 . diverter member 
           36 . diverter member 
           38 . diverter member