METHOD OF OPERATING AN INK JET

Abstract: A method of operating a demand or impulse ink jet comprising an ink jet ink chamber and an ink drop ejection orifice includes the steps of initiating filling by decreasing the pressure within the chamber; retracting the ink meniscus at the orifice to a predetermined position as the pressure is decreased; initiating firing of a first ink droplet by increasing the pressure within the chamber when the meniscus is retracted to said predetermined position; moving the menis­ cus forward through the orifice while the pressure is increased so as first to form and then to project an ink droplet outwardly from the orifice; and repeating the foregoing steps for each of a series of ink droplets.

Description: This invention relates to ink jets, and more particularly, to ink jets of the demand type or impulse type. Ink jets of the demand type include a transducer which is coupled to a chamber adapted to be supplied with ink. The chamber includes an orifice for ejecting droplets of ink when the transducer has been driven or pulsed by an appropriate drive voltage. The pulsing of the ink jet abruptly reduces the volume of the jet so as to advance the meniscus away from the chamber and form a droplet of ink from that meniscus which is ejected from the ink jet. Demand ink jets typically operate by reducing or contracting the volume of the chambers in the rest state to a lesser volume in the active state when a droplet is fired. This contraction in the active state is followed by an expansion of the volume when the jet is returned to the rest state and the chamber is filled. Such a mode of operation may be described as a fire-before-fill mode. There is a problem associated with the typical demand ink jet, i.e., a fire-before-fill jet. In many instances, such a jet will fire with the meniscus in the equilibrium state. Such a position is not particularly efficient from an operating standpoint since a greater volume contraction is necessary to generate a droplet of the same size and velocity because of the fluidic impedance of the droplet as compared with a droplet which is projected from a retracted meniscus wherein the fluidic impedance of the orifice is lessened. Finally, the typical fire-before-fill demand ink jet suffers from an instability of the drop break-off process. When the drop emerges from the orifice upon contraction of the chamber volume from an unretracted meniscus position which is necessary to avoid variations in droplet velocity and size, the droplet is more likely to attach to the edge of the orifice. This creates drop aiming problems which may be caused by geometric imperfections in the orifice edge. Firing from the equilibrium position of the meniscus is also more likely to result in ink spillover which will wet the face of the orifice as the droplet emerges also creating irregularities in droplet projection. Another disadvantage of such spillover is the probability of paper dust adhering to the jet face and causing a failure. U.S. Patent No. 4 284 996 to Greve relates to a method for driving an ink jet comprising a compression chamber for the ink, the volume of which is charged by means of a piezo-electric transducer by expanding the chamber in preparation for printing, contracting the chamber to print, and expanding the chamber again in preparation for another cycle of printing. SUMMARY OF THE INVENTION It is an object of this invention to provide a method of operating a demand ink jet wherein droplets of the same size are generated at various frequencies or projection rates. It is also an object of this invention to provide a method for operating a demand ink jet wherein the same droplet velocity is achieved for various frequencies or droplet projection rates. It is a further object of this invention to provide a method for operating a demand ink jet with greater operating efficiency. It is a still further object of this invention to provide a method of operating a demand ink jet capable of high frequency and/or droplet projection rates. It is a still further object of this invention to provide a demand ink jet characterized by stability in the drop break-off process. It is another object of this invention to provide a method of operating a demand ink jet wherein drop aiming is optimized. It is yet a further object of this invention to provide a method of operating a demand ink jet wherein the spilling over of ink and the wetting of the face of an orifice is minimized. According to the invention, there is provided a method of operating a demand or impulse ink jet, comprising an ink jet chamber (10) coupled to a transducer, the volume of the chamber varying in response to the state of energisation of the transducer, and an ink drop ejection orifice (14), which method comprises the following steps: initiating filling of the chamber by energising the transducer such that the volume of the chamber expands beyond the volume existing when the transducer is not energised, thereby by decreasing the pressure in the chamber; retracting the meniscus (26) at the orifice to a predetermined position as the pressure is decreased; initiating firing of a first ink droplet by deenergising the transducer such that the volume of the chamber returns to substantially the volume existing when the transducer is not energised, thereby increasing the pressure within the chamber when the meniscus is retracted to said predetermined position; and moving the meniscus forward through the orifice while the pressure is increased so as first to form and then to project an ink droplet outwardly from the orifice. Preferably, the method includes the step of forming an unretracted meniscus after projecting each said droplet of ink from the orifice prior to said retracting step. Suitably, the meniscus is retracted to said predetermined position over a range of frequencies extending upwardly from zero to in excess of 5 kHz. The meniscus may be retracted to said predetermined position over a range of frequencies extending upwardly from zero to 5 kHz. Preferably, the time lapse between initiating filling and initiating firing is substantially constant for each said droplet of ink. Preferably, the time lapse between initiating filling and initiating firing is 5 to 500 µ seconds. More preferably, the time lapse between initiating filling and initiating firing is 10 to 75 µ seconds. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a wave form diagram representing chamber volume as a function of time in prior art ink jets; FIG. 2 is a diagrammatic waveform representing meniscus position as a function of time in prior art ink jets; FIGs. 3(a-e) and FIGs. 4(a-e) represent the excitation of a meniscus and the formation of a droplet as a function of initial meniscus position; FIG. 5 is a diagrammatic representation of drop velocity as a function of frequency in prior art ink jets; FIG. 6 is a partially schematic, cross-sectional view of an ink jet capable of operating in accordance with this invention where the jet is in the rest state; FIG. 7 is a diagrammatic representation of a transducer voltage as a function of time for an ink jet operated in accordance with this invention; FIG. 8 is a diagrammatic representation of chamber volume as a function of time for an ink jet operated in accordance with this invention; FIG. 9 is a diagrammatic representation of meniscus position as a function of time for an ink jet operated in accordance with this invention; FIG. 10 is a partially schematic, cross-sectional diagram of the ink jet of FIG. 6 in the active state; and FIG. 11 is a diagrammatic representation of drop velocity as a function of frequency in an ink jet operated in accordance with this invention. The invention will now be described, non-limitatively, with reference to the drawings, the preferred embodiment being described with reference to Figs. 6ff. FIG. 1 depicts chamber volume v as a function of time t in a demand ink jet operating in a fire-before-fill mode. Referring to FIG. 1, the time t₀ represents the onset of the active state of the ink jet whereupon the volume of ink is reduced rapidly until time t₁. This rapid reduction in volume produces the projection of a droplet on or about time t₁. The contracted volume of the chamber continues with slight fluctuation until time t₂ whereupon the contracted volume begins to expand until time t₃. At time t₃ marking the beginning of a rest state, the volume of the chamber is identical to that at time t₀. As shown in FIG. 1, the rest state continues for time d t between times t₃ and t₅ whereupon an active state is initiated resulting in the projection of another droplet. Operation at high droplet projection rates or frequencies will necessitate very short dead times d t corresponding to the inactive state. In other words, it may be necessary to initiate the active state so as to again contract the volume of the chamber at an earlier time t₄ as depicted by dotted lines in FIG. 1. Generally speaking, higher droplet projection rates and/or frequencies are desirable but achieving such rates and/or frequencies with demand ink jets operating in a fire-before-fill mode as depicted by the waveform in FIG. 1 may create difficulties which will now be discussed with respect to FIGs. 2 through 4. FIG. 2 depicts the meniscus position p as a function of time as the demand ink jet discussed with respect to FIG. 1 moves between the rest and active states. In this connection, it will be understood that the times t₀ through t₅ of FIG. 2 are coincident with the times t₀ through t₅ of FIG. 1 and the meniscus position p as depicted in FIG. 2 is a function of the chamber volume v as depicted in FIG. 1. At time t₀, the meniscus position p is at equilibrium corresponding with the position of the meniscus when the ink jet is in the rest state. As the ink jet moves into the active state and the chamber volume v contracts rapidly between times t₀ and t₁, the meniscus position moves forward resulting in the ultimate ejection of a droplet of ink at time t₁. Immediately upon ejection of the droplet at time t₁, the meniscus position p returns essentially to an equilibrium state as shown at time t₂ while the volume v is still in the contracted state. At time t₂, when the chamber volume v is expanding back to the volume of the ink jet in the rest state, the meniscus position retracts and is still in the retracted position at time t₃ when the active state of the ink jet has terminated. During the rest state corresponding to the dead time d t , the meniscus position advances back to the equilibrium position corresponding to the position of the meniscus in the rest state. As shown in FIG. 2, t₅ has been chosen such that the meniscus position at time t₅ has had an opportunity to return to the equilibrium position prior to the onset of the next active state and the ejection of another droplet of ink. However, if the next active state were to begin at time t₄ resulting in the firing of a droplet of ink, the meniscus position would not yet have returned to the equilibrium state and the meniscus would abruptly advance at time t₄ as shown in FIG. 2 with the result that the meniscus would reach a somewhat different position than the meniscus reached as a result of delaying the onset of the active state until time t₅. This variation in the position of the meniscus as a function of the duration of the dead time d t produces a variation in the droplet size and velocity which is undesirable in achieving the optimum in ink jet printing. The adverse effects with respect to droplet size may be readily appreciated with reference to FIGs. 3 and 4. As shown in FIG. 3, a droplet of ink is fired when the meniscus is in an initial equilibrium position as shown in FIG. 3a. In particular, FIG. 3a shows a meniscus in the position depicted in FIG. 2 at time t₅. FIGs. 3b through 3d show the advancement of the meniscus following time t₅ including the formation of a droplet. FIG. 3e shows the ultimate droplet ejected. If, however, the meniscus is at least partially retracted as at time t₄ depicted in FIG. 4(a), a droplet of somewhat different size is formed as depicted by FIGs. 4b through 4e. More particularly, the formation of a droplet at the center of the meniscus in FIG. 4b results in a somewhat smaller droplet as depicted by FIG. 4e. It will, therefore, be appreciated by reference to FIGs. 3 and 4 that droplets of different size may be generated utilizing a typical demand ink jet as a function of the dead time d t or duration of the rest state. Where high droplet projection rates or frequencies are desired, diminution of the dead time d t or duration of the active state will produce smaller droplets. On the other hand, larger droplets will be produced where the duration of the rest state or dead time d t is of some threshold duration. FIG. 5 depicts a difference in velocity as a function of frequency which in turn is a function of the dead time d t . As shown, the droplet velocity increases from 0 kHz. up to 7 kHz. In other words, as the dead time d t is shortened so as to increase frequency, the droplet velocity varies as shown in FIG. 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 6 discloses a demand ink jet representing a preferred embodiment of the invention. The jet includes a variable volume chamber 10 formed within a housing 12 which includes an orifice 14. The transducer 16 is coupled to the chamber 10 through a diaphram 18. The volume of the chamber is varied in response to the state of energization of the transducer 16 which is controlled by the application of an electric field as a result of a drive voltage V applied between an electrode 20 connected to a supply of the voltage V and an electrode 22 connected to ground. A supply port 24 supplies ink to the chamber 10. A meniscus of ink 26 is formed at the orifice 14. As the volume of the chamber 10 expands and contracts decreasing and increasing the pressure within the chamber respectively, the meniscus 26 moves into and out of the chamber 10 respectively. As shown in FIG. 6, the ink jet is in the rest or inactive state. In this state, the transducer 16 is unenergized and the diaphram 18 is substantially undeformed such that the volume of the chamber 10 is substantially uncontracted. In the inactive or rest state, the meniscus 26 is in a position of equilibrium as shown in FIG. 6. By applying a voltage V such as that shown in the waveform of FIG. 7, the ink jet shown in FIG. 6 may be activated so as to project droplets from the orifice 14. More particularly, a voltage V is applied to the electrodes 20 and 22 as depicted by the waveform of FIG. 7 at time t₀ so as to change the ink jet from the rest state to the active state. The active state continues through times t₁ and t₂ to time t₃ while the voltage waveform as shown in FIG. 7 is applied. At time t₃, the voltage waveform goes to zero as shown in FIG. 7 and the rest or inactive state is resumed until time t₅ when the voltage waveform again becomes positive so as to place the ink jet in the active state. The voltage waveform as depicted in FIG. 7 produces the changes in volume of the chamber 10 as depicted by FIG. 8 with concommitant changes in pressure within the chamber 10. More particularly, the volume of the chamber expands and the pressure decreases beginning at time t₀ at the onset of the active state and the conclusion of the rest state with the maximum volume of the chamber occurring at times t₁ and t₂. During this time, filling of the chamber occurs. By time t₃, the voltage V applied to the electrodes 20 and 22 of the ink jet as shown in FIG. 6 has been reduced to zero such that the volume of the chamber 10 suddenly returns to the volume existing during the rest state with a rapid increase in pressure. Firing of a droplet occurs coincident with this increase in pressure. The volume remains constant until time t₅ when a positive voltage is again applied to electrodes 20 and 22 so as to expand the volume of the chamber with a resultant reduction in the pressure within the chamber. During the time between t₃ and t₅, the ink jet is in the rest state for a duration of dead time designated d t . In accordance with this invention, the duration of the time d t may be varied without adversely affecting the operation of the ink jet, i.e., the firing of droplets of ink. More particularly, the positive-going voltage of waveform may be applied beginning at time t₄ rather than t₅ with a resulting increase in the expansion of the volume of the chamber beginning at time t₄ rather than time t₅. This, in turn, will result in a shortened dead time d t . Because the ink jet is operated in a fill-before-fire mode, i.e., filling is initiated at the conclusion of the rest state and the onset of the active state rather than initiating firing at the conclusion of the rest state and the onset of the active state, the drop velocity and size will not vary. In other words, droplet size and velocity are substantially constant. In this connection, it will be appreciated that filling and not firing is initiated at time t₀ and time t₅. In contrast, a fire-before-fill mode of operation as depicted in FIG. 1 would result in firing at time t₀ rather than filling. The particular reasons for achieving uniform droplet velocity and size may be best appreciated by reference to FIG. 9 wherein it will be seen that the position of the meniscus is always in a state of retraction at the onset of firing which occurs at time t₂ as time t₇. Moreover, firing is initiated not only when the meniscus is retracted but when the meniscus is in substantially the same retracted position. In other words, the degree of retraction is controlled so that the meniscus is always in the same retracted position at the onset of firing as shown in FIG. 4 to assure uniformity in droplet size and droplet velocity. This is accomplished by synchronizing firing at times t₂ and t₇ with the filling beginning at times t₀ and t₅, i.e., there is a fixed time duration between filling and firing regardless of droplet projection rates or frequencies. Referring again to FIG. 9, it will be seen that the duration of the dead time d t which varies with frequency has no adverse effect on the position of the meniscus at the time of firing. If the rest state ends and the active state begins at time t₅, the meniscus will be in the position shown at time t₇ when firing of the droplet is initiated. On the other hand, if the rest state ends at time t₄ and the dead time d t is shortened accordingly, the meniscus is in an identical position at time t₆. As a consequence, droplet velocity and size will necessarily remain substantially constant since the meniscus is in the same position regardless of the duration of the dead time d t . In terms of the position of the meniscus 26 shown in FIG. 10, the meniscus will be in the same position whether the active state begins at time t₅ or an earlier time t₄. FIG. 11 depicts a substantially constant droplet velocity over a predetermined frequency range extending upwardly from zero kHz. Preferably, the droplet velocity is substantially constant from zero to 5 kHz. with a constant velocity up to 7 kHz. preferred. Above 7 kHz. as shown in FIG. 11, the velocity may vary as a result of the phasing of the transducer resonance which is excited by firing. Variations in the volume of ink as a function of time have been discussed with respect to FIG. 8 with these variations producing the change in meniscus as a function of time as shown in FIG. 9. As mentioned previously, the variations in volume produce changes in pressure within the chamber. For example, as the volume within the chamber contracts, the pressure is increased. On the other hand, if the volume expands, the pressure is decreased. By comparing FIGs. 1 and 2 with FIGs. 8 and 9, it will be appreciated that a fill-before-fire mode of operation in accordance with this invention is advantageous as compared with a fire-before-fill mode since the meniscus is always in a retracted position regardless of the frequency. In the fire-before-fill mode as depicted in FIG. 2, the meniscus is not in a retracted position at the time of initiating firing, i.e., at time t₅, where the dead time d t exceeds some predetermined limit. Obviously, at the time of initiating firing after a long rest state, the meniscus will be in the same position as shown in FIG. 2 at time t₅. Thus, the meniscus will not be retracted. On the other hand, the meniscus is always retracted in a fill-before-fire mode as depicted in FIG. 9 since the meniscus must be retracted before firing can occur even after the end of a rest state. It will also be observed with reference to FIG. 9 that the meniscus always returns to the unretracted equilibrium state as soon as firing is completed. Since the meniscus always retracts from the equilibrium state at the time of filling, the amount of meniscus retraction is always equal and the meniscus position at the time of firing is, therefore, always the same from droplet to droplet. As shown in FIG. 9, the time duration between time t₀ and t₂ is the same as the duration of the time between time t₅ and t₇ or between time t₄ and t₆. These time durations correspond to the time lapse between initiating filling and initiating firing. By making these time lapses substantially equal and thereby synchronizing firing with filling, the meniscus position at the time of initiating firing is repeatable so as to assure uniform droplet size and velocity. It will, therefore, be appreciated that this invention involves the controlling of the retracted meniscus position prior to firing so as to achieve uniformity in droplet velocity and size. As described herein, this uniformity in droplet size and velocity is achieved in the preferred embodiment of the invention by establishing a fixed time duration between the initiation of filling and the initiation of firing. This time duration is preferably greater than 5 but less than 500 µ sec. For example, a time duration of 10 to 75 µ sec has been found to be particularly desirable. By assuring that the meniscus is always fired from a retracted position, greater jet operating efficiency is achieved as the overall orifice channel length is effectively shortened resulting in reduced fluidic impedance. As a consequence, less transducer displacement is necessary to generate a drop of given size and velocity. As discussed above, droplet repetition rate in a fire-before-fill mode is limited by the time required for the meniscus to recover to equilibrium upon cessation of the volume displacement cycle unless differences in droplet size and velocity can be tolerated. In the fill-before-fire mode of this invention, less liquid volume is pulled from the orifice during expansion of the chamber and is driven outwardly through the orifice during contraction of the chamber. This is because the meniscus, being in equilibrium at the state of the cycle, presents a higher fluidic impedance to expansion than to contraction. The difference between the volume driven out through the orifice on contraction and the volume pulled in through the orifice on expansion constitutes a portion, or possibly all, of the drop volume that will not need to be refilled after cessation of the volume displacement cycle. Elimination of the refill requirement permits shorter dead times d t between volume displacement cycles and hence higher repetition rates. Finally, when a droplet emerges from an initial retracted meniscus position, attachment of the emerging droplet to the orifice edge is avoided. This reduces the tendency toward drop misaim that can be caused by geometric imperfection in the orifice edge and it also reduces the tendency of ink to spill over and wet the face as the droplet is emerging which can also result in misaim. As was described in the foregoing, a droplet is projected outwardly from a meniscus as the meniscus moves forward from a retracted position as shown in FIG. 3(a-e). It will be understood that the term droplet is not intended to denote or connote a necessarily spherical volume of ink. Rather, the volume of ink may be elongated as in the form of a ligament. It will also be understood that the particular configuration of the ink jet chamber and the orifice may vary. For example, a slightly modified orifice and chamber may be utilized wherein the chamber walls taper into the orifice walls rather than the more abrupt juncture of the walls as depicted in FIGs. 1 and 10. Regardless of the configuration of the walls in the orifice, the meniscus moves between an equilibrium state as depicted in FIG. 6 and a retracted state as depicted in FIG. 10. The term active state and the term rest state have been utilized. It is not intended that the term active state will necessarily connote the application of a potential across the transducer, nor is the term rest state intended to connote the absence of such a potential across the transducer. Rather, the active state is intended to connote the quiescent state of the ink jet to which the device returns during dead time when there is no demand for a droplet of ink. On the other hand, the active state is that period of time coinciding with demand for a droplet of ink. Although particular embodiments of the invention have been shown and described, it will be understood that various modifications may be made which will fall within the scope of the invention as set forth in the appended claims.

Claims: A method of operating a demand or impulse ink jet, comprising an ink jet chamber (10) coupled to a transducer, the volume of the chamber varying in response to the state of energisation of the transducer, and an ink drop ejection orifice (14), which method comprises the following steps: initiating filling of the chamber by energising the transducer such that the volume of the chamber expands beyond the volume existing when the transducer is not energised, thereby by decreasing the pressure in the chamber; retracting the meniscus (26) at the orifice to a predetermined position as the pressure is decreased; initiating firing of a first ink droplet by deenergising the transducer such that the volume of the chamber returns to substantially the volume existing when the transducer is not energised, thereby increasing the pressure within the chamber when the meniscus is retracted to said predetermined position; and moving the meniscus forward through the orifice while the pressure is increased so as first to form and then to project an ink droplet outwardly from the orifice. A method as claimed in claim 1, including the step of forming an unretracted meniscus (26) after projecting each said droplet of ink from the orifice (14) prior to said retracting step. A method as claimed in any preceding claim, wherein the time lapse between initiating filling and initiating firing is substantially constant for each said droplet of ink. The method of claim 3 wherein the time lapse between initiating filling and initiating firing is 5 to 500 µ seconds. The method of claim 3, wherein the time lapse between initiating filling and initiating firing is 10 to 75 µ seconds. The method of claim 1 or claim 2, wherein the meniscus is retracted to said predetermined position over a range of frequencies extending upwardly from zero to 5 kHz. The method of claim 1, wherein the step of filling includes energising the transducer by applying a voltage to the transducer so that the chamber expands and the step of firing includes deenergising the transducer by reducing the applied voltage so that the chamber contracts. The method of claim 7, wherein the step of firing includes deenergising the transducer by reducing the applied voltage to the point that no voltage is applied. The method of any preceding claim, wherein the step of projecting the ink droplet outwardly from the orifice includes a predetermined velocity and/or predetermined droplet size; and repeating the foregoing steps for each of a series of ink droplets, each ink droplet having substantially said predetermined velocity and/or said predetermined droplet size at frequencies of droplet ejection extending over a frequency range from 0 to 7 kHz.

Inventors: HOWKINS, STUART DAVID
Applicants: DATAPRODUCTS CORPORATION
Application year: 1985
Main classification: B41J2
IPC codes: B41J2