EP0115180B1

EP0115180B1 - Operating an ink jet

Info

Publication number: EP0115180B1
Application number: EP83307850A
Authority: EP
Inventors: Stephen Jay Liker
Original assignee: Dataproducts Corp
Current assignee: Ricoh Printing Systems America Inc
Priority date: 1982-12-27
Filing date: 1983-12-22
Publication date: 1990-04-04
Anticipated expiration: 2003-12-22
Also published as: ATE51582T1; EP0115180A2; CA1232490A; JPS59133066A; JPH07108568B2; DE3381406D1; EP0115180A3

Abstract

For controlling the volume of ink droplets ejected from a drop on demand ink jet apparatus including a transducer (204) operable for producing a pressure disturbance within an associated ink chamber (200) for ejecting an ink droplet from an associated orifice (202), the transducer (204) is operated in an iterative manner for producing a plurality of successively equal or higher or lower amplitude pressure disturbances, or some combination thereof, within the ink chamber (200), for causing a plurality of successively equal or higher or lower velocity ink droplets, or some combination thereof, to be ejected from the orifice (202) of the ink jet apparatus, within a time period permitting the ink droplets to either merge in flight or upon striking a recording medium.

Description

The present invention relates to controlling the volume of ink droplets ejected from a drop on demand ink jet apparatus, and more specifically though not exclusively to a method for operating an ink jet apparatus for providing selective control within a range of either the volume of the ink droplets ejected by the apparatus or the amount of ink striking a desired location on a recording medium.
The design of practical ink jet devices and apparatus for producing a single droplet of ink on demand is relatively new in the art. In prior drop on demand ink jet apparatus, the volume of each individual ink droplet is typically dependent upon the geometry of the ink jet apparatus, the type of ink used, and the magnitude of the pressure force developed within the ink chamber of the ink jet ejecting an ink droplet from an associated orifice. The effective diameter and design of the orifice, the volume and configuration of the ink chamber associated with the orifice, the tranducer design, and the method of coupling the transducer to the ink chamber, are all factors determining the volume of individual ink droplets ejected from the orifice. Typically, once the mechanical design of an ink jet apparatus is frozen, control over the volume of the ejected ink droplets can only be obtained from a narrow range by varying the amplitude of the electrical pulses or dry voltage applied to the individual transducers of the ink jet apparatus or array.
Both US-A-4184 168 and DE-A-2 615 713 describe such prior ink jet printing systems, wherein the amount of liquid in each ejected ink droplet may be varied dependent upon the amplitude of an ink jet control pulse. DE-A-2 615 713 suggest the use of means for modulating the amplitude of an alternating sinusoidal signal of constant amplitude with a video signal whose highest frequency is below the frequency of the sinusoidal signal so that the amount of liquid in each ejected liquid droplet varies in accordance with the amplitude of the video signal. US-A-4 184 168 relates to an ink jet head driving circuit, wherein a comparator is employed to compare a video signal with a reference voltage and to deliver a high-level output when the level of the video signal is higher than or equal to the reference voltage or a low-level output when the level of the video signal is lower than the reference voltage. A pulse synchronization circuit receives the output from the comparator and a printing sync pulse, and delivers an output pulse in synchronism with the printing sync pulse only when the pulse synchronization circuit receives high-level or low-level output from the comparator. According to both documents, only a single ink droplet is ejected per interval of the pre-set production rate; the only variation is with respect to the amount of liquid ejected in each ink droplet.
According to the invention from one aspect there is provided a method for controlling the volume of ink droplets ejected from a print head of a drop-on-demand ink jet apparatus including transducer means operable for producing a pressure disturbance within an associated ink chamber for ejecting an ink droplet from an associated orifice, said transducer means being operated in an iterative manner at a drop-on-demand production rate, for producing a plurality of successive pressure disturbances within said ink chamber at said drop-on-demand production rate, the method being characterized in that it comprises causing a plurality of ink droplets to be ejected from said orifice along substantially the same trajectory within a time period permitting said droplets to merge while air-borne thereby controlling the size of an ink spot produced by the merged droplets upon striking a print medium.
According to the invention from another aspect there is provided apparatus for controlling the volume of ink droplets ejected from a print head of a drop-on-demand ink jet apparatus including transducer means operable for producing a pressure disturbance within an associated ink chamber for ejecting an ink jet droplet from an associated orifice, and controller means operable for operating said transducer means at a drop-on-demand drop production rate in an iterative manner, for producing a plurality of successive pressure disturbances within said ink chamber at said drop-on-demand production rate, characterized in that said controller means is further operative to operate said transducer means so as to cause ejection of a plurality of ink droplets from said orifice along substantially the same trajectory within a time period permitting said droplets to merge while air-borne, thereby controlling the size of an ink spot produced by the merged droplets upon striking a print medium.
Various optional features of the invention are set forth in the dependent Claims 3 to 10 hereto.
It has been found that when putting the invention into effect, broader control of the boldness and toning of printing can be obtained. The volume of ink striking a recording medium at a given point is thereby partly determined by the number of ink droplets merged while airborne.
The invention will be better understood by referring, by way of example, to the accompanying drawings, in which:

Figure 1 is a sectional view of one form of ink jet apparatus in accordance with the invention;
Figure 2 is an enlarged view of a portion of the section shown in Figure 1;
Figure 3 is an exploded perspective view of the ink jet apparatus shown in Figures 1 and 2;
Figure 4 is a partial sectional/schematic diagram view of the transducer shown in Figures 1 and 3, with the transducer in the de-energised state;
Figure 5 is a partial sectional/schematic diagram or view of the transducer of Figure 4 in the energised state;
Figure 6 shows the wave shapes for electrical pulses of one embodiment of the invention;
Figure, 7 shows a typical ejection of an ink droplet from an orifice;
Figure 8 shows the ejection of an ink droplet from an orifice at a time when the previously ejected ink droplet is still in flight;
Figure 9 shows the merging of two ink droplets while in flight;
Figure 10 shows a typical ink droplet formed after the merger of a number of ink droplets just prior to striking a recording medium;
Figure 11 shows the waveshapes for electrical pulses for another embodiment of the invention;
Figure 12 shows the waveshapes for electrical pulses for yet another embodiment of the invention;
Figure 13 shows a waveshape for electrical pulses for causing ejection of two droplets; and
Figures 14 and 15 show waveshapes which in themselves do not fall within the scope of the present invention but in some combination of waveshapes selected from Figures 13 to 15 to produce a plurality of ink droplets constitute further embodiments of the invention.

Preferred ways of performing the present invention will now be described in the context of the ink jet apparatus of Figs. 1 to 5. However, the invention may be performed using a broad range of drop on demand ink jet apparatus. Accordingly, the ink jet apparatus to be discussed herein is presented for purposes of illustration and example only, and is not meant to be limiting. Also, only the basic mechanical features and operation of this apparatus are discussed in the following paragraphs, and reference is made to U.K. patent application 2094233A (published on 15.9.82) for further details concerning this apparatus. The reference designations used in Figures 1 to 5 are the same as used in Figures 7 to 11 of that published application, in order to facilitate any referencing back to that application or the patent that may issue therefrom.
With reference to Figures 1 to 3, the illustrative ink jet apparatus includes a chamber 200 having an orifice 202 for ejecting droplets of ink in response to the state of energization of a transducer 204 for each jet in an array of such jets (see Fig. 3). The transducer 204 expands and contracts (in directions indicated by the arrows in Fig. 2) along its axis of elongation, and the movement is coupled to the chamber 200 by coupling means 206 which includes a foot 207, a visco-elastic material 208 juxtaposed to the foot 207, and a diaphragm 210 which is preloaded to the position shown in Figures 1 and 2.
Ink flows into the chamber 200 from an unpressurized reservoir 212 through restricted inlet means provided by a restricted opening 214. The inlet 214 comprises an opening in a restrictor plate 216 (see Fig. 3). As shown in Figure 2, the reservoir 212 which is formed in a chamber plate 220 includes a tapered edge 222 leading into the inlet 214. As shown in Fig. 3, the reservoir 212 is supplied with a feed tube 223 and a vent tube 225. The reservoir 212 is compliant by virtue of the diaphragm 210, which is in communication with the ink through a large opening 227 in the restrictor plate 216 which is juxtaposed to an area of relief 229 in the plate 226.
One extremity of each one of the transducers 204 is guided by the cooperation of a foot 207 with a hole 224 in a plate 226. As shown, the feet 207 are slideably retained within the holes 224. The other extremities of each one of the transducers 204 are compliantly mounted in a block 228 by means of a compliant or elastic material 230 such as silicon rubber. The compliant material 230 is located in slots 232 (see Fig. 3) so as to provide support for the other extremities of the transducers 204. Electrical contact with the transducers 204 is also made in a compliant manner by means of a compliant printed circuit 234, which is electrically coupled by suitable means such as solder 236 to an electrode 260 of the transducers 204. Conductive patterns 238 are provided on the printed circuit 234.
The plate 226 (see Figures 1 and 3) includes holes 224 at the base of a slot 237 which receive the feet 207 of the transducers 204, as previously mentioned. The plate 226 also includes a receptacle 239 for a heater sandwich 240, the latter including a heater element 242 with coils 244, a hold down plate 246, a spring 248 associated with the plate 246, and a support plate 250 located immediately beneath the heater 240. The slot 253 is for receiving a thermistor 252, the latter being used to provide monitoring of the temperature of the heater element 242. The entire heater 240 is maintained within the receptacle in the plate 226 by a cover plate 254.
As shown in Fig. 3, the variously described components of the ink jet apparatus are held together by means of screws 256 which extend upwardly through openings 257, and screws 258 which extend downwardly through openings 259, the latter to hold a printed circuit board 234 in place on the plate 228. The dashed lines in Fig. 1 depict connections 263 to the printed circuits 238 on the printed circuit board 234. The connections 263 connect a controller 261 to the ink jet apparatus, for controlling the operation of the latter.
The controller 261 is programmed to at an appropriate time, via its connection to the printed circuits 238, apply a voltage to a selected one or ones of the hot electrodes 260 of the transducers 204. The applied voltage causes an electric field to be produced transverse to the axis of elongation of the selected transducers 204, causing the transducers 204 to contract along their elongated axis. When a particular transducer 204 so contracts upon energization (see Fig. 5), the portion of the diaphragm 210 located below the foot 207 of the transducer 204 moves in the direction of the contracting transducer 204, thereby effectively expanding the volume of the associated chamber 200. As the volume of the particular chamber 200 is so expanded, a negative pressure is initially created within the chamber, causing ink therein to tend to move away from the associated orifice 202, while stimultaneously permitting ink from the reservoir 212 to flow through the associated restricted opening or inlet 214 into the chamber 200. Given sufficient time, the newly supplied ink completely fills the expanded chamber and orifice, providing a "fill before fire" cycle. Shortly thereafter the controller 261 is programmed to remove the voltage or drive signal from the particular one or ones of the selected transducers 204, causing the transducer 204 or transducers 204 to return to their deenergized states as shown in Fig. 4. Specifically, the drive signals are terminated in a step like fashion, causing the transducers 204 to very rapidly expand along their elongated axis, whereby via the visco-elastic material 208 the feet 207 of the transducers 204 push against the area of the diaphragm 210 beneath them, causing a rapid contraction or reduction of the volume of the associated chamber or chambers 200. In turn, this rapid reduction in the volume of the associated chambers 200, creates a pressure pulse or positive pressure disturbance within the chambers 200, causing an ink droplet to be ejected from the associated orifices 202. Note that as shown in Figure 5, when a given transducer 204 is so energized, it both contracts or reduces its length and increases its thickness. However, the increase in thickness is of no consequence to the illustrated ink jet apparatus, in that the changes in length of the transducer control the operation of the individual ink jets of the array. Also note, that with present technology, by energizing the transducers for contraction along their elongated axis, accelerated aging of the transducers 204 is avoided, and in extreme cases, depolarization is also avoided.
For purposes of illustration, assume that the pulses shown in Figure 6 are applied via controller 261 to one of the transducers 204. As shown, the first and second pulses 1 and 3 respectively each have an exponential leading edge and a substantially linear trailing edge, respectively, peak amplitudes +V_i, +V₂ volts respectively, and pulse widths of T_i, T₂, respectively. Note that the shapes of the pulses 1, 3, respectively may be other than as illustrated herein, depending upon the particular ink jet device being driven and the particular application. In this example, the peak amplitude +V₂ of pulse 3 is greater than the peak amplitude V, of pulse 1, and the fall time for the trailing edge of pulse 3 is less than the fall time for the trailing edge of pulse 1. Since the degree of contraction of the selected transducer 204 is directly related within a range to the amplitude of the pulse applied to the transducer, the greater the amplitude, the greater the degree of contraction. Accordingly, upon termination of a particular operating or control pulse, the magnitude of the pressure disturbance produced in the associated chamber 200 will be directly related within a range to the amplitude of the previously applied control pulse. Also, the greater the slope or the less the fall time of the trailing edge of the control pulse, the more rapid the expansion or elongation of the selected transducer 204 to its rest state upon termination of the control pulse. Correspondingly, the greater the rate of expansion of the transducer 204, the greater the magnitude of the resulting pressure disturbance within the associated chamber 200. Assume that the amplitudes +V_i and +V₂ of pulses 1, 3, respectively, are large enough to ensure ejection of an ink droplet from associated orifice 202 upon termination of these pulses, respectively.
With reference to Figure 7, assume that pulse 1 is applied to a selected one of transducers 204. Upon termination of pulse 1, a typical ink droplet 5 will be ejected from the associated orifice 202. Substantially upon the termination of pulse 1 assume that pulse 3 is applied to the selected transducer 204. Shortly after the termination of pulse 3, a second ink droplet 7 will be ejected from the associated orifice 202 as shown in Figure 8, for example. Ink droplets 7 will have a substantially greater velocity than the air-borne ink droplet 5 because the amplitude of pulse 3 is greater of that than pulse 1 and the fall time of pulse 3 is less than that of pulse 1. Note that as previously explained though, the velocity of the second ink droplet 7 will be greater than that of ink droplet 5 so long as at least one of either the amplitude of pulse 3 is greater than that of pulse 1 even if the fall times of these pulses are equal, or the fall time of pulse 3 is less than that of pulse 1 even if their amplitudes are equal. Accordingly, either amplitude control of the control pulses, or trailing edge fall time control of the control pulses or a combination of the two can be used to produce a higher velocity second droplet 7 as illustrated in Figure 8, for example. By properly controlling the pulse parameters, the velocity of the second ink droplet 7 can be made high enough to cause droplet 7 to catch up with droplet 5 while each is air-borne, causing these droplets to begin to merge together as shown in Figure 9. Assuming sufficient flight time, the merger of droplets 5 and 7 may result in a droplet shape as shown in Figure 10 prior to the merged droplets striking a recording medium. In this manner, the size of the ink droplet or volume of ink striking a recording media at a particular point is substantially increased relative to using only a single droplet, and such control of the volume of ink directly provides control of the boldness of printing. Typical values for the parameters of pulses 1, 3 used by the inventor in conducting his experiments, were 28 volts and 30 volts for +V,, +V₂, respectively; 60 microseconds for each one of the pulse widths T, and T₂; and fall times of 2 microseconds and 1 microsecond for pulses 1, 3, respectively. The viscosity of the ink in this example was 12 centipoise. For the particular ink jet device operated by the present inventor, the approximate diameter of droplet 5 was 1.8 mils, for the second ink droplet 7 was 2.2 mils, and for the merged ink droplet 9 was 4.0 mils. Other ink droplet diameters or volumes may be obtained within a range via control of the amplitudes and fall times of pulses 1 and 3, as previously mentioned. ,
Within a range, control of the size of ink droplets ejected from the ink jet device can be controlled by adjusting the amplitudes and fall times of the control pulses applied to the ink jet device. The range of control of the volume of ink or ultimate ink droplet size striking a recording media is substantially extended via another embodiment of the present invention for merging a plurality of ink droplets in flight.
In Figure 11, the amplitudes +V₁, +V₂ of pulses 11, 13, respectively, are shown to be equal (typically 30 volts, for example). In this example, the trailing edge of pulse 11 is about 10 microseconds in fall time, whereas the trailing edge of pulse 13 has a fall time of about 1 microsecond. Accordingly, the ink droplet resulting from the application of pulse 11 to a selected transducer 204 will have a velocity that is substantially slower than the velocity of the following ink droplet resulting from the application of pulse 13 to the transducer 204. Accordingly, only fall time control is being used to adjust the velocities of the ink droplets resulting from the application of pulses 1 and 13. In this example, it is assumed that the second ejected higher velocity ink droplet will merge with the first ejected ink droplet while air-borne, as previously described.
In Figure 12, a third control or firing pulse 15 has been added following the termination of pulse 13. In one experiment with a given ink jet device, the present inventor set the amplitude of pulses 11, 13, 15 all at 30 volts (+V₁, +V₂ and +V₃ all equal 30 volts), with pulses 11, 13 and 15 typically having exponential fall times of 10 microseconds, 5 microseconds and 1 microsecond, respectively; and pulse widths of 60 microseconds, 40 microseconds and 30 microseconds, respectively, for example. When applied to a selected transducer 204 of the given ink jet device, pulse 11 caused a first ink droplet to be ejected, pulse 13 caused a second ink droplet of greater velocity than the first to be ejected, and pulse 15 caused a third ink droplet of even greater velocity to be ejected, whereby all of these ink droplets were of such relative velocities that they merged in flight prior to striking a recording media. In this manner, an even greater range of control can be obtained for adjusting the size of an ink droplet in an ink jet system. Depending upon the distance of the selected ink jet orifice 202 from the recording medium, the relative speeds of movement of the recording medium and/or the ink jet head, and the design of the particular ink jet device, it is possible that an even greater number of ink droplets can be ejected at correspondingly greater velocities in order to permit merger in flight, providing even greater control of ink droplet size from one marking position to another on a recording medium.
Note that in practice, an ink droplet is not ejected immediately after the termination of a particular firing pulse. For example, if the pulses 1, 3 of Figure 6 are applied to a transducer 204 of the ink jet device used by the present inventor in his experiments, an ink droplet 5 is ejected 4 microseconds after the termination of pulse 1, and the second ink droplet is ejected 3 microseconds after the termination of pulse 3. The velocity of the first ejected ink droplet was measured to be 3.5 meters per second and of the second ejected ink droplet 5.0 meters per second.
With reference to Figure 13, the combination of waveshapes shown caused the ink jet apparatus to emit two droplets, which are merged upon striking on a print medium to produce dots varying in diameter from 5.3 to 5.6 milliinches, for producing very bold print. Typically, T" T₂, T₃ and T₄ are 80, 4, 18 and 6 microseconds, respectively, with the amplitudes of pulses 17 and 19 at 110 volts, and pulse 21 at about 73 volts, for producing the previous dot diameter range on a particular type of paper (Hammermill XEROCOPY, manufactured by Hammermill Papers Co., Inc., Erie, PA), using an ink having a wax base. The type of paper and ink formulation affects the dot diameter, in a given application. Typically, the fall time of pulses 17 and 19 are 9 microseconds and 1.0 microseconds, respectively. Under the conditions indicated above, shortly after termination of pulse 17, a first droplet having a velocity ranging from 8 to 10 meters per second was produced. Also the combination of pulses 19 and 21, caused a second droplet to be produced about 2 microseconds after the termination of pulse 19. Pulse 21 is not of sufficient amplitude to cause a third droplet to be produced, but does cause the second droplet to break off earlier from the orifice of the ink jet relative to operating without pulse 21. Also, pulse 21 permits higher frequency operation of the ink jet apparatus, and reduce ink blobbing problems at the orifice. Using the pulse time periods and amplitudes mentioned above, the velocity of the second droplet is typically 6 to 8 meters per second. The slower velocity of the second droplet relative to the first droplet is caused by the presence of pulse 21. In accordance with the invention, it is necessary for the droplets to merge while air-borne. In this example by increasing the amplitude of pulse 19, the velocity of the second droplet can be increased. Also, by varying the delay time T₂ between the termination of pulse 17 and initiation of pulse 19, the boldness can be modulated within a range.
By using various combinations of the waveforms of Figures 13, 14 and 15, desired shading can be accomplished. Such shading is known as half-toning. In Figure 14, by using only pulse 17 to operate the ink jet apparatus, a dot having a diameter range of 3.3 to 3.5 milliinches can be obtained. Such dot diameters produce much less bold print relative to operating the ink jet apparatus via the combination of pulses 17,19 and 21. With reference to Figure 15, the combination of pulses 17 and 21, as shown, operated the ink jet for producing an ink droplet having diameters ranging from 2.9 to 3.0 milliinches. This combination produces a very light print. It is stressed that the waveforms of Figure 14 or 15 on their own, which produce only a single droplet, do not lie within the scope of the invention.
The controller 261 can be provided via hardwired logic, or by a microprocessor programmed for providing the necessary control functions, or by some combination of the two, for example. Note that a Wavetek Model 175 waveshape generator, manufactured by Wavetek, San Diego, California, was used by the present inventor to obtain the waveshapes shown in Figures 6,11,12, 13, 14 and 15. In a practical system, a controller 261 would typically be designed for providing the necessary waveshapes and functions, as previously mentioned, for each particular application.

Claims

1. A method for controlling the volume of ink droplets ejected from a print head of a drop-on-demand ink jet apparatus including transducer means (204) operable for producing a pressure disturbance within an associated ink chamber (200), for ejecting an ink droplet from an associated orifice (202), said transducer means (204) being operable by controller means (261) in an iterative manner at a drop-on-demand production rate, for producing a plurality of successive pressure disturbances within said ink chamber (200) at said drop-on-demand production rate, the method being characterized in that it comprises causing a plurality of ink droplets to be ejected from said orifice (202) along substantially the same trajectory, within a time period permitting said droplets to merge while air-borne, thereby controlling the size of an ink spot produced by the merged droplets upon striking a print medium.

2. Apparatus for controlling the volume of ink droplets ejected from a print head of a drop-on-demand ink jet apparatus including transducer means (204) operable for producing a pressure disturbance within an associated ink chamber (200), for ejecting an ink jet droplet from an associated orifice (202), and controller means (261) operable for operating said transducer means (204) at a drop-on-demand drop production rate in an iterative manner, for producing a plurality of successive pressure disturbances within said ink chamber (200) at said drop-on-demand production rate, characterized in that said controller means (261) is further operative to operate said transducer means (204) so as to cause ejection of a plurality of ink droplets from said orifice (202) along substantially the same trajectory, within a time period permitting said droplets to merge while air-borne, thereby controlling the size of an ink spot produced by the merged droplets upon striking a print medium.

3. A method according to claim 1 or apparatus according to claim 2, characterized in that said transducer means (204) is responsive to an electrical pulse for producing the pressure disturbance within an associated ink chamber (200), the magnitude of the pressure disturbance being directly proportional to the amplitude of the electrical pulse, the slope of the trailing edge of said electrical pulse or some combination thereof, and the transducer means (204) is operated by applying successive electrical pulses (1, 3; 11, 13; 11, 13, 15; 17, 19) to said transducer means (204) so as to cause ejection of said plurality of ink droplets each at a desired velocity.

4. A method or apparatus according to claim 3, characterized in that said controller (261) is operative to produce electrical pulses (1; 11; 17) at said drop-on-demand production rate and is further operative to produce selectively at least one additional electrical drive signal (3; 13; 19) each timed to be applied substantially upon termination of said immediately preceding electrical drive signal or with an adjusted time delay (T₂ in Fig. 13) with respect to said immediately preceding electrical drive signal, said adjusted time delay being short with respect to said drop-on-demand drop production rate, said transducer means being actuated with each of said electrical drive signals to produce a quantity of ink having a controlled volume from said orifice (202).

5. A method or apparatus according to claim 4, characterized in that the amplitude of each one of said electrical pulses is adjusted for obtaining a desired velocity for an associated ink droplet, whereby the magnitudes of the pressure disturbances produced by said transducer means (204) are directly proportional to the amplitudes of said electrical pulses, respectively.

6. A method or apparatus according to claim 4 or 5, characterized in that fall time of the trailing edge of each one of said electrical pulses is adjusted for obtaining a desired velocity for an associated ink droplet, whereby the magnitudes of the pressure disturbances produced by said transducer means (204) are inversely proportional to the fall times of the trailing edges of said electrical pulses, respectively.

7. A method or apparatus according to any one of claims 3 to 6, characterized in that said print head has a plurality of orifices (202), a plurality of ink chambers (200) each associated with a particular one of said orifices, respectively, and a plurality of transducer means (204) each being selectively operable for producing a pressure disturbance within an associated ink chamber (200), for ejecting an ink droplet from an associated orifice (202), wherein a selected one of said plurality of transducer means is operated in an iterative manner causing a predetermined plurality of ink droplets to all be successively ejected from an associated orifice along the same trajectory, within a time period permitting said droplets to merge while air-borne, whereby upon predetermined relative movement between said print head and said print medium a plurality of ink spots are printed upon said print medium for printing desired images of predetermined boldness.

8. A method or apparatus according to Claim 7, characterized in that selected ones of said plurality of transducer means (204) are operated simultaneously for ejecting from each one of said respective orifices (202) a predetermined number of ink droplets to merge while air-borne, thereby controlling the boldness of each ink spot printed on said print medium during each cycle of printing.

9. A method or apparatus according to any one of claims 3 to 8, characterized in that a secondary pulse (21 in Fig. 13) is applied immediately after a given one or ones of said electrical pulses for causing earlier breakoff from said orifice (202) of the ink droplet or droplets associated with said given one or ones of said electrical pulses, relative to the time of breakoff of said droplet or droplets in the absence of said secondary pulse.

10. A method or apparatus according to any one of claims 4 to 9, characterized in that the delay time is controlled between said successive electrical pulses for controlling the boldness of printing.