CA1232490A

CA1232490A - Method for operating an ink jet apparatus

Info

Publication number: CA1232490A
Application number: CA000444199A
Authority: CA
Inventors: Stephen J. Liker
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1982-12-27
Filing date: 1983-12-23
Publication date: 1988-02-09
Also published as: DE3381406D1; EP0115180A3; JPH07108568B2; EP0115180B1; ATE51582T1; EP0115180A2; JPS59133066A

Abstract

ABSTRACT OF THE DISCLOSURE

A method for controlling the volume of ink droplets ejected from a drop on demand ink jet apparatus including the transducer operable for producing a pressure disturbance within an associated ink chamber for ejecting an ink droplet from an associated orifice, the method comprising the steps of operating the trans-ducer in an iterative manner for producing a plurality of successively equal or higher or lower amplitude pressure disturbances within the ink chamber, or some combination thereof, for causing a plurality of succes-sively equal or higher or lower velocity ink droplets, or some combination thereof, to be ejected from the orifice of the ink jet apparatus, within a time period permitting the ink droplets to either merge in flight or at the point of striking a recording medium.

Description

2~9~) _ 2 Field of Invention

3 The field of the present invention relates

4 generally to ink jet apparatus, and more specifically

5 to a method for operating an ink jet apparatus for

6 providing selective control within a range of either the

7 volume of the ink droplets ejected by the apparatus

8 and/or the amount of ink striking a desired point on a

9 recording medium.

The design of practical ink jet devices and 11 apparatus for producing a single droplet of ink on 12 demand is relatively new in the art. In prior drop on 13 demand ink jet apparatus, the volume of each individual 14 ink droplet is typically dependent upon the geometry of 15 the ink jet apparatus, the type of ink used, and the 16 magnitude of the pressure force developed within the ink 17 chamber of the ink jet rejecting an ink droplet from an 18 associated orifice. The effective diameter and design 19 of the orifice, the volume and configuration of the ink 20 chamber associated with the orifice, the transducer 21 design, and the method of coupling the transducer to the 22 ink chamber, are all factors determining the volume of 23 individual ink droplets ejected from the orifice.
24 Typically, once the mechanical design of an ink jet 25 apparatus it frozen, control over the volume of the 26 ejected ink droplets can only be obtained over a narrow 27 range by varying the amplitude of the electrical pulses I or dry voltage applied to the individual transducers of 29 the ink jet apparatus or array.

The present inventor discovered that by 31 operating the transducer of an ink jet in an iterative 32 manner, for causing a plurality of successively higher, I I

~23;~9~

1 lower, or equal velocity ink droplets or some combine-2 lion thereof, to be ejected from the orifice of the ink 3 jet, within a time period permitting the droplets to 4 either merge in flight prior to striking a recording 5 medium, or to each strike the recording medium at the 6 same point, that broader control of the boldness and 7 toning of printing could be obtained. The volume of ink 8 striking a recording medium at a given point is thereby 9 partly determined by the number of ink droplets merged

10 prior to striking or at the point of striking.

11 In the drawing, wherein like items have common

12 reference designations:

13 Figure 1 is a sectional view of an illustrated

14 ink jet apparatus;

Figure 2 is an enlarged view of a portion of 16 the section shown in Figure l;
;

17 Figure 3 is an exploded projectile or pie-18 tonal view of the ink jet apparatus, including the 19 embodiments shown in Figures 1 and 2;

Figure 4 is a partial sectional/schematic 21 diagram view of the transducer shown in Figure l and 3, 22 with the transducer in the de-energized state;

23 Figure 5 is a partial sectional/schematic 24 diagram or view of the transducer of Figure 4 in the 25 energized state;

26 Figure 6 shows the wave shapes for electrical 27 pulses of one embodiment of the invention;

28 Figure 7 shows a typical ejection of an ink 29 droplet from an orifice;

~Z3Z~9C~

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 wave shapes for electrical pulses for another embodiment of the invention;
Figure 12 shows the wave shapes for electrical pulses for yet another embodiment of the invention; and Figures 13, 14 and 15 show wave shapes for other embodiments of the invention.
The resent invention was discovered during development of improved methods for operating the thus-trative~ ink jet apparatus of Canadian Patent No.
1,174,516 which is shown in Figures through S. However, the present inventor believes that the various embody-mints of his invention illustrated and claimed herein are applicable for use with a broad range of ink jet apparatus (especially drop on demand ink jet apparatus).
Accordingly, the ink jet apparatus to be discussed here-in is presented for purposes of illustration of the method of the present invention, 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 the pro-piously mentioned patent for greater details concerning this apparatus. The reference designations used in Figures 1 through 5 are the same as used in the patent, in order to facilitate any referencing back to that ; patent that may issue therefrom.
:
By ~3L232~

With reference to Figures 1 through 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-elas-tic material 208 juxtaposed to the foot 207, and a diaphragm 210 which is reloaded to the position shown in Figures 1 and 2.
Ink flows into the chamber 200 from an unpres-surized reservoir 212 through restricted inlet means provided by a restricted opening 214. The inlet 214 comprises an opening in a restructure 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 Yin-tune of the diaphragm 210, which is in communication with the ink through a large opening 227 in the restructure 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 slide ably retained within the holes 224. The other extremities of each one of the transducers 204 are I

B

~L~329L9~

1 compliantly mounted in a block 228 by means of a come 2 pliant or elastic material 230 such as silicon rubber.
3 The compliant material 230 is located in slots 232 4 (see Fig. 3) so as to provide support for the other extremities of the transducers 204~ Electrical contact 6 with the transducers 204 is also made in a compliant 7 manner by means of a compliant printed circuit 234, 8 which is electrically coupled by suitable means such 9 as solder 236 to an electrode 260 of the transducers 204. Conductive patterns 238 are provided on the 11 printed circuit 234.

12 The plate 226 (see Figures 1 and 3) includes 13 holes 224 at the base of a slot 237 which receive the 14 feet 207 of the transducers 204, as previously men-toned. The plate 226 also includes a receptacle 239 16 for a heater sandwich 240, the latter including a heater 17 element 242 with coils 244, a hold down plate 246, 18 a spring 248 associated with the plate 246, and a 19 support plate 250 located immediately beneath the heater 240. The slot 253 is for receiving a thermistor 252, 21 the latter being used to provide monitoring of the 22 temperature of the heater element 242. The entire 23 heater 240 is maintained within the receptacle in the 24 plate 226 by a cover plate 254.

As shown in Fig. 3, the variously described 26 components of the ink jet apparatus are held together 27 by means of screws 256 which extend upwardly through 28 openings 257, and screws 258 which extend downwardly 29 through openings 259, the latter to hold a printed circuit board 234 in place on the plate 228. The dashed 31 lines in Fig. 1 depict connections 263 to the printed 32 circuits 238 on the printed circuit board 234. The 33 connections 263 connect a controller 261 to the ink jet 34 apparatus, for controlling the operation of the latter.

~32~L9~3 1 The controller 261 is programmed to at an 2 appropriate time, via its connection to the printed 3 circuits 238, apply a voltage to a selected one or ones 4 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 7 transducers 204, causing the transducers 204 to contract 8 along their elongated axis. When a particular trays-9 dicer 204 so contracts upon energization (see Fig. 5), the portion of the diaphragm 210 located below the foot 11 207 of the transducer 204 moves in the direction of 12 the contracting transducer 204, thereby effectively 13 expanding the volume of the associated chamber 200. As 14 the volume of the particular chamber 200 is so expanded, a negative pressure is initially created within the 16 chamber, causing ink therein to tend to move away from 17 the associated orifice 202, while simultaneously per-18 milting ink from the reservoir 212 to flow through the 13 associated restricted opening or inlet 214 into the chamber 200. Given sufficient time, the newly supplied 21 ink completely fills the expanded chamber and orifice, 22 providing a "fill before fire" cycle. Shortly there-23 after, the controller 261 is programmed to remove the 24 voltage or drive signal from the particular one or ones of the selected transducers 204, causing the transducer 26 204 or transducers 204 to return to their deenergized 27 states as shown in Fig. 4. Specifically, the drive 28 signals are terminated in a step like fashion, causing 29 the transducers 204 to very rapidly expand along their elongated axis, whereby via the visco-elastic 31 material 208 the feet 207 of the transducers 204 push 32 against the area of the diaphragm 210 beneath them, 33 causing a rapid contraction or reduction of the volume 34 of the associated chamber or chambers 200. In turn, this rapid reduction in the volume of the associated 36 chambers 200, creates a pressure pulse or positive ~Z32~g~

I pressure disturbance within the chambers 200, causing an 2 ink droplet to be ejected from the associated orifices 3 202. Note that as shown in Figure 5, when a given 4 transducer 204 is so energized, it both contracts or reduces its length and increases its thickness. However, 6 the increase in thickness is of no consequence to the 7 illustrated ink jet apparatus, in that the changes in 8 length of the transducer control the operation of the g individual ink jets of the array. Also note, that with present technology, by energizing the transducers for 11 contraction along their elongated axis, accelerated 12 aging of the transducers 204 is avoided, and in extreme 13 cases, depolarization is also avoided.

14 For purposes of illustration, assume that the pulses shown in Figure 6 are applied via controller 261 16 to one of the transducer 204. As shown the first and 17 second pulses 1 and 3 respectively each have an expo-18 nential leading eye and a substantially linear trailing 19 edge, respectively, peak amplitudes + Al, + V2 volts respectively, and pulse widths of To, To, respectively.
21 Note that the shapes of the pulses 1,3, respectively, 22 may be other than as illustrated herein, depending upon 23 the particular ink jet device being driven and the 24 particular application. In this example, the peak amplitude plus + V2 of pulse 3 is greater than the peak 26 amplitude Al of pulse 1, and the fall time for the 27 trailing edge of pulse 3 is less than the fall time 28 for the trailing edge of pulse 1. Since the degree of 29 contraction of the selected transducer 204 is directly related within a range to the amplitude of the pulse 31 applied to the transducer, the greater the amplitude, 32 the greater the degree of contraction. Accordingly, 33 upon termination of a particular operating or control 34 pulse/ the magnitude of the pressure disturbance pro-duped in the associated chamber 200 will be directly ~Z32~

1 related within a range to the amplitude of the previous-2 lye applied control pulse. Also, the greater the slope 3 or the less the fall time of the trailing edge of the 4 control pulse, the more rapid the expansion or elonga-lion of the selected transducer 204 to its rest state 6 upon termination of the control pulse. Correspondingly, 7 the greater the rate of expansion of the transducer 204, 8 the greater the magnitude of the resulting pressure g disturbance within the associated chamber 200. Assume that the amplitudes + Al and + V2 of pulses 1,3, respect 11 lively, are large enough to ensure ejection of a ink 12 droplet from associated orifice 202 upon termination 13 of these pulses, respectively.

14 With reference to Figure 7, assume that pulse 1 is applied to a selected one of transducers 204. Upon 16 termination of pulse 1, a typical ink droplet 5 will be 17 ejected from the associated orifice 202. Substantially 18 upon the termination of pulse 1, assume that pulse 3 is 19 applied to the selected transducer 204. Shortly after the termination of pulse 3, a second ink droplet 7 will 21 be ejected from the associated orifice 202 as shown in 22 Figure 8, for example. Ink droplet 7 will have a 23 substantially greater velocity than the air-borne ink 24 droplet 5 because the amplitude of pulse 3 is greater of that than pulse 1 and the fall time of pulse 3 is less 26 than that of pulse 1. Note that as previously explained 27 though, the velocity of the second ink droplet 7 will be 28 greater than that of ink droplet 5 so long as at least 29 one of either the amplitude of pulse 3 is greater than that of pulse 1 even if the fall times of these pulses 31 are equal, or the fall time of pulse 3 is less than 32 that of pulse 1 even if their amplitudes are equal.
33 Accordingly, either amplitude control of the control 34 pulses, or trailing edge fall time control of the control pulses or a combination of the two can be used ~;~3Z~9~) g 1 to produce a higher velocity second droplet 7 as thus-2 treated in Figure 8, for example. By properly control-3 lying the pulse parameters, the velocity of the second 4 ink droplet 7 can be made high enough to cause droplet 7 to catch up with droplet 5 while each is air-borne, 6 causing these droplets to begin to merge together as 7 shown in Figure 9. Assuming sufficient flight time, the 8 merger of droplets 5 and 7 may result in a droplet shape 9 as shown in Figure 10 prior to the merged droplets striking a recording medium. Alternatively depending 11 upon the relative speeds (successively higher or lower) 12 of the droplets and movement of the recording media, the 13 droplets can be made to strike the recording media at 14 the same point or spot, without merging while air-borne, thereby obtaining the same result. In this manner, the 16 size of the ink droplet or volume of ink striking a 17 recording media at a particular point is substantially 18 increased relative to using only a single droplet, and 19 such control of the volume of ink directly provides control of the boldness of printing. Typical values for 21 the parameters of pulses 1,3 used by the inventor in 22 conducting his experiments, were 28 volts and 30 volts 23 for Al, V2, respectively; 60 microseconds for each 24 one of the pulse widths To and To; and fall times of 2 microseconds and 1 microsecond for pulses 1,3, respect 26 lively. The viscosity of the ink in this example was 12 27 centipoise. For the particular ink jet device operated 28 by the present inventor, the approximate diameter of 29 droplet 5 was 108 miss, for the second ink droplet 7 was 2.2 miss, and for the merged ink droplet 9 was 4.0 miss.
31 Other ink droplet diameters or volumes may be obtained 32 within a range via control of the amplitudes and fall 33 times of pulses 1 and 3, as previously mentioned.

34 Within a range, control of the size of ink droplets ejected from the ink jet device can be con-3L;~3Z~9~

1 trolled by adjusting the amplitudes and fall times of 2 the control pulses applied to the ink jet device. The 3 range of control of the volume of ink or ultimate ink 4 droplet size striking a recording media is substantially extended via another embodiment of the present invention 6 for merging a plurality of ink droplets in flight or at the point of striking a recording media.

8 In Figure 11, the amplitudes Al, V2 of 9 pulses 11, 13, respectively, are shown to be equal (typically 30 volts, for example). In this example, the 11 trailing edge of pulse 11 is about 10 microseconds 12 in fall time, whereas the trailing edge of pulse 13 has 13 a fall time of about 1 microsecond. Accordingly, the 14 ink droplet resulting from the application of pulse 11 to a selected transducer 204 will have a velocity that 16 is substantially slower than the velocity of the follow-17 in ink droplet resulting from the application of pulse 18 3 to the transducer 204. Accordingly, only fall time 19 control is being used to adjust the velocities of the ink droplets resulting from the application of pulses 1 21 and JO In this example, it is assumed that the second 22 ejected higher velocity ink droplet will merge with the 23 first ejected ink droplet while air-borne or at the 24 point of striking a recording media, as previously described.

26 In Figure 12, a third control or firing pulse 27 15 has been added following the termination of pulse 13.
28 In one experiment with a given ink jet device, the 29 present inventor set the amplitude of pulses 11, 13, 15 all at 30 volts (+ Al, V2 and V3 all equal 30 volts), 31 with pulses 11, 13 and 15 typically having exponential 32 fall times of 10 microseconds, 5 microseconds and 1 33 microsecond, respectively; and pulse widths of 60 34 microseconds, 40 microseconds and 30 microseconds, ~L~32~90 1 respectively, for example. When applied to a selected 2 transducer 204 of the given ink jet device, pulse 11 3 caused a first ink droplet to be ejected, pulse 13 4 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 7 all of these ink droplets were of such relative vowels-8 ties that they merged in flight prior to striking a g recording media. In this manner, an even greater range of control can be obtained for adjusting the size of an 11 ink droplet in an ink jet system. Depending upon the 12 distance of the selected ink jet orifice 202 from 13 the recording medium, the relative speeds of movement of 14 the recording medium and/or the ink jet head, and the design of the particular ink jet device, it is possible 16 that an even greater number of ink droplets can be 17 ejected at correspondingly greater velocities in order 18 to permit merger in flight or at the point of striking, lo providing even greater control of ink droplet size from one marking position to another on a recording medium.

21 Note that in practice, an ink droplet is not 22 ejected immediately after the termination of a portico-23 far firing pulse. For example, if the pulses lo off Figure 6 are applied to a transducer 204 of the ink jet device used by the present inventor in his experiments, 26 an ink droplet 5 is ejected 4 microseconds after the 27 termination of pulse 1, and the second ink droplet is 28 ejected 3 microseconds after the termination of pulse 29 3. The velocity of the first ejected ink droplet was measured to be 3.5 meters per second and of the second 31 ejected ink droplet 5.0 meters per second.

32 With reference to Figure 13, the combination 33 of wave shapes shown cause the ink jet apparatus to emit I two droplets which merge at a common point of striking guy 1 on a print medium to produce dots varying in diameter 2 from 5.3 to 5.6 milliinches, for producing very bold 3 print. Typically, To, To, To, and To are 80, 4, 18 and 4 6 microseconds, respectively, with the amplitudes of pulses 17 and 19 at 110 volts, and pulse 21 at about 6 73 volts, for producing the previous dot diameter range 7 on a particular type of paper (~ammermill XEROCOPY, 8 manufactured by Hammer mill Papers Co., Inc., Erie, PA), g using an ink having a wax base. The type ox paper and ink formulation affects the dot diameter in a given 11 application. Typically, the fall time of pulses 17 and 12 19 are 9 microseconds and 1.0 microseconds, respectively 13 Under the conditions indicated above, shortly after I termination of pulse 17, a first droplet having a velocity ranging from 8 to 10 meters per second was 16 produced. Also, the combination of pulses 19 and 21, 17 caused a second droplet to be produced about 2 micro-18 seconds after the termination of pulse 19. Pulse 21 19 is not of sufficient amplitude to cause a third droplet to be produced, but does cause the second droplet to 21 break off earlier from the orifice of the ink jet rota-22 live to operating without pulse 21. Also, pulse 21 23 permits higher frequency operation of the ink jet 24 apparatus, and reduced ink bobbing problems at the orifice. Using the pulse time periods and amplitudes 26 mentioned above, the velocity of the second droplet is 27 typically 6 to 8 meters per second. The slower velocity 28 of the second droplet relative to the first droplet is 29 caused by the presence of pulse 21. In this example, by increasing the amplitude of pulse 19, the velocity 31 of the second droplet can be increased Also, by 32 varying the delay time To between the termination of 33 pulse 17 and initiation of pulse 19, the boldness can be 34 modulated within a range.

~..23~24~

1 In Figure 14, by using only pulse 17 to 2 operate the ink jet apparatus, dots having a diameter 3 range of 3.3 to 3.5 milliinches can be obtained. Such 4 dot diameters produce much less bold print relative to operating the ink jet apparatus via the combination of 6 pulses 17, 19, and 21.

7 With reference to Figure 15, the combination 8 of pulses 17 and 21, as shown, operated the ink jet for 9 producing ink droplets having diameters ranging from 2.9 to 3.0 milliinches. This combination produces a very 11 light print 12 By using various combinations of the waveforms 13 of Figures 13, 14, and 15, desired shading can be 14 accomplished. Such shading is known as half-toning.
Note that with respect to Figure 13, that although the 16 second droplet is lower in velocity than the first 17 droplet, they are merged at a common point of impact as 18 the point medium.

19 As previously mentioned, depending upon the relative speeds of the ink droplets, the ink jet head, 21 and the recording medium, the droplets can be made to 22 strike the recording medium at substantially the same 23 spot or point, and are thereby merged at that point for I producing a desired dot size. Accordingly, the shapes of the waveforms used to drive the ink jet apparatus can 26 be designed to cause successively produced ink droplets 27 to have successively higher or lower relative velocities, 28 or some combination thereof, so long as system timing 29 permits the droplets to strike the recording medium at substantially the same point. In this manner, one 31 droplet or a plurality of ink droplets can be selective-32 lye chosen for printing a dot of desired boldness at a 33 point on a recording medium ,",' ~z~z~

1 The controller 261 can be provided via hard-2 wired logic, or by a microprocessor programmed for 3 providing the necessary control functions, or by same 4 combination of the two, for example. Note that a Wavetek Model 175 wave shape generator, manufactured by 6 Wavetek~ San Diego, California, was used by the present 7 inventor to obtain the wave shapes shown in Figures 6, 8 11, 12, 13, 14, and 15. In a practical system, a g controller 261 would typically be designed for providing the necessary wave shapes and functions, as previously 11 mentioned, for each particular application.

12 Although particular embodiments of the present 13 inventive method for operating an ink jet apparatus for 14 extending the range of control of the volume of ink or ink droplet diameter striking a recording media at a 16 given point have been shown and described, other embody-17 mints, which fall within the true spirit and scope of 18 the appended claims may occur to those of ordinary skill 19 in the art.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for controlling the volume of ink producing a spot on a recording medium via control of the number of ink droplets ejected from individual orifices in an array of said orifices in a drop-on-demand ink jet apparatus, the apparatus including trans-ducer means operable for producing a pressure distur-bance within an associated ink chamber for ejecting an ink droplet from at least one associated orifice, the method comprising the steps of:
operating said transducer means for producing a first pressure disturbance for ejecting a first ink droplet from said orifice;
operating said transducer means for producing a second pressure disturbance related in amplitude to the amplitude of said first disturbance for ejecting a second ink droplet from said orifice having the same trajectory as said first droplet and having a velocity related to that of said first droplet so as to cause said first and second droplets to strike said recording medium at substantially the same location, thereby pro-ducing a bolder ink spot upon said recording medium than would be obtained using only one of said droplets.

2. The method of claim 1 wherein said second pressure disturbance is selected to have a greater am-plitude than said first so as to cause said first and second droplets to merge in flight.

3. The method of claim 1 wherein said first and second pressure disturbance have amplitudes related to each other so as to cause said first and second drop-lets to merge at said recording medium.

4. The method of claim 2, wherein said trans-ducer means is responsive to an electrical pulse for producing the pressure disturbance within an assoc-iated ink chamber, the magnitude of the pressure dis-turbance being directly proportional to the slope of the trailing edge of said electrical pulse, wherein said operating step further includes the step of:
applying successive electrical pulses having either one of successively greater or reduced or equal trailing edge slopes, or some combination thereof, to said transducer means.

5. The method of claim 4, further including the step of shaping said electrical pulses to have expon-ential leading edges.

6. The method of claim 4, further including the step of shaping the trailing edges of said electrical pulses to be exponential.

7. The method of claim 6, further including the step of adjusting the amplitude of each one of said electrical pulses for obtaining a desired velocity for an associated ink droplet, whereby the magnitude of the pressure disturbances produced by said transducer means are directly proportional to the amplitudes of said electrical pulses, respectively.

8. The method of claim 4, further including the step of shaping the trailing edges of said electrical pulses to be substantially linear.

9. The method of claim 8, further including the step of adjusting the amplitudes of said electrical pulses, whereby the magnitudes of said pressure dis-turbances are directly propoxtional to the amplitudes of said pulses, respectively.

10. The method of claim 7, further including the step of applying a secondary pulse immediately after given ones of said electrical pulses for causing earli-er breakoff from said orifice of the ink droplets associated with said given ones of said electrical pulses, relative to the time of breakoff of said drop-lets absent the use of said secondary pulses.

11. The method of claim 10 further including the step of controlling the delay time between said successive electrical pulses for controlling the bold-ness of printing.

12. A drop-on-demand ink jet printing system comprising an ink jet having an ink cavity, an orifice communicating with said ink cavity, and transducer means in communication with said ink cavity, a source of electrical drive signals to force a single drop of ink from said orifice; the improvement comprising:
means for selectively producing at least one additional electrical drive signal each with a time delay with respect to the immediately preceding elec-trical drive signal, said time delay being short with respect to said drop-on-demand drop production rate;
and means to actuate said transducer means with each of said electrical drive signals to produce a quantity of ink having a predetermined volume from said orifice, said quantities of ink merging into a single drop of ink prior to or at the time the drop reaches the print medium for printing, whereby each ink drop can be produced having a selected one of a plurality of possible drop sizes.

13. The drop-on-demand ink jet printing system of claim 12 in which the cross-sectional dimension of said orifice is within the range of .025 to .075 mm.

14. The method of claim 3, wherein said trans-ducer means is responsive to an electrical pulse for producing the pressure disturbance within an associated ink chamber, the magnitude of the pressure disturb-ance being directly proportional to the scope of the trailing edge of said electrical pulse, wherein said operating step further includes the step of:
applying successive electrical pulses having either one of successively greater or reduced or equal trailing edge slopes, or some combination thereof, to said transducer means.

15. The method of claim 14, further including the step of shaping said electrical pulses to have expon-ential leading edges.

16. The method of claim 14, further including the step of shaping the trailing edges of said electrical pulses to be exponential.

17. The method of claim 16, further including the step of adjusting the amplitude of each one of said electrical pulses for obtaining a desired velocity for an associated ink droplet, whereby the magnitude of the pressure disturbances produced by said transducer means are directly proportional to the amplitudes of said electrical pulses, respectively.

18. The method of claim 14, further including the step of shaping the trailing edges of said electrical pulses to be substantially linear.

19. The method of claim 18, further including the step of adjusting the amplitudes of said electrical pulses, whereby the magnitudes of said pressure dis-turbances are directly proportional to the amplitudes of said pulses, respectively.

I

20. The method of claim 17, further including the step of applying a secondary pulse immediately after given ones of said electrical pulses for causing earlier breakoff from said orifice of the ink droplets assoc-iated with said given ones of said electrical pulses, relative to the time of breakoff of said droplets absent the use of said secondary pulses.

21. The method of claim 20, further including the step of controlling the delay time between said success-ive electrical pulses for controlling the boldness of printing.