CH667044A5 - METHOD FOR SELECTIVE, MULTI-CYCLIC, RESONANCE OPERATION OF AN INK JET DEVICE FOR THE CONTROL OF THE DOT SIZE. - Google Patents

Description

DESCRIPTION

The invention relates to a method according to the preamble of claim 1.

There is a need for a method of operating ink jet devices in a resonance mode to enable high resolution printing.

The implementation of practical ink jet devices and devices that serve to produce a single droplet of ink on demand is relatively new in the art. In the known "drop on demand" ink jet devices, the volume of each individual ink droplet typically depends on the geometry of the ink jet device, the type of ink used and the size of an overpressure developed in the ink chamber for ejecting an ink droplet from an associated opening. The effective diameter and structure of the orifice, the volume and structure of the ink chamber associated with the orifice, the transducer structure, and the method of coupling the transducer to the ink chamber are other factors that determine the volume of the individual ink droplets ejected from the orifice. In any such ink jet device, high resolution imaging requires relatively small or low volume ink droplets to be ejected from the device. Typically, such small size ink droplets are obtained by reducing the diameter of the openings of the ink jet device. However, it is difficult to make small-diameter jet orifices, and operation of an ink jet device equipped with such small-diameter orifices is adversely affected by orifice clogging problems (due to dried ink, impurities in the ink, paper dust, etc.) Effects due to a high ratio of surface tension to inertial forces, poor results, etc. negatively affected.

Many attempts have been made to control the print density as well as the print resolution or halftone in an ink jet printer. For example, in US Pat. No. 3,977,007, gray shading is reproduced in an ink jet printer by selectively changing the number of ink droplets deposited at a predetermined point in a dot matrix by one. U.S. Patent No. 4,018,383 teaches a method of eliminating satellite droplets in a continuous ink jet system, which method further selectively removes or traps the ink droplets during printing to control the density of the droplet streams. A continuous ink jet device disclosed in U.S. Patent No. 4,047,183 uses a laser to control the formation and shape of the ink droplets forming the ink droplet stream to sample the frequency components of a continuous ink jet stream to control the characteristics of a noise control signal controlling the device.

In US Pat. No. 4,281,333, the volume or size of the ink droplets ejected from a drop-on-demand ink jet device is controlled only by varying the amplitude or power envelope of the control signal waveform used to operate the ink jet device. In U.S. Patent 4,337,470, the pact size produced by an ink jet printer is controlled by varying the frequency of vibration of a vibrator which vibrates the ink in the ink head to eject droplets of ink. These droplets are electrostatically directed toward or away from a receiving medium to control the printing density. U.S. Patent No. 4,393,384 teaches a method of operating an ink jet device for the purpose of controlling the volume and speed of ink droplets generated for absolute control of print quality. This control is accomplished by controllably and successively first reducing the volume of the associated ink chamber, then increasing the volume, and then immediately decreasing the volume by an amount less than the first volume reduction, and then the volume of the ink chamber for ejection of

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Ink droplets enlarged. U.S. Patent 4,493,388 discloses a method of operating an ink jet device in which the oscillogram of the signal applied to the transducer has an interruption period which is longer than a predetermined period which is followed by the periods of three successive electrical signals, at least the amplitude or the width of the second of the three electrical signals is increased relative to the other two to prevent a decrease in the radius of the second ink droplet ejected after the interruption portion.

None of the patents briefly described above discloses that to operate an ink jet device, certain resonances of that device are excited to allow control over the size and volume of the ejected ink droplets.

The aim of the invention is to remedy the disadvantages mentioned.

The method according to the invention is characterized by the features of claim 1.

The invention is explained in more detail below with reference to the drawing. It shows:

1 shows an ink jet device in a sectional view;

FIG. 2 shows a detail of the ink jet device according to FIG. 1 in an enlarged view;

3 shows a projection illustration of the ink jet device in an exploded arrangement, the ink jet device comprising the exemplary embodiments shown in FIGS. 1 and 2;

Figure 4 shows the waveform for electrical pulses of a preferred embodiment;

5 shows a sinusoidal waveform for electrical signals of another embodiment of the invention;

6 shows a sine half-wave curve for a third exemplary embodiment of the invention;

Fig. 7 is a sine quarter wave curve for electrical pulses of a fourth embodiment of the invention;

8 is a sawtooth waveform for a fifth embodiment of the invention;

Fig. 9 is a triangular waveform for electrical pulses of a sixth embodiment of the invention;

Fig. 10 prints (A) to (F) obtained from the shown ink jet device using the method of the present invention;

11 are printouts (A) to (C) which illustrate the typical printout density control which can be achieved by using the shown inkjet device using the method according to the invention, and

12 shows droplets generated during the flight using the method according to the invention.

1-3, an ink jet device of U.S. Patent 4,459,601 is shown, which is incorporated by reference into this specification. The present invention was made while developing improved methods of operating the aforementioned ink jet device to achieve a high resolution printing process. However, the various exemplary embodiments of the invention, which are described below, can be used in a large number of inkjet devices (in particular in the case of “drop-on-demand” inkjet devices). The ink jet device explained below is therefore intended to explain the invention and does not imply any limitation. Accordingly, only the basic mechanical features and the operation of this device are discussed in the following paragraphs.

The ink jet device shown in FIGS. 1 through 3 includes a chamber 200 having an opening 202 for ejecting ink droplets in response to excitation of a transducer, with a transducer 204 provided for each jet of a series of such jets (see FIG. 3) is. The transducer 204 extends along its axis of extension and contracts along its axis of extension (in the directions shown by the arrows in FIG. 2), this movement being coupled to the chamber 200 via a coupling device 206. The coupling device 206 comprises a foot 207, a viscoelastic material 208 adjoining the foot 207 and a membrane 210 which is prestressed in the position shown in FIGS. 1 and 2.

Ink flows from a non-pressurized container 212 into chamber 200 through a restricted inlet means in the form of a restricted opening 214. The restricted opening 214 is formed by an opening provided in a flow restriction plate 216 (see FIG. 3). As can be seen from FIG. 2, the container 212 formed in a chamber plate 220 has a bevelled edge 222 which leads into the opening 214. As can be seen from FIG. 3, the container 212 is equipped with an inlet pipe 223 and an outlet pipe 225. The container 212 is flexible due to the membrane 210. This membrane 210 communicates with the ink via a large opening 227 provided in the flow restriction plate 216, which adjoins a recess 229 provided in the plate 226.

One end of each transducer 204 is guided by the cooperation of the foot 207 with a hole 224 provided in a plate 226. As shown, the feet 207 of the transducers 204 are slidably received in the holes 224. The other ends of transducers 204 are elastically secured in a block 228 using a resilient or resilient material 230 provided in slots 232 to provide support for the other ends of transducers 204. Also, electrical contact with transducers 204 is made resiliently by means of a resilient circuit board 234 which can be secured by a suitable means, e.g. a solder, is coupled to an electrode 260 of the transducer. Printed circuit boards 238 are provided on circuit board 234.

The plate 226 (see FIGS. 1 and 3) has holes 224 at the base of a slot 237 which, as mentioned above, receive the feet 207 of the transducers 204. The plate 226 is also equipped with a receptacle 239 for an intermediate heating layer 240. This intermediate heating layer 240 comprises a heating element 242 with spirals 244, a hold-down plate 246, a spring 248 assigned to the plate 246 and a support plate 250 arranged directly below the heating element 242. Furthermore, a slot 253 is provided for receiving a thermistor 252. Thermistor 252 is used to monitor the temperature of heating element 242. The entire heating intermediate layer 240 is held by a cover plate 254 in the receptacle 239 provided in the plate 226.

As can be seen in Figure 3, the various components of the ink jet device described are held together by means of screws 256 which extend upwardly through openings 257 and by means of screws 258 which extend downwardly through openings 259, the screws 258 hold circuit board 234 of plate 228 in place. The dashed lines in FIG. 1 show the connections 263 to the printed circuit 238 on the printed circuit board 234. The connections 263 couple a control device 261 to the ink jet device in order to control its operation.

In normal operation of the inkjet device, controller 261 is programmed to supply voltage to a particular or some of heated electrodes 260 of transducers 204 at a suitable time via its connection to printed circuit 238. The supplied voltage creates an electric field that is generated perpendicular to the extension axis of the selected transducers 204, causing the transducers 204 to contract along their elongated axis. When a particular transducer 204 contracts after excitation, the portion of the membrane 210 located under the foot 207 of the transducer 204 moves toward the contracting transducer 204, effectively increasing the volume of the attached chamber 200. As the volume of the particular chamber 200 expands in this way, a vacuum is initially created in the chamber, causing the therein

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Any ink present tends to move away from the opening 202, while at the same time ink can flow from the container 212 into the chamber 200 via the attached, restricted opening 214. The amount of ink flowing into the chamber 200 during refill is greater than the amount that flows back through the restricted opening 214 while the ink is being fired. The time between refilling and firing is not changed during blasting, which means that a «refilling before firing» cycle is provided. The controller is programmed to shortly thereafter remove the voltage or control signal from a particular one or some of the selected transducers 204, causing the transducer (s) 204 to expand very quickly along their elongated axes and across the viscoelastic material 208 and the feet 207 butt against the part of the membrane 210 below them. In this way, a rapid contraction or reduction in the volume of the associated chambers or chambers 200 is achieved. This rapid reduction in volume of the attached chambers 200 generates a pressure pulse or an excess pressure disturbance in the chamber 200, as a result of which an ink droplet is expelled from the associated openings 202. It should be noted that with such excitation of a particular transducer 204, it will both decrease in length and increase in thickness. However, the increase in its thickness is irrelevant for the illustrated ink jet device in that the changes in length of the transducer control the use of the individual ink jets in the ink jet row. It should also be noted that with the present technology, i.e. by exciting the transducer for the purpose of contraction along its elongated axes, accelerated aging of the transducers is prevented. In extreme cases, depolarization is also avoided.

As mentioned above, it is known that the droplet size produced by a pulse ink jet printer is closely related to the size of the opening of the associated ink jet device and that generally there are only slight variations in droplet size, e.g. by varying the control signal amplitude or control signal waveform. On the other hand, however, the droplet size must be controllable in a wide range for high-quality halftone printing. It is also for certain inks that do not spread widely on paper, such as Wax-based inks require larger ink droplets to be produced than the current ink jet device methods to achieve desired dot diameters.

According to the invention, it was recognized during operation of the ink jet device described above that by synchronously exciting one or a combination of the fluidic and mechanical resonance frequencies of the ink jet device to produce a dominant resonance frequency disturbance in the corresponding associated chamber and the ink, and by generating a single-cycle or subharmonic generation Cycle of the dominant resonance frequency can control the volume of the ejected ink droplets. According to the invention, it was further found that by repeating this mode of operation in an iterative or successive manner, with each subsequent cycle being synchronous with the dominant resonance frequency of the ink jet device, a large number of ink droplets can be ejected within a period of time which enables the droplets can fuse during their flight through the air or on the recording medium. This allows substantial control over the dot size resulting on the recording medium relative to the dot size achieved by a single ink droplet. The resulting point size depends on how often the method according to the invention for operating the device is repeated within a given time period. FIG. 12 shows nine droplets 301 to 309 in flight to create a dot on a recording medium using the method according to the invention.

According to the invention, it was also recognized that for the illustrative ink jet device of this example, the Helmholtz resonance frequency is the dominant resonance frequency of the ink jet device in question. Other ink jet devices that can also be operated using the method according to the invention can have a different resonance frequency than the Helmholtz resonance frequency as the dominant resonance frequency. For the purpose of further describing and clarifying the method according to the invention for operating an inkjet device, it is assumed that the Helmholtz resonance frequency is the dominant resonance frequency. However, such an assumption is not a limitation on the scope and use of the invention.

The present method is a multi-pulse method of operating an ink jet device that uses the dominant resonant frequency of the ink jet device to excite controllable volume ink droplets by pulsing transducer 204 (in this example) with a repetition frequency of the dominant resonant frequency using one single pulse or a large number of pulses depending on the required point size. Where the Helmholtz frequency is the dominant frequency, this frequency results from the interaction of the compliance of the ink chamber 200 (in this example) and the ink or fluid inertia, expressed by the formula where C is the ink chamber compliance, L is the inertia and 1 / L is, for example, [1 / L opening 202 + 1 / L limited opening 214].

Laboratory tests and analysis have shown that the inkjet device shown has a Helmholtz frequency of approximately 30 kHz. As can be seen from FIG. 4, essentially rectangular or square wave pulses were used to operate the ink jet device shown in accordance with the method according to the invention. The pulse characteristics of this particular waveform, which were found to provide substantial control over the size of the ejected ink droplet, were determined for the various time periods shown as follows: Ti = 1.0 microsecond pulse rise time, T2 = 13.0 microsecond pulse duration, T3 = 1.0 microsecond pulse decay time and T4 = 15.0 microsecond dead time, whereby a pulse repetition rate close to the 30 kHz Helmholtz resonance frequency of the device shown is provided. Note that in this example, dead time T4 is required to make the control signal supplied to a transducer in phase with the natural vibration of the ink fluid contained in ink chamber 200. It was found that by supplying two pulses, as shown in Fig. 4, to a transducer 204, the volume of the final ink droplet ejected was approximately twice that obtained using a single pulse over the same period in which the two impulses were supplied. It has also been found that the droplet volume appears to increase linearly in direct accordance with the number of such pulses applied to transducer 204. By supplying two or more pulses of appropriate amplitude having the waveforms shown in Fig. 4 and the characteristics described above, it was further determined that this multi-pulse method results in the ink droplets fusing in flight or upon hitting the recording medium, resulting in results in an increased dot size on the recording medium compared to using a single pulse to create such a dot on the medium.

It should be noted that the waveform of FIG. 4 and the waveforms of FIGS. 5 to 9 described below are under laboratory conditions.

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Test conditions can be generated with a commercially available wave generator. For example, in a practical device, controller 261 must be specially designed or programmed to produce the desired waveforms and the number of pulses required to generate a particular spot size on the recording medium.

Tests performed have shown that the illustrated device, which, as mentioned above, has a Helmholtz frequency of 30 kHz using any combination of pulse duration T2 and dead time T4, which is between 8.0 microseconds and 16.0 microseconds can move, can work,

the rise time Ti and the fall time T3 being set to, for example, one microsecond. The lower limit of this range is determined by the response time of transducer or transducers 204, whereas the upper limit of this range is determined by the design of the inkjet device, the effectiveness of the control or operation of the device at or near its Helmholtz frequency limited. The complexity of the electronic structure of the control device 261 is reduced if the waveform of the control pulses, like that in FIG. 4, essentially corresponds to that shown, the total pulse duration (Tj + T2 + T3) and the dead time T4 being essentially the same. Optimal operation of the ink jet device shown was also achieved if the total periodicity of the pulse train (Tj + T2 + T3 + T4) was made substantially equal to the reciprocal of the dominant resonance frequency, in this example 1 / FH. It has also been found that due to the response time constraints of transducers 204 coupled with the relatively high frequency of the dominant resonant frequency mode of control and operation of the ink jet device using the multi-pulse method of the present invention, many other different waveforms other than those of the 4 are, but have the same or similar periodicity, can be used. Other waveforms that have been found to provide satisfactory point size control using the method of the present invention include, for example, a sine waveform, a sine half waveform, a quarter sine waveform, a sawtooth waveform, and a triangular waveform as shown in FIG 5 to 9 is shown in this order. When using such alternative waveforms to operate the device shown, the 20 kHz Helmholtz frequency of the device was determined as the dominant frequency, as mentioned above. Accordingly, 5% Ts can be made substantially equal to 30 kHz for the sinusoidal waveform of FIG. Similarly, for the sine half-wave curve of FIG. 6, the pulse duration T6 and the dead time T should be approximately 15 microseconds. Similar remarks apply to the pulse durations T8, T10 and Ti2 of FIGS. 7 to 9 and the dead times T9, Ttl and T13 of FIGS. 7 to 9.

When testing various pulse or waveforms, it was found that the rectangular or square waveform with a much lower voltage pulse amplitude can be used than any other waveforms tested, e.g. 5 to 9, probably due to the faster rise and fall times. In fact, it has been found that the sine quarter wave curve of Fig. 7 requires 20% greater amplitude pulses than the substantially square or rectangular pulses of Fig. 4 in order to obtain equivalent printing from the illustrated ink jet device. As mentioned above, in general, the waveform of FIG. 4 relative to the other waveforms of FIGS. 5 through 9 and relative to still other different waveforms is much easier to do electronically.

During testing of the method according to the invention and during operation of the inkjet device shown, it was also discovered that, due to the dominance of the Helmholtz frequency in the device under test, the multi-pulse method according to the invention can also be provided by changing the periodicity of the

Control impulses are based on subharmonic cycles of the Helmholtz frequency. It is believed that the same result for the dominant resonance frequency would be obtained from some other ink jet devices if tested using the method of the invention. However, using the example of a 30 kHz main Helmholtz frequency in a particular ink jet device, a subharmonic frequency would result in control pulse widths that would be very large, resulting in an undesirable reduction in the usable pressure frequency of the particular device or ink jet device. Accordingly, the inventors tested an ink jet device that is similar to the device shown, but has a smaller ink chamber 200 (relatively low compliance) to provide a Helmholtz resonance frequency of about 100 kHz. With the aid of the method according to the invention, this device could be operated at a satisfactory pressure, specifically using multiple pulses with a periodicity of 30 microseconds, which corresponds to the third subharmonic of the 100 kHz Helmholtz resonance or main frequency. Multiple pulses that have a periodicity that has been made subharmonic to 100 kHz, e.g. of 20 kHz were tested, but the performance at this subharmonic level was found to be relatively poor.

As shown in Fig. 10, tapes of successive dots were printed by using a continuously larger number of multiple pulses to print each dot in the corresponding tapes of views (A) to (F). The multiple pulses used to generate the point bands in FIG. 10 were four-part sine waves according to FIG. 7, the pulse durations Ts and the dead times T9 each being 15 microseconds. The voltage amplitude of the pulses was kept constant at about 33 volts. In the point band of view (A), only such a pulse was used to achieve the points shown. The points of the band shown in view (B) were generated during the same cycle time as those in view (A), but two multiple pulses were used instead of one pulse to generate each point of the band shown in view (B). Similarly, the dots of the tapes shown in views (C) through (F) were generated using 3, 4, 5 and 6 multiple pulses, respectively, using the same time cycle for printing each dot. As expected, the bands of views (A) to (F) are consequently fatter due to the continuously larger dot size obtained with the aid of the method according to the invention.

Similar multiple pulses were used to produce the font sets shown in views (A) through (C) of Fig. 11 with increasing fatness. The characters printed in view (A) required a control pulse to generate each of the dots that make up a particular character, whereas two pulses were used to generate each of the individual dots in the font of view (B) and three pulses to generate each of the individual dots that form the characters of view (C) were used.

The résumé of the operation of the invention is that in the operation of an ink jet device for creating a printing dot on a recording medium, a certain period of time is allocated, depending on the ink jet printing system, in order to provide the ink droplet or the ink droplets for printing the dot on the recording medium deliver. The fatness of a particular dot can be controlled by controlling the volume or number of ink droplets produced by the ink jet device during the allotted time to create that dot. The invention provides a method for operating an inkjet device, which generates a control pulse or a plurality of control pulses for operating the device in order to generate ink droplets with known ink volumes

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during a given dot generation time by carefully controlling the shape and periodicity of the control pulses used, thereby making the periodicity of the control pulses used substantially equal to the dominant resonance frequency of the ink jet device.

The controller 261 can perform e.g. with hard-wired logic, or with a programmed microprocessor, or a combination of both. To create the waveforms shown in FIGS. 4 through 9, the inventors used a Wavetek waveform generator, Model 175, manufactured by Wavetek, San Diego, California. In a practical embodiment of the device, controller 261 would typically be designed to provide the required waveforms and functions, as discussed above, for each particular application.

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Claims (7)

667 044 2nd PATENT CLAIMS A method of operating an ink jet device to control the size of the ink dot printed on a recording medium, the ink jet device comprising transducer means for generating a positive pressure disturbance in an associated chamber filled with ink to eject a droplet of ink from an associated orifice, characterized in that one (1) actuates the transducer means to synchronously excite one or a combination of the fluid or mechanical resonance frequencies of the ink jet device to produce a dominant resonance frequency in the chamber and in the associated ink, and (2) generate either a single cycle or a subharmonic cycle of the dominant resonance frequency to substantially predictably control the volume of an ink droplet ejected from the orifice due to the resulting pressure disturbance generated in the chamber, and (3) repeating steps (1) and (2) continuously and in synchronism with the dominant resonance frequency as often as desired to produce a plurality of ink droplets within a period of time that allows the droplets to be in the air or on can fuse with the recording medium, and thus controllably increase the dot size resulting on the recording medium relative to the dot size achieved by a single ink droplet. 2. The method according to claim 1, characterized in that the converter device for generating the pressure disturbance is responsive to an electrical signal, wherein in step (1) the period of the electrical signal is either substantially equal to the period of the Helmholtz resonance frequency or substantially the same the period of a subharmonic of the Helmholtz frequency and supplies the electrical signal to the converter device. 3. The method according to claim 2, characterized in that the electrical signal is shaped such that it essentially represents a pulse with an exponential leading edge and a stepped trailing edge. 4. The method according to claim 1, characterized in that the converter device for generating the pressure disturbance responds to an electrical signal, wherein in step (1) the electrical signal is also essentially a square wave, a rectangular wave, a triangular wave, one Sine half waveform, a full sine waveform, a quarter sine waveform, a sine waveform with less than a quarter wave or a pulse with an exponential leading edge and a step-shaped trailing edge and feeds this electrical signal to the converter. 5. The method according to claim 4, characterized in that one makes the period of the electrical signal either equal to a period of the selected fluidic and mechanical resonance frequencies of the ink jet device or equal to a combination of the periods of the selected fluidic and mechanical resonance frequencies of the ink jet device. 6. The method according to claim 5, characterized in that one further selectively controls the electrical signal to “ON” and “OFF” to control the dot size of each individual ink droplet. 7. The method according to claim 6, characterized in that the period of the electrical signal is substantially equal to the period of the Helmholtz resonance frequency of the ink jet device or the period of a subharmonic of the Helmholtz frequency.