Method and apparatus for high resolution ink jet printing

Abstract

Method and apparatus for making high resolution color prints using ink jet techniques. The method and apparatus of providing for precise control of the number of droplets of printing fluid which is deposited in a pixel on recording paper. The precise control is achieved by incremental control of the print pulse width or time duration. Further precision can be obtained by synchronizing droplet formation with for example, the leading edge of the print pulse. The number of droplets charged during the time duration of the print pulse are the number of droplets which impinge on the paper and at the pixel location. The number of droplets creates therefore a color density. Use of a plurality of nozzles and ink colors will permit the production of very high resolution and high quality color prints.

Description

FIG. 7 shows another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The methods and apparatus of this invention can be accomplished using electrode systems very different from these systems conventionally used to control the continuous jet as used in drum plotters and as described by Hermanrud & Hertz in Journal of Appl. Photogr. Eng. 5 No. 4 (1979). However, for the sake of simplicity an electrode system used with a single jet, and as shown in FIG. 1 and described in U.S. Pat. No. 3,916,421, included herein by reference thereto, will be used when explaining the invention. Here an ink jet 11 issues under high pressure from the nozzle 1 and breaks up into a train of drops 11b at the point of drop formation 11a inside the control electrode 2. This train of normally uncharged drops 11b travels in a line or along an initial axis towards the recording medium or paper 3 which is mounted on or otherwise affixed to a rotating drum 4 of a drum plotter (not shown). On the way from nozzle 1 toward paper 3, the drops 11b pass a transverse electric field generated between the positively charged high voltage electrode 5 and the lower part 2a of the control electrode 2.

Now, if a positive control voltage is applied to the control electrode 2 via an amplifier 6 while the ink is grounded via the electrode 7, an electric field is established at the point of drop formation causing each of the drops 11c formed at the point of drop formation 11a to be positively charged. Because of the charge, these drops 11c are deflected into the catcher 8 and cannot reach the recording paper 3. When the control voltage is decreased from such a positive level so as to cause all the drops 11c to fall into catcher 8 a lesser positive voltage level is reached which will permit the drops 11c to reach the paper 3. Thus, it is obvious and apparent from FIG. 1 that the length of time during which the signal voltage or print pulse applied to electrode 2 is zero, or less than the point cut off control voltage, determines the number of drops 11c that reach the recording paper 3, i.e., the drops 11c formed during that period of lesser positive voltage are not charged or are not sufficiently charged, as a result of the charge on the droplet directing electrode 5, so that they get deflected into catcher 8.

In an actual embodiment of the invention an ink jet 11 having a diameter of 10 μm and a velocity of 40 meters per second is used. Such a jet will form into approximately 10⁶ drops 11c per second at the point of drop formation 11a. If the drum 4 of the drum plotter rotates with a surface velocity of 3.3 meters per second and the size of a pixel (picture element) is assumed to be 0.1×0.1 millimeters, the time required in order to print a pixel is about 30 microseconds. Since 30 drops are formed during the 30 microsecond interval of time, all of these 30 drops will be deposited within a certain pixel if the control voltage supplied to the control electrode 2 was zero or less than the print cut-off voltage during the 30 microseconds. Thus a maximum color density will be generated in the pixel. If the print pulse duration is shorter than 30 microseconds, less than 30 drops will be deposited in the pixel, and thus the actual number of drops and therefore the color density of the pixel will depend on the duration i.e., the length of the print pulse or control voltage. Thus by controlling the length of the print pulses the density or saturation of the color can be controlled in each pixel of the picture generated on the recording paper 3. Since a plurality, typically 3 or 4 or more jets 1 having different colors can be arranged side by side on the drum plotter apparatus as described by Hermanrud and Hertz in Journal of Appl. Photogr. Eng. 5 No. 4 (1979), full color pictures can be generated where the hue and color density can be varied continuously in each pixel.

While the principle of the invention, i.e., varying the color density in each pixel by controlling the number of drops deposited in each pixel, is simple, its realization meets with many difficulties. First, the drop generation rate has to be as large as possible to provide for the generation of a nearly continuous variation of the color density. A high drop generation rate can be attained by using a small nozzle 1, thus producing a high speed jet 11. For letter sized pictures the nozzle size should be about 10 μm and the speed of the jet 11 should be about 40 meters per second. Further, by stimulating the drop formation process by applying ultrasonic vibrations generated by the oscillator 9 and the transducer 10 to the nozzle 1, as exemplified in FIG. 1, the number of drops formed per second can be increased. A further consequence of the ultrasonic vibrations is that the size of each drop 11c usually becomes relatively constant resulting in a decrease in the deleterious effects of air resistance such as drop merger. In the 10 μm jet described above ultrasonic frequencies between 1 and 1.5 MHz (megahertz) will create 10⁶ to 1.5×10⁶ drops per second. Therefore, it is an advantage to stimulate the jet 11 by ultrasonic vibrations.

One of the difficulties encountered in attempting to create exactly the same color density in each pixel is created by the tendency for drops 11c to merge, or groups of drops to merge due to air resistance encountered on the flight from the point of drop formation 11a toward the recording paper 3. Such merging results in slight misplacement of the drops on the recording paper 3 since the paper is traveling at a near constant rate while the drops 11c would arrive at the paper 3 at an irregular rate. This slight misplacement results in a certain amount of graininess, especially in the lighter shades of a color.

The merging of the drops can be partially counteracted by using a voltage slightly different from zero for the print pulses 20 and 22 as shown in FIGS. 2 and 3. In FIG. 2 the voltage applied to the control electrode 2 is switched between +20 and +150 volts and the number of droplets 11c which will impinge on the paper 3 is directly related to the width of the pulses. If the voltage is a positive 150 volts, the drops 11c are strongly charged positively and essentially all of the drops 11c are therefore deflected into the catcher 8. If, however, during the voltage of the print pulses 20 (the negatively directed portion of the control signal) is only 20 volts the drops 11c are only slightly charged and their deflection in the transverse electrical field created by the as illustrated by arrows 20 in FIG. 6 an air flow having about the same velocity as the jet velocity could be generated close to the jet axis following the jet 11 from the nozzle 1 to the recording paper 3 in FIG. 1, which would also tend to eliminate or substantially reduce the effect of air resistance.

It is obvious that the invention described above can be applied to different electrode systems controlling one or a plurality of ink jets in one or several colors as generally illustrated in FIG. 7. Also it can be used with other ink jet control mechanisms, e.g., drop on demand or thermal ink jets (bubble jets). It can be used for a single or full color image printing by either drum, flat bed or other plotters on any kind of record receiving surface. Further, its use is not limited to the production of pictures and the detailed description of the invention given above is to be regarded as an example only. Having described this invention, it will be apparent to those skilled in the art that various modifications may be made hereto without departing from the spirit and scope of this invention as defined in the appended claims.

Claims

1. In an improved ink jet apparatus, for depositing an amount of a printing fluid of at least one color onto a pixel, said pixel being in a predetermined position on a recording medium;

droplet formation means to form droplets of said fluid said droplets being formed at a drop formation point, said droplet formation means comprising at least one nozzle means to create at least one liquid jet of fluid;

at least one means for charging said droplets substantially at said drop formation point with a predetermined and appropriate voltage level sufficient to reduce droplet merging;

means to control the location of said drop formation point;

a droplet interceptor means comprising at least one droplet directing electrode means and a droplet catcher means, said droplet directing electrode means used for applying a charge of sufficient magnitude to cause those droplets to be intercepted to be deflected into said droplet catcher means;

at least one vibrator means driven by a signal of predetermined amplitude and frequency and disposed relative to said at least one nozzle means to generate substantially uniform sized and uniformly spaced droplets, and amplitude also influencing said drop formation point location;

said improvement comprising in combination:

means for controlling said deposited amount of printing fluid by controlling the number of said droplets, said number being a predetermined number of said droplets, said predetermined number of droplets dependent upon at least one color density to be recorded and controllably deposited within said pixel whereby each such pixel having said predetermined number of droplets therein contributes toward the creation of a specific density of printing fluid.

2. The improved apparatus according to claim 1 wherein said means for controlling is an electrical print pulse having an appropriate predetermined voltage level and a time duration thereby causing said predetermined number of said droplets to impinge on said recording medium.

3. The improved apparatus according to claim 2 wherein said appropriate predetermined voltage level is zero.

4. The improved apparatus according to claim 2 wherein said appropriate predetermined voltage level is about +20 volts.

5. The improved apparatus according to claim 2 wherein said appropriate predetermined voltage level is about -20 volts.

6. The improved apparatus according to claim 2 further comprising means to synchronize said print pulse with the mechanical vibrations generated by said at least one vibrator means.

7. The improved apparatus according to claim 6 further comprising means to vary by increments said print pulse time duration where said time duration increments are in direct relation to the number of said droplets required to create a particular color density and in inverse relation to a drop formation rate.

8. The improved apparatus according to claim 1 further comprising a means for creating an air flow in a direction of travel of said droplets from said nozzle to said recording medium.

9. The improved apparatus according to claim 1 further comprising a means for at least partially evacuating a volume of space surrounding said liquid jet of fluid and said droplets formed therefrom at least from said drop formation point to a location proximate to said predetermined pixel position on said recording medium.

10. In an improved method for depositing an amount of a printing fluid of at least one color onto a pixel said pixel being in a predetermined position on a recording medium;

forming droplets of said fluid at a drop formation point using a droplet formation means comprising at least one nozzle means to create at least one liquid jet of fluid;

charging using at least one droplet charging means said droplets substantially at said drop formation point with a predetermined and appropriate voltage level sufficient to reduce droplet merging;

controlling a location of said drop formation point;

intercepting droplets not being deposited onto said pixel using a droplett interceptor means said interceptor means comprising at least one droplet directing electrode means and a droplet catcher means, said droplet directing electrode means used for applying a charge of sufficient magnitude to cause those droplets to be intercepted to be deflected into said droplet catcher means;

vibrating said droplet formation means using a vibrator means driven by a signal of predetermined amplitude and frequency and disposed relative to said at least one nozzle means to generate substantially uniformly sized and uniformly spaced droplets said amplitude also influencing said drop formation point location;

said improvement comprising in combination:

controlling said deposited amount of printing fluid by controlling the number of said droplets, said number being a predetermined number of said droplets, said predetermined number of droplets dependent upon at least one color density to be recorded and controllably deposited within said pixel whereby each such pixel having said predetermined number of droplets therein contributes toward the creation of a specific density of printing fluid.

11. The improved method according to claim 10 wherein said controlling is by applying an electrical print pulse having an appropriate and predetermined voltage level and a predetermined time duration to said at least one droplet charging means.

12. The improved method according to claim 11 wherein said appropriate and predetermined voltage level is zero.

13. The improved method according to claim 11 wherein said appropriate and predetermined voltage level is about +20 volts.

14. The improved method according to claim 11 wherein said appropriate and predetermined voltage level is about -20 volts.

15. The improved method according to claim 11 further comprising the step of synchronizing by electronic means said print pulse with the mechanical vibrations generated by said at least one vibrator means.

16. The improved method according to claim 15 further comprising the step of varying by increments said print pulse time duration where said time duration increments are in direct relation to the number of said droplets required to create a particular color density and in the inverse relation to a drop formation rate.

17. The improved method according to claim 10 further comprising the step of producing an air flow in a direction of travel of said droplets from said nozzle to said recording medium.

18. The improved method according to claim 10 further comprising the step of at least partially evacuating the air in a space surrounding said liquid jet of fluid and said droplets formed therefrom at least from said droplet formation point to a location proximate to said predetermined pixel position on said recording medium.

19. An improved ink jet printing apparatus for depositing a controlled amount of a printing fluid into each of a plurality of pixel areas on a recording medium, said apparatus comprising:

drop formation means for forming a continuous stream of drops of said fluid from an ink jet by a repetitive drop formation process, said drop formation means including at least one nozzle for forming said stream and vibrator means, responsive to a vibration signal and disposed relative to said nozzle, for controlling said drop formation process so that said drops are formed at a drop formation point at a predetermined, substantially uniform rate "f", "f" being sufficiently large such that the variation in the possible number of drops that can be deposited in each pixel area provides for a nearly continuous variation of possible densities for each of said pixel areas, wherein said drops are of a substantially uniform size and sufficiently small so that each drop, disposed on a recording medium, is undetectable by the unaided eye at a normal viewing distance from said recording medium;

means for providing fluid to said nozzle;

charging means for selectively charging each of the drops;

means for generating said vibration signal;

signal control means, connected to said charging means, for generating (i) a deflection signal for charging said drops to a first magnitude at approximately said drop formation point so as to prevent said drops from reaching said recording medium, and (ii) a print signal for charging said drops to a second magnitude sufficient to permit said drops to reach said medium;

drop interceptor means for intercepting said drops charged to said first magnitude so as to prevent said drops charged to said first magnitudes from reaching said recording medium;

means for providing relative motion between the drops of said stream charged to said second magnitude and said recording medium so that said drops can be precisely placed on said recording medium; and

means for depositing precisely a predetermined number of drops in a pixel by controlling and synchronizing the phase between the formation of the first of said predetermined number of drops and the start of the print signal for charging said predetermined number of drops and by controlling the duration of said print signal to be equal to an integer multiple "n" of the period "1/f" of said vibration signal where the integer multiple "n" equals the predetermined number of drops, wherein said means for depositing includes means for adjusting the phase between the formation of the first of said predetermined number of drops and the start of the print signal corresponding to said predetermined number of drops.

20. The apparatus according to claim 19, wherein said means for depositing further includes a digital signal to pulse duration signal converter for translating digital data into electrical pulses of different fixed durations, said durations being integer multiples "n" of the period "1/f" of said vibration signal.

21. The apparatus according to claim 20, wherein said means for depositing further includes means for generating a time digital signal in response to said relative motion, a source of digital data, said digital data indicative of the number of drops to be deposited in each pixel, said digital signal being operative to initiate the transfer of data associated with each pixel from said source to said converter.

22. The apparatus according to claim 19, wherein said means for depositing further includes means for generating a digital control signal, said digital control signal being operative to set the duration of said print signal associated with each pixel.

23. The apparatus according to claim 19, wherein said means for adjusting the phase between the formation of the first of said predetermined number of drops and the start of the print signal further includes an adjustable signal delay circuit responsive to a timing pulse, said timing pulse being generated by a circuit responsive to said vibration signal.

24. The apparatus according to claim 19, wherein said means for adjusting the phase between the formation of the first said predetermined number of drops and the start of the print signal includes electronic drop formation process sensing means for generating a phase adjustment signal, said phase adjustment signal operative to adjust the start of the print signal relative to the vibration signal of said drop of formation process.

25. The apparatus according to claim 19, wherein said drops are sufficiently small so that pixels of predetermined size and containing a preselected maximum number of said drops can not be resolved by the unaided human eye at a normal viewing distance.

26. The apparatus according to claim 25, wherein the predetermined size of said pixel is on the order of about 0.1 mm×0.1 mm.

27. The apparatus according to claim 25, wherein the preselected maximum number of drops is about 30 drops.

28. The apparatus according to claim 19, wherein the diameter of said ink jet is on the order of about 10 μm.

29. The apparatus according to claim 19, wherein the diameter of said ink jet is on the order of about 3 μm.

30. The apparatus according to claim 19, wherein said stream of drops is formed at a velocity on the order of about 40 meters per second.

31. The apparatus according to claim 19, wherein said stream of drops is formed at a velocity on the order of between about 50 and about 100 meters per second.

32. The apparatus according to claim 19, wherein said vibration signal is on the order of between about 1 MHz and about 1.5 MHz and the rate at which said drops are formed by said drop formation process is on the order of between about 1 MHz and 1.5 MHz.

33. The apparatus to claim 19, further including means for reducing the air resistance encountered by said stream of drops.

34. The apparatus to claim 33, wherein said means for reducing the air resistance includes means for evacuating, at least partially, the space through which said stream of drops travels.

35. The apparatus according to claim 33, wherein said means for reducing the air resistance includes means for creating air flow in the same direction of travel as said stream of drops.

36. The apparatus according to claim 35, wherein the velocity of said air flow is substantially the same as the velocity of said stream of drops.

37. The apparatus according to claim 19, wherein said drop formation means includes a plurality of nozzles for separately forming a corresponding plurality of said streams.

38. The apparatus according to claim 19, wherein said drop formation means includes a plurality of nozzles for separately forming a corresponding plurality of said streams of several colors.

39. A method of ink jet printing for controlling the number of drops from a stream of drops toward a recording medium to be deposited in successive pixel areas on a recording medium said method comprising the steps of:

generating said stream of drops by a drop formation process of repetitively forming said drops at a controlled frequency "f", "f" being sufficiently larger such that the variation in the possible number of drops that can be deposited in each pixel area provides for a nearly continuous variation of possible densities for each of said pixel areas, wherein the drops are of a uniform size and sufficiently small so that each drop, disposed on the recording medium is undetectable by the unaided eye;

selectively generating a print signal for charging selected drops in said stream to a charge level to allow them to reach said recording medium and selectively generating a deflection signal for charging selected drops in said stream to a charge level to prevent them from reaching said medium;

synchronizing said print signal with said drop formation process such that each print signal starts at a selected but adjustable phase relative to said drop formation process; and

controlling the duration of said print signal to be an integral multiple of the period of said drop formation process.

40. The method of claim 39, including the step of adjusting the start of said print signal relative to and in response to said drop formation process.

41. The method of claim 39, further including the step of providing an oscillator signal of a controlled frequency "f" for driving a crystal so that the crystal vibrates so as to control the drop formation process, generating a repetitive clock pulse indicative of a preselected phase of said oscillator signal, and selectively initiating said print signal by adjustably delaying said clock signal.

42. The method of claim 41, wherein said step of adjusting delaying is carried out in response to said drop formation process.

43. A method of ink jet printing in which variable amounts of ink are deposited on subsequent pixel areas of a recording medium, said method comprising the steps of:

a) generating a least one ink jet propagating in a predetermined direction toward said recording medium to a point of drop formation, said ink jet disintegrating at said point of drop formation into a train of drops of a uniform size and sufficiently small so that each drop, disposed in the recording medium is undetectable by the unaided eye at a normal viewing distance from the recording medium;

b) controlling the time and the rate and, thus, the period of the formation of said drops "1/f" where "f" is sufficiently large such that the variation in the possible number of drops that can be deposited in each pixel area provides for a nearly continuous variation of possible densities for each of the pixel areas;

c) producing relative motion between the propagating direction of said jet and said recording medium;

d) producing an electrical deflection field across the path of the drops between said point of drop formation and said recording medium;

e) selectively charging the drops forming at said point of drop formation by an electrical signal containing print pulses, the charge state which the drops obtain during each print pulse allowing the drops having this charge state to proceed through said deflection field to the corresponding pixel area of said recording medium while the drops generated when no print pulse is present obtain another charge state which in cooperation with said deflection field prevents those drops from reaching said recording medium; and

f) controlling the duration of each print pulse as a function of the amount of ink to be deposited on the pixel area corresponding to said print pulse,

wherein the duration of each print pulse is controlled in steps of an integer multiple of said drop formation period "1/f", and the time relation is synchronized between the time drops are formed during the drop formation process and the start of each print pulse.

44. The method as claimed in claim 43, wherein drops charged by the control signal level between said print pulses are prevented from reaching the recording medium and the print pulses comprise a small offset voltage which still allows the drops to reach the recording medium but reduces the tendency of adjacent drops to merge.

45. An ink jet printing apparatus comprising:

a) means for generating at least one ink jet propagating in a predetermined direction toward a recording medium to a point of drop formation, where said ink jet disintegrates into a train of drops of a uniform size and sufficiently small so that each drop disposed in the recording medium is undetectable by the unaided eye at a normal viewing distances from the recording medium;

b) means for controlling the time and the rate and, thus, the period of the formation of said drops "1/f" where "f" is sufficiently large such that the variation in the possible number of drops that can be deposited in each pixel area provides for a perceptible nearly continuous variation of possible densities for each of the pixel areas;

c) means for producing relative motion between said jet propagating direction and said recording medium;

d) means for producing an electrical deflection field across the path of the drops between said point on drop formation and said recording medium;

e) electrode means positioned at said point of drop formation;

f) circuit means for applying an electrical signal containing print pulses to said electrode means for selectively charging the drops formed at said point of drop formation, wherein said print pulses, when present, cause the drops formed by said signal to attain a first charge state which allows the drops to proceed through said deflection field to said recording medium, and when absent, cause the drops formed by said signal to attain a second charge state which in cooperation with said deflection field prevents those drops from reaching said recording medium;

g) means for controlling the duration of each print pulse as a function of the amount of ink to be deposited on a pixel area of said recording medium which corresponds to the respective print pulse; and

h) means for controlling the time relationship between the commencement of each print pulse and the time the drops are formed during said drop formation process; wherein said means for controlling the duration of each print pulse comprises circuit means controlled by the means for controlling the period of drop formation so that the duration of said print pulses is varied by integer multiples of said drop formation period "1/f" and said print pulse is synchronized with respect to the time the drops are formed during the drop formation process.

46. The apparatus as claimed in claim 45, further comprising means for controlling the time relationship between the commencement of each print pulse and the time the drops are formed during said drop formation process.

47. The apparatus as claimed in claim 46, wherein said time relationship control means comprises a delay circuit coupled between said means for controlling the drop formation process and said means for controlling the duration of said print pulses, the delay of said delay circuit being controllable by a control signal.