Patent Grant
8696094
Reference is made to commonly-assigned, U.S. patent application Ser. No. 13/530,161, entitled “CONTROLLING DROP CHARGE USING DROP MERGING DURING PRINTING”, filed Jun. 22, 2012.
This invention relates generally to the field of digitally controlled printing systems, and in particular to continuous printing systems in which a liquid stream breaks into drops some of which are electrostatically deflected.
Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet (DOD) or continuous ink jet (CIJ).
The first technology, “drop-on-demand” ink jet printing, provides ink drops that impact upon a recording surface using a pressurization actuator, for example, a thermal, piezoelectric, or electrostatic actuator. One commonly practiced drop-on-demand technology uses thermal actuation to eject ink drops from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink drop. This form of inkjet is commonly termed “thermal ink jet (TIJ).”
The second technology commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source to produce a continuous liquid jet stream of ink by forcing ink, under pressure, through a nozzle. The stream of ink is perturbed in a manner such that the liquid jet breaks up into drops of ink in a predictable manner. Printing occurs through the selective deflecting and catching of undesired ink drops. Various approaches for selectively deflecting drops have been developed including electrostatic deflection, air deflection, and thermal deflection mechanisms.
One well-known problem with any type inkjet printer, whether drop-on-demand or continuous ink jet, relates to the accuracy of dot positioning. As is well-known in the art of inkjet printing, one or more drops are generally desired to be placed within pixel areas (pixels) on the receiver, the pixel areas corresponding, for example, to pixels of information comprising digital images. Generally, these pixel areas comprise either a real or a hypothetical array of squares or rectangles on the receiver, and printer drops are intended to be placed in desired locations within each pixel, for example in the center of each pixel area, for simple printing schemes, or, alternatively, in multiple precise locations within each pixel areas to achieve half-toning. If the placement of the drop is incorrect and/or their placement cannot be controlled to achieve the desired placement within each pixel area, image artifacts may occur, particularly if similar types of deviations from desired locations are repeated on adjacent pixel areas.
In a first electrostatic deflection based CIJ approach, the liquid jet stream is perturbed in some fashion causing it to break up into uniformly sized drops at a nominally constant distance, the break off length, from the nozzle. A charging electrode structure is positioned at the nominally constant break off point so as to induce a data-dependent amount of electrical charge on the drop at the moment of break off. The charged drops are then directed through a fixed electrostatic field region causing each droplet to deflect proportionately to its charge. The charge levels established at the break off point thereby cause drops to travel to a specific location on a recording medium or to a gutter for collection and recirculation. This approach is disclosed by R. Sweet in U.S. Pat. No. 3,596,275, issued Jul. 27, 1971, Sweet '275 hereinafter. The CIJ apparatus disclosed by Sweet '275 consisted of a single jet, i.e. a single drop generation liquid chamber and a single nozzle structure. A disclosure of a multi-jet CIJ printhead version utilizing this approach has also been made by Sweet et al. in U.S. Pat. No. 3,373,437 issued Mar. 12, 1968, Sweet '437 hereinafter. Sweet '437 discloses a CIJ printhead having a common drop generator chamber that communicates with a row (an array) of drop emitting nozzles each with its own charging electrode. This approach requires that each nozzle have its own charging electrode, with each of the individual electrodes being supplied with an electric waveform that depends on the image data to be printed. This requirement for individually addressable charge electrodes places limits on the fundamental nozzle spacing and therefore on the resolution of the printing system.
One known problem with these conventional CIJ printers is variation in the charge on the print drops caused by image data-dependent electrostatic fields from neighboring charged drops in the vicinity of jet break off and electrostatic fields from adjacent electrodes associated with neighboring jets. These input image data dependent variations are referred as electrostatic cross talk. Katerberg disclosed a method to reduce the cross-talk interactions from neighboring charged drops by providing guard gutter drops between adjacent print drops from the same jet in U.S. Pat. No. 4,613,871. However, electrostatic cross talk from neighboring electrodes limits the minimum spacing between adjacent electrodes and therefore resolution of the printed image. Thus, the requirement for individually addressable charge electrodes in traditional electrostatic CIJ printers places limits on the fundamental nozzle spacing and therefore on the resolution of the printing system. A number of alternative methods have been disclosed to overcome the limitation on nozzle spacing by use of an array of individually addressable nozzles in a nozzle array and one or more common charge electrodes at constant potentials. This is accomplished by controlling the jet break off length as described by Vago et al. in U.S. Pat. No. 6,273,559 and by B. Barbet and P. Henon in U.S. Pat. No. 7,192,121. T. Yamada disclosed a method of printing using a charge electrode at constant potential based on drop volume in U.S. Pat. No. 4,068,241. B. Barbet in U.S. Pat. No. 7,712,879 disclosed an electrostatic charging and deflection mechanism based on break off length and drop size using common charge electrodes at constant potentials. M. Piatt and R. Fagerquist in U.S. Pat. No. 7,938,516 disclosed an approach to produce selective charging and deflection of droplets formed at different phases (time) of a common charge electrode.
Other known problems with electrostatic deflection based CIJ printing systems include electrostatic interactions between adjacent drops which cause alterations of their in-flight paths and result in degraded print quality and drop registration. P. Ruscitto in U.S. Pat. No. 4,054,882 described a method of non sequential printing of ink drops issuing sequentially from a nozzle so that drops issuing sequentially from the nozzle are never printed adjacent to one another. This is done by applying multiple voltage states to deflection electrodes in sequence and requires different voltage state waveforms dependent on the image sequence to be printed. V. Bischoff et al. in U.S. Pat. No. 3,827,057 and J. Zaretsky in U.S. Pat. No. 3,946,399 described arrangements for compensating the charge to be applied to a drop being formed to correct for the effects of the charge on the drop which was just previously formed by altering the voltage applied during formation of the present drop.
High speed and high quality inkjet printing requires that closely spaced drops of relatively small volumes are accurately directed to the receiving medium. Since ink drops are usually charged there are drop to drop interactions between adjacent drops from adjacent nozzles in a CIJ printer. These interactions can adversely affect drop placement and print quality. In electrostatic based CIJ printer systems using high density nozzle arrays the main source of drop placement error on a receiver is due to electrostatic interactions between adjacent charged print drops.
As the pattern of drops traverse from the printhead to the receiving medium (throw distance), through an electrostatic deflection zone, the relative spacing between the drops progressively changes depending on the print drop pattern. When closely spaced print drops from adjacent nozzles are similarly charged while traveling in air, electrostatic interactions will cause the spacing of these adjacent neighboring print drops to increase as the print drops travel toward the receiving medium. This results in printing errors which are observed as a spreading of the intended printed liquid pattern in an outward direction and are termed “splay” errors or cross-track drop placement errors herein. Since splay errors increase with increasing throw distance it is required that the throw distance be as short as possible which adversely affects print margin defined as the separation between print drops and gutter drops.
As such, there is an ongoing need to provide a high print resolution continuous inkjet printing system that electrostatically deflects selected drops using an individually addressable nozzle array and a common charge electrode with reduced drop placement errors caused by electrostatic interactions having a simplified design, improved print image quality and improved print margin.
It is an object of the invention to overcome at least one of the deficiencies described above by using mass charging and electrostatic deflection with a CMOS-MEMS printhead to create high resolution high quality prints while maintaining or improving drop placement accuracy and minimizing drop volume variation of printed drops.
The present invention improves CIJ printing by decreasing drop to drop electrostatic interactions, thus resulting in improved drop placement accuracy over previous CIJ printing systems. When two adjacent drops having opposite charge states on them are combined to form a print drop the combined charge will be lower on the print drops and close to 0 which will effectively remove most of the electrostatic interactions between adjacent print drops. The present invention also reduces the complexity of control signals sent to stimulation devices associated with nozzles of the nozzle array. This helps to reduce the complexity of charge electrode structures and enables using increased spacing between the charge electrode structures and the nozzles. The present invention also allows for longer throw distances by lowering the electrostatic interactions between adjacent print drops.
According to an aspect of the present invention, a method of ejecting liquid drops includes providing liquid under pressure sufficient to eject a liquid jet through a nozzle of a liquid chamber; input image data; a drop formation device; a velocity modulation device; a charging device; and a deflection device. The charging device includes a charge electrode associated with the liquid jet and a source of varying electrical potential between the charge electrode and the liquid jet. The drop formation device is associated with the liquid jet. A first catcher and a second catcher are provided. The first catcher is located on a first side of the liquid jet and the second catcher is located on a second side of the liquid jet.
The liquid jet is modulated using the drop formation device to cause portions of the liquid jet to break off into one or more clusters of drops traveling along a path with each cluster of drops being separated on average by a drop cluster period and each cluster of drops including a first drop and a second drop. A charging waveform is provided to the charge electrode of the charging device using the source of varying electrical potential of the charging device. The charging waveform includes a first voltage state and a second voltage state having opposing polarities when compared to each other; has a period that is equal to the drop cluster period; and is independent of the input image data. The charging device and the drop formation device are synchronized with each other to produce a first charge state on the first drop of each drop cluster and produce a second charge state on the second drop of each drop cluster. The first drop having a first charge state is caused to be deflected away from the path and toward the first catcher using the deflection device and the second drop having a second charge state is caused to be deflected away from the path and toward the second catcher using the deflection device. A relative velocity of the first drop and the second drop of a selected drop cluster is modulated using the velocity modulation device in response to input print data to cause the first drop and the second drop to form a merged drop traveling along the path. The merged drop has a third charge state that prevents the merged drop from being deflected to the first catcher by the deflection device and prevents the merged drop from being deflected to the second catcher by the deflection device.
According to another aspect of the present invention, a continuous liquid ejection system includes a liquid chamber in fluidic communication with a nozzle, the liquid chamber containing liquid under pressure sufficient to eject a liquid jet through the nozzle and a processor is configured to provide input image data. A drop formation device, associated with the liquid jet, modulates the liquid jet to cause portions of the liquid jet to break off into one or more clusters of drops traveling along a path with each cluster of drops separated on average by a drop cluster period and each cluster of drops including a first drop and a second drop. A charging device includes a charge electrode associated with the liquid jet; and a source of varying electrical potential between the charge electrode and the liquid jet that provides a charging waveform to the charge electrode. The charging waveform includes a first voltage state and a second voltage state having opposing polarities when compared to each other and a period that is equal to the drop cluster period. The charging waveform is independent of the input image data. The charging device and the drop formation device are synchronized to produce a first charge state on the first drop of each drop cluster and produce a second charge state on the second drop of each drop cluster.
A first catcher is located on a first side of the liquid jet and a second catcher located on a second side of the liquid jet. A deflection device causes the first drop having a first charge state to be deflected away from the path and toward the first catcher and the second drop having a second charge state being deflected away from the path and toward the second catcher. A velocity modulation device modulates a relative velocity of the first drop and the second drop of a selected drop cluster in response to input print data to cause the first drop and the second drop to form a merged drop traveling along the path. The merged drop includes a third charge state that prevents the merged drop from being deflected to the first catcher by the deflection device and prevents the merged drop from being deflected to the second catcher by the deflection device.
In the detailed description of the example embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements.
The example embodiments of the present invention are illustrated schematically and not to scale for the sake of clarity. One of the ordinary skills in the art will be able to readily determine the specific size and interconnections of the elements of the example embodiments of the present invention.
As described herein, example embodiments of the present invention provide a printhead or printhead components typically used in inkjet printing systems. In such systems, the liquid is an ink for printing on a recording media. However, other applications are emerging, which use inkjet print heads to emit liquids (other than inks) that need to be finely metered and be deposited with high spatial resolution. As such, as described herein, the terms “liquid” and “ink” refer to any material that can be ejected by the printhead or printhead components described below.
Continuous ink jet (CIJ) drop generators rely on the physics of an unconstrained fluid jet, first analyzed in two dimensions by F. R. S. (Lord) Rayleigh, “Instability of Jets,” Proc. London Math. Soc. 10 (4), published in 1878. Lord Rayleigh's analysis showed that liquid under pressure, P, will stream out of a hole, the nozzle, forming a liquid jet of diameter dj, moving at a velocity vj. The jet diameter dj is approximately equal to the effective nozzle diameter dn and the jet velocity is proportional to the square root of the reservoir pressure P. Rayleigh's analysis showed that the jet will naturally break up into drops of varying sizes based on surface waves that have wavelengths λ longer than πdj, i.e. λ≧πdj. Rayleigh's analysis also showed that particular surface wavelengths would become dominate if initiated at a large enough magnitude, thereby “stimulating” the jet to produce mono-sized drops. Continuous ink jet (CIJ) drop generators employ a periodic physical process, a so-called “perturbation” or “stimulation” that has the effect of establishing a particular, dominate surface wave on the jet. The stimulation results in the break off of the jet into mono-sized drops synchronized to the fundamental frequency of the perturbation. It has been shown that the maximum efficiency of jet break off occurs at an optimum frequency Fopt which results in the shortest time to break off. At the optimum frequency Fopt (optimum Rayleigh frequency) the perturbation wavelength λ is approximately equal to 4.5 dj. The frequency at which the perturbation wavelength λ is equal to πdj is called the Rayleigh cutoff frequency FR. since perturbations of the liquid jet at frequencies higher than the cutoff frequency won't grow to cause a drop to be formed.
The drop stream that results from applying Rayleigh stimulation will be referred to herein as creating a stream of drops of predetermined volume. While in prior art CIJ systems, the drops of interest for printing or patterned layer deposition were invariably of unitary volume, it will be explained that for the present inventions, the stimulation signal can be manipulated to produce drops of predetermined multiples of the unitary volume. Hence the phrase, “streams of drops of predetermined volumes” is inclusive of drop streams that are broken up into drops all having one size or streams broken up into drops of planned different volumes.
In a CIJ system, some drops, usually termed “satellites” much smaller in volume than the predetermined unit volume, can be formed as the stream necks down into a fine ligament of fluid. Such satellites may not be totally predictable or may not always merge with another drop in a predictable fashion, thereby slightly altering the volume of drops intended for printing or patterning. The presence of small, unpredictable satellite drops is, however, inconsequential to the present invention and is not considered to obviate the fact that the drop sizes have been predetermined by the synchronizing energy signals used in the present invention. Drops of predetermined volume each have an associated portion of the drop forming waveform responsible for the creation of the drop. Satellite drops don't have a distinct portion of the waveform responsible for their creation. Thus the phrase “predetermined volume” as used to describe the present invention should be understood to comprehend that some small variation in drop volume about a planned target value may occur due to unpredictable satellite drop formation.
The example embodiments discussed below with reference to
A continuous inkjet printing system 10 is illustrated in
The RIP or other type of processor 16 converts the image data to a pixel-mapped image page image for printing. Image data can include raw image data, additional image data generated from image processing algorithms to improve the quality of printed images, and data from drop placement corrections, which can be generated from many sources, for example, from measurements of the steering errors of each nozzle in the printhead 12 as is well-known to those skilled in the art of printhead characterization and image processing. The information in the image processor 16 thus can be said to represent a general source of data for drop ejection, such as desired locations of ink droplets to be printed and identification of those droplets to be collected for recycling.
During printing, recording medium 19 is moved relative to printhead 12 by means of a media transport system 22 which are electronically controlled by media transport controller 21. A logic controller 17, preferably micro-processor based and suitably programmed as is well known, provides control signals for cooperation of transport controller 21 with the ink pressure regulator 20 and stimulation controller 18. The stimulation controller 18 comprises one or more stimulation waveform sources 56 that generate drop formation waveforms in response to the print data and provide or applies the drop formation waveforms 55, also called stimulation waveforms, to the stimulation device(s) 59 also called drop formation device(s) 59 associated with each nozzle 50 or liquid jet 43. In response to the energy pulses of applied stimulation waveforms, the drop formation device 59 perturbs the continuous liquid stream 43, also called a liquid jet 43, to cause individual liquid drops to break off from the liquid stream. The drops break off from the liquid jet 43 at a distance BL from the nozzle plate. The information in the image processor 16 thus can be said to represent a general source of data for drop formation, such as desired locations of ink droplets to be printed and identification of those droplets to be collected for recycling.
It can be appreciated that different mechanical configurations for media transport systems 19 can be used. For example, in the case of a page-width printhead, it is convenient to move recording medium 19 pasta stationary printhead 12. On the other hand, in the case of a scanning-type printing system, it is more convenient to move a printhead along one axis (i.e., a main-scanning direction) and move the recording medium along an orthogonal axis (i.e., a sub-scanning direction), in relative raster motion.
Drop forming pulses are provided by the stimulation controller 18, which can be generally referred to as a drop controller, and are typically voltage pulses sent to the printhead 12 through electrical connectors, as is well-known in the art of signal transmission. However, other types of pulses, such as optical pulses, can also be sent to printhead 12, to cause printing and non-printing drops to be formed at particular nozzles, as is well-known in the inkjet printing arts. Once formed, printing drops travel through the air to a recording medium and later impinge on a particular pixel area of the recording medium or are collected by a catcher as will be described.
Referring to
Depending on the type of transducer used, the transducer can be located in or adjacent to the liquid chamber that supplies the liquid to the nozzles to act on the liquid in the liquid chamber, be located in or immediately around the nozzles to act on the liquid as it passes through the nozzle, or located adjacent to the liquid jet to act on the liquid jet after it has passed through the nozzle. The drop formation waveform source 56 supplies a drop formation waveform having a fundamental frequency fo and a fundamental period of τo=1/fo to the drop formation transducer, which produces a modulation with a wavelength λ in the liquid jet. The modulation grows in amplitude to cause portions of the liquid jet break off into drops. Through the action of the drop formation device, a sequence of drops are produced at a fundamental frequency fo with a fundamental period of τo=1/fo.
In
Also shown in
The drop formation dynamics of drops forming from a liquid stream being jetted from an inkjet nozzle can be varied by altering the waveforms applied to the respective drop formation transducer associated with a particular nozzle orifice. Changing at least one of the amplitude, duty cycle or timing relative to other pulses in the waveform or in a sequence of waveforms can alter the drop formation dynamics of a particular nozzle orifice. It has been found that the drop forming pulses of the drop formation waveform can be adjusted modulate the velocity of consecutive drops 35 and 36 in a drop cluster 34 such that the drops in the drop cluster merge to form a merged drop 38 as shown in
In various embodiments of this invention, the voltage on the charging electrode 44 is controlled by the charging pulse source 51 which provides a two voltage state waveform operating at the charging waveform frequency fcw. In
In the printer, sequences of print or non print drops are generated in response to the input image data. During printing, communication signals from the stimulation controller 18 applied to the drop formation stimulation waveform source 56 are used to determine the order of formation of print and non-print drops, and the waveform source 56 provides different print drop and non-print drop stimulation waveforms 55 to the drop formation device. In the practice of this invention, the merged drops 38 are print drops and the non merged first and second drops 35 and 36 drops are non-print drops. Thus, the selected drop clusters correspond to print drops in the image data.
The liquid jets are modulated using the drop formation device to selectively cause portions of the liquid jet to break off into one or more clusters of drops traveling along a path using the drop formation device associated with the liquid jet, each cluster of drops separated on average by a drop cluster period, each cluster of drops including a first drop 36 and a second drop 35 in response to the input image data. When the input image data calls for a print drop to be formed, the drop forming waveform is selected or modified such that the first and second drops of the drop cluster are made to combine (merge) with each other to form a merged drop 38 as shown in
In an alternate embodiment the cluster of drops includes a third drop 49 in addition to first drop 36 and second drop 35. This example embodiment of the invention is described in more detail with reference to
As stated above a drop merging mechanism comprises a drop velocity modulation transducer associated with the liquid jet. The drop velocity modulation transducer can be one of a thermal device, a piezoelectric device, a MEMS actuator, an electrohydrodynamic device, an optical device, an electrostrictive device, and combinations thereof. Depending on the type of transducer used, the transducer can be located in or adjacent to the liquid chamber that supplies the liquid to the nozzles to act on the liquid in the liquid chamber, be located in or immediately around the nozzles to act on the liquid as it passes through the nozzle, or located adjacent to the liquid jet to act on the liquid jet after it has passed through the nozzle. The drop velocity modulation device is employed to alter or modulate the velocity of the first drop, the second drop, or both drops in a drop cluster to cause the first and second drop in a drop cluster to merge. As small changes in the amplitude, the duty cycle and waveform timing of the energy pulses transferred to the liquid jet to form the drops affect the velocity of the formed drops, the velocity of one or both drops in a drop cluster can be modulated and is accomplished by altering the characteristics of the energy transferred to the liquid jet that create the perturbations on the liquid jet that cause the drops to break off from the liquid stream. The drop velocity modulation waveform depends on the print or image data and is only applied when drop clusters are produced.
In the embodiment shown in
In the various embodiments of the invention, the continuous liquid ejection system 40 includes a printhead 12 comprising a liquid chamber 24 in fluid communication with an array of one or more nozzles 50 for emitting liquid jets 43. Liquid is supplied under a pressure sufficient to eject liquid jets through the nozzles of the liquid chamber. Associated with each liquid jet is a stimulation transducer 59. In the embodiments shown, the stimulation transducer 59 is formed in the wall around the nozzle 50. Separate stimulation transducers 59 can be integrated with each of the nozzles in a plurality of nozzles. The stimulation transducer 59 is actuated by a drop formation waveform source 56 which provides the periodic stimulation of the liquid jet 43 in the form of drop stimulation waveforms which are dependent on the input image data. In these embodiments, the periodic stimulation of the liquid jets 43 causes the jets to break off into sequences of drop clusters 34 traveling along a path, or into sequences of drop clusters that merge to form merged drops 38 travelling along the path. The embodiment shown in
The energy of the stimulation waveforms applied to the liquid jets is controlled so that all drops break off from the liquid stream 43 adjacent to the charge electrode 44 which is common to all of the nozzles of the plurality of nozzles in the printhead 12. The charging waveform source 51 supplies a time varying electrical potential (charge electrode waveform 97) between the charging electrode 44 and the liquid jet 43 which is usually grounded. The charge electrode waveform has a period, called the charging waveform period and includes a first distinct voltage state and a second distinct voltage state. The timing of the stimulation waveforms applied to the drop formation devices and the timing of the charging pulse source applied to the charge electrode are synchronized to produce a first charge state on the first drop 36 of a drop cluster indicated by a negative sign and a second charge state on the second drop 35 of a drop cluster indicated by a positive sign. When the velocity of the first and second drops of the drop cluster are modulated, the second drops 35 and first drops 36 of drop clusters are subsequently merged to form merged drops 38, having a third charge state shown as having neutral charge. Preferably the third charge state is a zero or near zero net charge state.
The first, second and merged drops, 35, 36, and 38 pass between optionally provided deflection electrodes 53. The voltage between the deflection electrodes produces an electric field in the space between the electrodes that causes negatively charged drops to be deflected toward the one catcher 47-1 as they travel along the negatively charged drop trajectory 26, while the positively charged drops are deflected toward the other catcher 47-2 as they travel along the positively charged drop trajectory 25. The merged drops 38, having zero charge are undeflected, travel along merged drop trajectory 28, passing both catchers to strike the print media 19. In the embodiment shown, the deflection electrodes are biased to approximately equal and opposite voltages relative to the grounded drop generator. This reduces the amount of charge induced on the drops as they break off due to the electric fields produced by the deflection electrodes. While the deflection mechanism shown here is made up of two deflection electrodes, other deflection mechanisms can be used. For example, instead of using deflection electrodes that are distinct from both the charge electrodes and the catcher, alternate configurations include configurations in which the two catchers are biased to different voltages such that the electric field between the catchers deflects the charged drops such that some drops are deflected to one of the catchers and the oppositely charged drops are deflected to the other catcher.
The drops that strike either catcher form a liquid film 48 on the catcher face; the liquid film flows down the catcher face 52 and into the liquid recovery channel 58. Liquid from the liquid recovery channel 58 is typically returned to the ink reservoir 11 via the ink recycling unit 15. While Coanda catchers are shown, other catcher types, such as a knife edge catcher can be used.
The relatively non-charged merged drops are printed and highly charged drops are guttered and recycled. Due to the minimal charge on print drops they will travel along a trajectory which is substantially coincident with the initial path. When print drops contact the recording medium 19 while it is moving at a relative velocity vm with respect to the printhead 12 they form printed drops 46 on the recorded medium in regions corresponding to the input image data.
In practice, the print drops may be slightly deflected away from the catcher and allowed to hit the recording medium. For proper operation of the printhead 12 shown in
In the illustrated drop charging waveforms of
Section B of
This embodiment, having three drops in the drop clusters, provides more consistent placement of the print drops within each pixel region on the print media than the two drops per drop cluster at certain print speeds. On the other hand, drop clusters with two drops per drop cluster provides more consistent placement of the print drops within each pixel region on the print media than the three drops per drop cluster at other print speeds. Furthermore drop clusters with two drops per drop cluster enable higher speed printing than drop clusters having three drops per drop cluster. The use of two drop clusters is preferred when the rate at which pixels are moved past the printhead is one half the rate of drop formation or when two drops are formed per nozzle during the time interval for a pixel to pass the printhead. The use of three drop clusters is preferred when the rate at which pixels are moved past the printhead is one third the rate of drop formation or when three drops are formed per nozzle during the time interval for a pixel to pass the printhead. As print speeds intermediate between these print speeds, a mix of two drop clusters and three drops clusters can be used; the mix ratio depending on the print speed, the drop formation rate, and the resolution, number of pixels per inch, in the print media travel direction. In certain embodiments, the printing system determines mix ratio of different charge electrode waveforms based on a measurement of the print speed. In other embodiments, the print system includes a clock that creates clock pulse at the fundamental period. The printing system determines on a pixel by pixel basis the number of clock pulses in each pixel time interval and a charge electrode waveform and the drop cluster size having a corresponding period is selected for that pixel interval. The selection of charge electrode waveforms and the corresponding drop cluster size depends on the print speed and/or the number of fundamental drops formed in the pixel time interval, but not on the print data for the pixel. That is, the charge electrode waveform used during a particular pixel interval doesn't depend on whether a drop is to be printed on that pixel or not.
The printing system can include a drop charge sensor to determine the charge of the merged drops. Based on the measured charge, the control can make voltage adjustments to one or both of the first and the second voltage states to drive the charge of the merged drop closer to zero. This can be beneficial as lower charge amplitudes on the print drops reduce the electrostatic drop-drop interactions that can affect drop placement on the print media.
The embodiment shown in
In step 165, a charging device is provided. The charging device includes a charge electrode and a source of time varying electrical potential. The charge electrode is common to and associated with each of the liquid jets. The source of time varying electrical potential applies a charge electrode waveform between the charge electrode and the liquid jets. The charge electrode waveform includes a first distinct voltage state and a second distinct voltage state and has a period that is equal to the drop cluster period. This results in a time varying electrical potential in the vicinity of drop break off from the liquid jets. The charge electrode waveform is independent of the image data applied to the drop formation devices of the nozzles.
In step 170, the charging device and the drop formation device are synchronized so that the first voltage state is active when first drops break off from the jets and the second voltage state is active when second drops break off from the liquid. This produces a first charge state on the first drop of the clusters of drops, and a second charge state on the second drop of the clusters of drops When the third drops are present in the clusters of drops the charging device and the drop formation device are synchronized to produce one of the first charge state and the second charge state on the third drops.
In step 175 the first and second drops of drop clusters are merged to form print drops. Drop merging mechanisms used in this invention include varying the velocity of the first and second drops of a drop cluster with a separate drop velocity modulation transducer, using drop velocity modulation pulses applied to the drop formation transducer, by electrostatic attraction of oppositely charged drops of the drop pair or by combinations of any two or more approaches. Drop merging can be accomplished by applying velocity modulation pulses to the drop formation transducers or to separate velocity modulation transducers associated with each of the nozzles in a nozzle array and/or by electrostatic attraction. Application of the drop merging mechanism causes the first drop and the second drop of the drop clusters to combine with each other to form a merged drop which has a third charge state.
In step 180, charged first drops, second drops and third drops are deflected. A deflection mechanism includes an electrostatic deflection device which causes the first drop to begin traveling along a first trajectory and causes the second drop to begin traveling along a second trajectory, the first and second trajectories being different when compared to each other. The third drops travel along one of the first or second trajectories. The merged print drops are not deflected and continue to travel along their original trajectories.
In step 185, drops traveling along the first trajectory and the second trajectory are intercepted by catchers for recycling. These drops are non print drops. The print drops that are not deflected are not intercepted by the catcher, and are allowed to contact the recording medium and are printed.
Generally this invention can be practiced to create print drops in the range of 1-100 pl, with nozzle diameters in the range of 5-50 μm, depending on the resolution requirements for the printed image. The jet velocity is preferably in the range of 10-30 m/s. The fundamental drop generation frequency is preferably in the range of 50-1000 kHz. The specific selection of these drop size, drop speed, nozzle size and drop generation frequency parameters is dependent on the printing application.
The invention allows drops to be selected for printing or non-printing without the need for a separate charge electrode to be used for each liquid jet in an array of liquid jets as found in conventional electrostatic deflection based ink jet printers. Instead a single common charge electrode is utilized to charge drops from the liquid jets in an array. This eliminates the need to critically align each of the charge electrodes with the nozzles. Crosstalk charging of drops from one liquid jet by means of a charging electrode associated with a different liquid jet is not an issue. Since crosstalk charging is not an issue, it is not necessary to minimize the distance between the charge electrodes and the liquid jets as is required for traditional drop charging systems. The common charge electrode also offers improved charging and deflection efficiency thereby allowing a larger separation distance between the jets and the electrode. Distances between the charge electrode and the jet axis in the range of 25-300 μm are useable. The elimination of the individual charge electrode for each liquid jet also allows for higher densities of nozzles than traditional electrostatic deflection continuous inkjet system, which require separate charge electrodes for each nozzle. The nozzle array density can be in the range of 75 nozzles per inch (npi) to 1200 npi.
The invention has been described in detail with particular reference to certain example embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3373437 | Sweet et al. | Mar 1968 | A |
| 3596275 | Sweet | Jul 1971 | A |
| 3656171 | Robertson | Apr 1972 | A |
| 3827057 | Bischoff et al. | Jul 1974 | A |
| 3846800 | Chen | Nov 1974 | A |
| 3946399 | Zaretsky | Mar 1976 | A |
| 4054882 | Ruscitto | Oct 1977 | A |
| 4068241 | Yamada | Jan 1978 | A |
| 4217595 | Horike et al. | Aug 1980 | A |
| 4350986 | Yamada | Sep 1982 | A |
| 4613871 | Katerberg | Sep 1986 | A |
| 6059393 | Takahashi | May 2000 | A |
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| Number | Date | Country | |
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| 20140009523 A1 | Jan 2014 | US |
