Patent Grant
7938516
Reference is made to commonly assigned co-pending U.S. patent application Ser. No. 12/187,593 filed in the name of Piatt et al and entitled “Continuous Inkjet Printing System and Method for Producing Selective Deflection of Droplets Formed From Two Different Break Off Lengths” and filed concurrently herewith.
The present invention relates to the field of continuous inkjet printing systems and methods. Specifically, the invention is for an apparatus and method for selectively generating droplets that are formed during different phases of a common charge electrode and selectively deflecting droplets formed by an inkjet printhead in accordance with the charges the droplets acquire at different phases of the common electrode.
Continuous inkjet (CIJ) printing systems create printed materials by forcing ink, under pressure, through a nozzle. The flow of ink may be disrupted in a manner such that the flow breaks up into droplets of ink in a predictable manner. Printing occurs through the selective deflecting and catching of undesired ink droplets. In U.S. Pat. No. 6,273,559 filed in the names of Vago et al. there are described continuous inkjet printing techniques one of which is referred to as the binary continuous inkjet technique. In the binary continuous inkjet technique electrically conducting ink is pressurized and discharged through a calibrated nozzle and the ink jets formed are broken off at two different time intervals. Droplets to be printed or not printed are created with periodic stimulation pulses at a nozzle. The droplets to be printed are each created with a periodic stimulation pulse that is relatively strong and causes the ink jet stream forming that droplet to separate at a relatively short break off length. The droplets that are not to be printed are each created with a periodic stimulation pulse that is relatively weak and causes the droplet to separate at a relatively long break off length. Electrodes are positioned just downstream of the nozzle and provide a charge to each droplet that is formed. The longer break off length droplets are selectively deviated from their path by a deflection device because of their charge and are deflected by the deflection device towards a catcher surface where they are collected in a gutter and returned to a reservoir for reuse.
The binary CIJ printheads may be operable in a manner such that the liquid jets may be said to have associated therewith a wavelength □ that is the distance between successive ink droplets or ink nodes in that liquid jet. The wavelength, λ, is equal to the speed of the jet divided by the frequency of the stimulation signals, assuming one stimulation signal at each nozzle during a stimulation cycle It is thus possible to modulate the liquid jets break off points such that there exist a first and a second liquid break off points such that the break off points differ by a distance measured related to this wavelength. For example, in the aforementioned Vago et al. patent the longer and shorter break off length droplets have a distance between two jet break off points of less than λ. The longer break off length droplets have a break off point or droplet formation point d2 that is spaced from the location d1 where the shorter break off length droplets form by a distance less than λ. In Vago et al. there is mention made of prior art wherein the delta difference between d2 and d1 is λ and that this creates problems when there is a transition at a nozzle from creation of a longer break off length droplet followed by a shorter break off length droplet. The problem recognized by Vago et al. is that of the tendency of the longer break off length droplet and the shorter break off length droplet to simultaneously detach; i.e. two droplets break off from the jet concurrently. Where the delta difference is slightly greater than λ the two droplets may temporarily be combined and alter the trajectory of the droplets. There is thus the suggestion by Vago et al. to avoid the use of having droplet separation distance differences between the longer break off length droplets and shorter break off length droplets be greater than or equal to λ. To this end the specification of Vago et al. is directed to the teaching of using a significantly smaller break off separation distance between the longer break off length droplets and the shorter break off length droplets.
To enable droplet selection based on such small break off length differences as taught by Vago et al. it is necessary to establish electric fields having a sharp gradient along the jet trajectory. Vago et al. is able to achieve these high gradients by utilizing two sets of charge electrodes that were closely spaced along the drop trajectory. One of the electrode pairs was biased at +300 volts relative to the drop generator and the second electrode pair biased to −300 volts relative to the drop generator. To alter the break off length locations as described in the Vago et al. specification requires two stimulation amplitudes (a print and a non-print stimulation amplitude) to be employed. Limiting the break off length locations difference to less than λ restricts the stimulation amplitudes difference that must be used to a small amount. For a printhead that has only a single jet, it is quite easy to adjust the position of the electrodes, the voltages on the charging electrodes, and print and non-print stimulation amplitudes to produce the desired separation of print and non-print droplets. However, in a printhead having an array of nozzles parts tolerances can make this quite difficult. The need to have a high electric field gradient in the droplet breakoff region makes the drop selection system sensitive to slight variations in charging electrode flatness, electrode thicknesses, and spacings that can all produce variations in the electric field strength and the electric field gradient at the droplet breakoff region for the different liquid jets in the array. As a result of such variations it can be quite difficult to adequately separate print and non-print droplets from the different nozzles in the array so that the non-print droplets can be caught even if the droplet generator and the associated stimulation devices were perfectly uniform down the nozzle array. But of course the droplet generator and the associated stimulation devices are not perfectly uniform down the nozzle array, but instead require different stimulation amplitudes from nozzle to nozzle to produce particular break off lengths These problems are compounded by ink properties that drift over time, and thermal expansion that can cause the charging electrodes to shift and warp with temperature. In such systems extra control complexity is required to adjust the print and non-print stimulation amplitudes from nozzle to nozzle to ensure the desired separation of print and non-print droplets.
It is therefore an object of the invention to overcome the aforesaid deficiencies by providing a common charge electrode that has a time varying electrical potential. This allows, as will be shown below, for less complexity in control of signals to the stimulation devices at the nozzles. This further enables the use of less complex charge electrode structures and larger spacing between the charge electrode structures and the nozzles.
In accordance with a first aspect of the invention there is provided a continuous inkjet system for selectively depositing liquid droplets upon a surface, the system comprising a liquid chamber including a nozzle, the liquid chamber containing liquid under pressure sufficient to produce a liquid jet through the nozzle. A source of varying electrical potential has a periodicity providing cycles each having a relatively high-voltage phase and a relatively low-voltage phase. A stimulation device is operatively associated with the liquid jet. The stimulation device is responsive to respective different types of stimulation signals and operable to produce a modulation in the liquid jet to selectively control droplet break off relative to phases of the cycle of the source. A first liquid droplet from the liquid jet has a first break off phase relative to a cycle of the source and a second liquid droplet from the liquid jet has a second break off phase relative to a cycle of the source. The first break off phase and the second break off phase have a difference such that the first break off phase coincides with the relatively high-voltage phase and the second break off phase coincides with the relatively low-voltage phase. A deflection mechanism includes a charge electrode electrically connected to the source of varying electrical potential. The charge electrode is operable to produce a charge differential between the first liquid droplet and the second liquid droplet. The deflection mechanism is operable to cause trajectories of the first liquid droplet and the second liquid droplet to diverge so that a trajectory of one droplet of the first and second liquid droplets causes the one droplet to be directed for collection and prevented from depositing on the surface and a trajectory of the other droplet of said first and second liquid droplets causes the other droplet to be directed for depositing upon the surface and wherein the electrical potential on the charge electrode varies with said periodicity and is independent of types of stimulation signals used to determine whether a droplet is to travel in accordance with the trajectory of the first liquid droplet or the trajectory of the second liquid droplet.
In accordance with a second aspect of the invention there is provided a continuous inkjet droplet generating method for selectively depositing liquid droplets upon a surface, the method comprising producing a liquid jet through a nozzle and providing a charge electrode connected to a source of varying electrical potential having a periodicity providing cycles each having a relatively high-voltage phase and a relatively low-voltage phase. A stimulation device associated with the liquid jet is operated to produce a modulation in the liquid jet to selectively control droplet break off relative to phases of the cycle of the source wherein a first liquid droplet from the liquid jet has a first break off phase relative to a cycle of the source and a second liquid droplet from the liquid jet has a second break off phase relative to a cycle of the source. The first break off phase and the second break off phase having a difference such that the first break off phase coincides with the relatively high-voltage phase and the second break off phase coincides with the relatively low-voltage phase. The charge electrode operates to produce a charge differential between the first liquid droplet and the second liquid droplet and droplets are selectively deflected to cause trajectories of the first liquid droplet and the second liquid droplet to diverge so that a trajectory of one droplet of the first and second liquid droplets causes the one droplet to be directed for collection and prevented from depositing on the surface and a trajectory of the other droplet of said first and second liquid droplets causes the other droplet to be directed for depositing upon the surface. The electrical potential on the charge electrode varies with said periodicity and is independent of types of stimulation signals used to determine whether a droplet is to travel in accordance with the trajectory of the first liquid droplet or the trajectory of the second liquid droplet.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.
In the detailed description of the preferred 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.
A continuous inkjet printing system 10 as illustrated in
One well-known problem with any type inkjet printer, whether drop-on-demand or continuous flow, relates to dot positioning. As is well-known in the art of inkjet printing, one or more droplets 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 droplets 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 droplet 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. The RIP or other type of processor 16 converts the image data to a pixel-mapped image page image for printing. During printing operation, a recording medium 19 is moved relative to printhead 12 by means of a plurality of transport rollers 22 which are electronically controlled by transport control system 21. A logic controller 17, preferably micro-processor based and suitably programmed as is well known, provides control signals for cooperation of transport control system 21 with the ink pressure regulator 20 and stimulation controller 18. The stimulation controller 18 comprises a droplet controller that provides the drive signals for ejecting individual ink droplets from printhead 12 to recording medium 19 according to the image data obtained from an image memory forming part of the image processor 16. Image data may 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.
It may be appreciated that different mechanical configurations for receiver transport control may be used. For example, in the case of a page-width printhead, it is convenient to move recording medium 19 past a 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 may be generally referred to as a droplet 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, the types of pulses, such as optical pulses, may also be sent to printhead 12, to cause printing and non-printing droplets to be formed at particular nozzles, as is well-known in the inkjet printing arts. Once formed, printing droplets travel through the air to a recording medium and later impinge on a particular pixel area of recording medium or are collected by a catcher as will be described.
With reference now to
In accordance with one aspect of the invention the ability to selectively charge droplets is dependent upon the creation of a jet break off time differences of less than one stimulation period. Unlike the art described by Vago et al., there is not a need to locate the droplet breakoff points in an electric field having a strong spatial gradient. As a result the system is much more robust than that described by Vago et al.
With reference now to
In the alternative and as illustrated in
Because droplets break off either when the charge electrode is at a relatively high electrical potential or at a relatively low electrical potential a charge differential is therefore produced between the first liquid droplets having a first breakoff phase and the second liquid droplets having a second break off phase. As a result of the charge differential, the deflection mechanism causes the first and second paths or trajectories of the first liquid droplets and the second liquid droplets, respectively to diverge. For descriptive simplicity, the term uncharged droplets is used in this specification for the droplets with significantly less charge.
It should be obvious, in view of the above description of the invention, to one skilled in the art that the charged droplets are not required to be the non-print droplets. Thus, the charged droplets may be the droplets that are printed while the non-charged droplets are the ones collected by the catcher. This is accomplished by positioning the catcher to intercept the path of the uncharged droplets rather than the path of the charged droplets.
With reference now to the chart of
With reference now to the chart of
The stimulation pulse produces a slight wiggle or perturbation in the diameter of the liquid stream so that a portion of the stream is made slightly narrower than normal and another portion is made wider than normal. The perturbation will grow exponentially with time, the narrower section getting even narrower and the wider section getting even wider. The surface tension of the liquid produces a slight pressure difference in the stream causing liquid to move from the narrower region to the wider region. As the liquid stream is moving, the perturbation moves with the liquid stream. As the perturbation grows, eventually the diameter of the narrower region becomes zero and the drop breaks off.
If the initial perturbation amplitude is made larger, by using higher amplitude stimulation pulses or longer stimulation pulses, less time is needed for the perturbation to grow to the point at which the drop breaks off. Therefore the use of longer and shorter stimulation pulses as in
Thus distinguishing between printed and catcher droplets is essentially provided by break off phase of the droplet vis-à-vis the voltage or electrical potential on the charge electrode(s). In one respect such occurs through a difference in break off lengths of the droplets that translates into a difference in break off time. In another respect it occurs through a difference in non-uniformity in stimulation pulse timing which also has the effect of changing break off time. In either form, the stimulation device is responsive to respective different types of stimulation signals (corresponding to pulses having different pulse amplitude, pulse width or duty cycle, or pulse phase) and is operable to produce a modulation of the liquid jet to selectively control droplet break off relative to the phase of the cyclically varying voltage from the charge pulse source. Thus, by cycling the voltage level to the charge electrode between low and high voltage on every stimulation cycle and independent of the print data, selective charging of droplets may be provided for by controlling timing of the break off of the droplet with the phase of the voltage on the charge electrode.
While the invention has been described with reference to printing systems and methods it is also known to use inkjet droplet generating devices for decorating pastries and other three-dimensional articles or for forming three-dimensional articles by building up droplets of material on a substrate. The term ink in this application is therefore not limited to colored liquids for printing on paper, but is intended to also refer to liquids appropriate to other such applications. In addition while the stimulation pulses have been illustrated as a single rectangular pulse being provided during each cycle other waveforms can be employed, such as bursts of pulses, ramped pulses, sinusoidal pulses, and pulses of various polarities can also be used dependent on the type of stimulation device. While in the embodiments described the stimulation devices have comprised resistive elements, other types of drop stimulation including optical, piezoelectric, MEMS actuator, electrohydrodynamic, etc. or combinations thereof also may be substituted. Such applications and substitutions are all contemplated by this invention. The stimulation controller may be remote from the stimulation device, or it may be fabricated along with the stimulation device on a common component such as a nozzle plate. While the catcher shown in the illustrations is a Coanda type catcher, other catcher types, such as a knife edge catcher can also be employed. The cyclically varying charge voltage supplied to the charge electrode(s) has been shown as a square wave having a 50% duty cycle, but other cyclically varying waveforms can also be employed such as having pulses with duty cycles greater or less than 50% or having non-zero rise and fall times. As noted above there is the advantage with the invention of use of a common charge electrode with plural nozzles. It will be understood that this does not limit the invention to all nozzles of a printhead being associated with one charge electrode. Thus, as an example only and not by way of limitation, a charge electrode may be associated with for example a set of 50 nozzles of the printhead and another charge electrode may be associated with a different set of 50 nozzles of that printhead.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
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