Patent Grant
7303265
This invention relates generally to the field of digitally controlled printing and liquid patterning devices, and in particular to continuous ink jet systems in which a liquid stream breaks into drops, some of which are selectively deflected.
Traditionally, digitally controlled liquid patterning capability is accomplished by one of two technologies. In each technology, a patterning liquid is fed through channels formed in a printhead. Each channel includes a nozzle from which drops of liquid are selectively extruded and deposited upon a medium. When color marking is desired, each technology typically requires independent liquid supplies and separate liquid delivery systems for each liquid color used during printing.
The first technology, commonly referred to as “drop-on-demand” ink jet printing, provides liquid drops for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying drop that crosses the space between the printhead and the pattern receiving media, striking the media. The formation of printed images or other patterns is achieved by controlling the individual formation of liquid drops, based on data that specifies the pattern or image.
Conventional “drop-on-demand” ink jet printers utilize a pressurization actuator to produce the ink jet drop at orifices of a print head. Typically, the pressurization is accomplished by rapidly displacing a portion of the liquid in individual chambers that supply individual nozzles. Displacement actuators are most commonly based on piezoelectric transducers or vapor bubble forming heaters (thermal ink jet). However, thermomechanical and electrostatic membrane displacement has also been disclosed and used.
U.S. Pat. No. 4,914,522 issued to Duffield et al., on Apr. 3, 1990 discloses a drop-on-demand ink jet printer that utilizes air pressure to produce a desired color density in a printed image. Liquid in a reservoir travels through a conduit and forms a meniscus at an end of an inkjet nozzle. An air nozzle, positioned so that a stream of air flows across the meniscus at the end of the liquid nozzle, causes the liquid to be extracted from the nozzle and atomized into a fine spray. The stream of air is applied at a constant pressure through a conduit to a control valve. The valve is opened and closed by the action of a piezoelectric actuator. When a voltage is applied to the valve, the valve opens to permit air to flow through the air nozzle. When the voltage is removed, the valve closes and no air flows through the air nozzle. As such, the liquid dot size on the image remains constant while the desired color density of the liquid dot is varied depending on the pulse width of the air stream.
The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing (CIJ), uses a pressurized liquid source which produces a continuous stream of liquid drops. This technology is applicable to any liquid patterning or selection application. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual drops. The drops are electrically charged and then deflected to an appropriate location by an electric field of self-image charge in a grounded conductor. When no drop deposition is desired at a particular location on the receiver medium, the drops are deflected into an liquid capturing mechanism, a drop catcher or gutter, and either recycled or discarded. When a print or pattern drop is desired, the drops are not deflected to the drop catcher and are allowed to strike the receiver media. Alternatively, deflected drops may be allowed to strike the media, while non-deflected drops are collected in the liquid capturing mechanism.
Conventional continuous ink jet printers utilize electrostatic charging devices and deflector plates that require addressable electrical components that must be very closely and precisely aligned to the continuous streams of patterning liquid without touching them. The patterning liquid must be sufficiently conductive to allow drop charging within a few microseconds. While serviceable, these electrostatic deflection printheads are difficult to manufacture at low cost and suffer many reliability problems do to shorting and fouling of the drop charging electrodes and deflection electric field plates. A continuous ink jet system that does not rely on drop charging would greatly simplify printhead manufacturing, and eliminate the need for highly conductive working fluids.
U.S. Pat. No. 3,709,432, issued to Robertson, on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced liquid drops through the use of transducers. The lengths of the filaments before they break up into liquid drops are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated uniformly across all the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into drops more than it affects the trajectories of the liquid drops themselves. By controlling the lengths of the filaments, the trajectories of the liquid drops can be controlled, or switched from one path to another. As such, some liquid drops may be directed into a catcher while allowing other liquid drops to be applied to a receiving member. The physical separation or amount of discrimination between the two drop paths is very small and difficult to control.
U.S. Pat. No. 4,190,844, issued to Taylor, on Feb. 26, 1980, discloses a single jet continuous ink jet printer having a first pneumatic deflector for deflecting non-printing drops to a catcher and a second pneumatic deflector for oscillating printing drops (Taylor '844 hereinafter). A printhead supplies a filament of working fluid that breaks into individual liquid drops. The liquid drops are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector has a diaphragm that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the liquid drop is to be deposited on the medium or not. The second pneumatic deflector is a continuous type having a diaphragm that varies the amount a nozzle is open depending on a varying electrical signal received the central control unit. This deflects printed liquid drops vertically so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time, being built up by repeated traverses of the printhead.
While this method does not rely on electrostatic means to affect the trajectory of drops it does rely on the precise control and timing of the first (“open/closed”) pneumatic deflector to create printed and non-printed liquid drops. Such a system is difficult to manufacture and accurately control. The physical separation or amount of discrimination between the two drop paths is erratic due to the uncertainty in the increase and decrease of air flow during switching resulting in poor drop trajectory control and imprecise drop placement. Pneumatic operation requiring the air flows to be turned on and off is necessarily slow in that an inordinate amount of time is needed to perform the mechanical actuation as well as time associated with the settling any transients in the air flow. Further, it would be costly to manufacture a closely spaced array of uniform first pneumatic deflectors necessary to extend the Taylor '844 concept to a plurality of closely spaced jets.
U.S. Pat. No. 5,963,235 issued to Chwalek, et al., on Oct. 5, 1999 discloses a continuous ink jet printer that uses a micromechanical actuator that impinges a curved control surface against the continuous stream filaments prior to break-up into droplets (Chawlek '235 hereinafter). By manipulating the amount of impingement of the control surface the stream may be deflected along multiple flight paths. While workable, this apparatus tends to produce large anomalous swings in the amount of stream deflection as the surface properties are affected by contact with the working fluid.
U.S. Pat. No. 6,509,917 issued to Chwalek et al., on Jan. 21, 2003, discloses a continuous ink jet printer that uses electrodes located downstream of the nozzle, closely spaced to the unbroken fluid column, to deflect the continuous stream filament before breaking into drops (Chawlek '917 hereinafter). By imposing a voltage on the electrodes drops may be steered along different deflection paths. This approach is workable however the apparatus prone to electrical breakdown due to a build up-of conductive debris around the deflection electrodes.
U.S. Pat. No. 6,474,795 issued to Lebens, et al., on Nov. 5, 2002 discloses a continuous ink jet printer that uses a dual passage way to supply fluid to each nozzle (Lebens '795 hereinafter). One fluid passageway is located off-center to the nozzle entry bore and has a micromechanical valve that regulates the amount of flow that is supplied. The off-center flow from this passageway causes the jet to be emitted at an angle. Thus by manipulating this valve, drops may be directed to different deflection pathways. This approach is workable however the printhead structure is more complex to fabricate and it is difficult to achieve uniform deflection from all of the jets in a large array of jets.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual liquid drops from a filament of working fluid and deflect those liquid drops (Chwalek '821 hereinafter). A printhead includes a pressurized liquid source and an asymmetric heater operable to form printed liquid drops and non-printed liquid drops. Printed liquid drops flow along a printed liquid drop path ultimately striking a print media, while non-printed liquid drops flow along a non-printed liquid drop path ultimately striking a catcher surface. Non-printed liquid drops are recycled or disposed of through a liquid removal channel formed in the catcher.
While the ink jet printer disclosed in Chwalek '821 works extremely well for its intended purpose, the amount of physical separation between printed and non-printed liquid drops is limited which may limit the robustness of such a system. Simply increasing the amount of asymmetric heating to increase this separation will result in higher temperatures that may decrease reliability. Therefore, an apparatus that amplifies the separation between print and non-printed drops would be useful in increasing the reliability of the system disclosed by Chwalek '821.
U.S. Pat. No. 6,505,921 issued to Chwalek, et al. on Jan. 14, 2003, discloses and claims an improvement over Chwalek '821 whereby a plurality of thermally deflected liquid streams is caused to break up into drops of large and small volumes, hence, large and small cross-sectional areas (Chwalek '921 hereinafter). Thermal deflection is used to cause smaller drops to be directed out of the plane of the plurality of streams of drops while large drops are allowed to fly along nominal “straight” pathways. A uniform gas flow is imposed in a direction perpendicular and across the array of streams of drops of cross-sectional areas. This perpendicular gas flow applies more force per mass to drops having smaller cross-sections than to drops having larger cross-sections, resulting in an amplification of the deflection acceleration of the small drops. Such gas flow deflection amplification can provide needed additional separation between drops to be captured in a gutter versus drops that are allowed to deposit on a medium. Chwalek '921 does not disclose designs for airflow plenums that optimize the airflow deflection achieved for a chosen magnitude of peak airflow velocity nor disclose designs to minimize unwanted sideways drop deflections or sensitivity to unintended air current perturbations.
U.S. Pat. No. 6,508,542 issued to Sharma, et al. on Jan. 21, 2003, also discloses and claims an improvement over Chwalek '821 that uses a gas flow to amplify the spatial separation between drops traveling along two diverging pathways, so as to improve the reliability of drop capture (Sharma '542 hereinafter). Sharma '542 teaches a gas flow that is emitted in close proximity to a gutter drop capture lip and that is generally opposed to both the nominal and thermally deflected flight paths of drops. The gas flow of Sharma '542 is illustrated as further splitting the drops into two pathways and is positioned so that the gas flow is losing convergence at a point where the thermally deflected drops are physically separating.
Effectively, the apparatus and method taught by Sharma '542 increases drop pathway divergence by reducing the drop velocity in the direction of the media and gutter. That is, by slowing the flying drops, more time is provided for the off-axis thermal deflection acceleration imparted at the nozzle to build up into more spatial divergence by the time the capture lip of the gutter is reached. The interaction of the gas flow of Sharma '524, and the diverging drop pathways, will also be very dependent on the time varying pattern of drops inherent in image or other pattern printing. Different drop sequences with be differently deflected, resulting in the addition of data dependent drop placement error for the printed drops. Further, the approach of Sharma '542 may be unsuitable to implement for a large array of jets as it is difficult to achieve sufficiently uniform gas flow behavior along a wide slit source so that the point of transition to incoherent gas flow would occur at the same distance from the nozzle for all jets of the array.
Notwithstanding the several inventions described above, there remains a need for a robust, high speed, high quality liquid patterning system. Such a system may be realized using continuous ink jet technology that does not rely on drop charging and electrostatic drop deflection. Further, such a system could be realized if sufficient drop deflection can be achieved to allow robust drop capturing without sacrificing print speed and pattern resolution by the formation of large volume drops or long flight paths from nozzle to medium. Finally, such a system requires simplicity of design that facilitates fabrication of large arrays of closely space jets.
The foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by constructing a drop deflector apparatus for a continuous drop emission system comprising a plurality of drop nozzles emitting a plurality of continuous streams of a liquid that breaks up into streams of drops having nominal flight paths that are substantially parallel and substantially within a nominal flight plane. An airflow plenum having an evacuation end connected to a negative pressure source and an impingement end having an opening located adjacent the nominal flight plane into which ambient air is drawn for the purpose of deflecting drops in an air deflection direction perpendicular to the nominal flight plane is provided. The opening is bounded by upstream, downstream, first and second walls wherein the upstream and downstream wall ends are spaced away from the nominal flight plane in the air deflection direction by a larger amount than are the first and second side wall edges.
The present inventions are also configured with an airflow plenum having through slots for the passage of drops so as to increase the amount of drop deflection achieved for a given maximum deflection air velocity and to provide a reduction in the affect of perturbing air currents that may be present around the nominal flight paths.
The present inventions are additionally comprised of drop synchronization apparatus adapted to break up continuous liquid streams into drops of large and small volumes according to liquid pattern data, the large and small drops being differently deflected by the air flow in the airflow plenum.
The present inventions are further comprised of a plurality of path selection elements for directing drops along different paths according to liquid pattern data, wherein drops following different paths are differently deflected by the air flow in the airflow plenum.
The present inventions also comprise drop capture apparatus adapted to catch and contain drops of small volume before exiting the air flow plenum.
The present inventions further include methods of forming a liquid pattern on a medium based on liquid pattern data comprising providing a plurality of drop nozzles emitting a plurality of continuous streams of drops of large and small drop volumes, according to liquid pattern data, having nominal flight paths that are substantially within a nominal flight plane and that impinge the medium. An air flow plenum having an evacuation end connected to a negative pressure source and an impingement end having a primary opening, an upstream slot opening through the upstream wall positioned and sized so that the plurality of streams of drops paths pass through, and a downstream slot opening through the downstream wall positioned and sized so that at least drops having a large drop volume pass through is provided. A negative pressure source is communicated to the evacuation end drawing ambient air into the airflow plenum via the primary opening, the upstream slot and the downstream slot, thereby deflecting drops having a small drop volume in an air deflection direction perpendicular to the nominal flight plane. Deflected drops having a small drop volume are captured in a drop capture apparatus. Drops having a large drop volume are allowed to impinge the media, forming the liquid pattern.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment 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:
a) and 2(b) show schematic plane views of a single thermal synchronization and path selection element and a portion of an array of such elements, respectively, according to a preferred embodiment of the present invention;
a), 3(b) and 3(c) show schematic cross-sections illustrating natural break-up, synchronized break-up, and synchronized and deflected break-up of continuous steams of liquid into drops, respectively;
a), 4(b) and 4(c) show representations of energy pulse sequences for stimulating synchronous break-up of a fluid jet by heater resistors and first deflection by heater resistors according to a preferred embodiment of the present invention;
a), 5(b) and 5(c) show representations of energy pulse sequences for stimulating synchronous break-up of a fluid jet by heater resistors resulting in drops of different predetermined volumes according to a preferred embodiment of the present inventions;
a) and 10(b) shows schematic side cross sectional views of air flow velocity vectors around airflow plenum wall edges of different shapes according to the present inventions;
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. Functional elements and features have been given the same numerical labels in the figures if they are the same element or perform the same function for purposes of understanding the present inventions. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
Referring to
The liquid pattern deposition system further includes a source of the image or liquid pattern data 410 which provides raster image data, outline image data in the form of a page description language, or other forms of digital image data. This image data is converted to bitmap image data by controller 400 and stored for transfer to a multi-jet drop emission printhead 10 via a plurality of printhead transducer circuits 412 connected to printhead electrical interface 20. The bit map image data specifies the deposition of individual drops onto the picture elements (pixels) of a two dimensional matrix of positions, equally spaced a pattern raster distance, determined by the desired pattern resolution, i.e. the pattern “dots per inch” or the like. The raster distance or spacing may be equal or may be different in the two dimensions of the pattern.
Controller 400 also creates drop synchronization signals to the printhead transducer circuits that are subsequently applied to printhead 10 to cause the break-up of the plurality of fluid streams emitted into drops of predetermined volume and with a predictable timing. Printhead 10 is illustrated as a “page wide” printhead in that it contains a plurality of jets sufficient to print all scanlines across the medium 300 without need for movement of the printhead itself.
Recording medium 300 is moved relative to printhead 10 by a recording medium transport system, which is electronically controlled by a media transport control system 414, and which in turn is controlled by controller 400. The recording medium transport system shown in
Pattern liquid is contained in a liquid reservoir 418 under pressure. In the non-printing state, continuous drop streams are unable to reach recording medium 300 due to a fluid gutter (not shown) that captures the stream and which may allow a portion of the liquid to be recycled by a liquid recycling unit 416. The liquid recycling unit 416 receives the un-printed liquid via printhead fluid outlet 245, reconditions the liquid and feeds it back to reservoir 418 or stores it. The liquid recycling unit may also be configured to apply a vacuum pressure to printhead fluid outlet 245 to assist in liquid recovery and to affect the gas flow through printhead 10. Such liquid recycling units are well known in the art. The liquid pressure suitable for optimal operation will depend on a number of factors, including geometry and thermal properties of the nozzles and thermal properties of the liquid. A constant liquid pressure can be achieved by applying pressure to liquid reservoir 418 under the control of liquid supply controller 424 that is managed by controller 400.
The liquid is distributed via a liquid supply line entering printhead 10 at liquid inlet port 42. The liquid preferably flows through slots and/or holes etched through a silicon substrate of printhead 10 to its front surface, where a plurality of nozzles and printhead transducers are situated. In some preferred embodiments of the present inventions the printhead transducers are resistive heaters. In other embodiments, more than one transducer per jet may be provided including some combination of resistive heaters, electric field electrodes and microelectromechanical flow valves. When printhead 10 is at least partially fabricated from silicon, it is possible to integrate some portion of the printhead transducer control circuits 412 with the printhead.
A secondary drop deflection apparatus, described in more detail below, is configured downstream of the liquid drop emission nozzles. This secondary drop deflection apparatus comprises an airflow plenum that generates air flows that impinge individual drops in the plurality of streams of drops flying along predetermined paths based on pattern data. A negative pressure source 420, controlled by the controller 400 through a negative pressure control apparatus 422, is connected to printhead 10 via negative pressure source inlet 99.
A front face view of a single nozzle 50 of a preferred printhead embodiment is illustrated in
Two resistive heaters, side one heater 30, and side two heater 38, are formed on a front face layer on opposite sides of the nozzle bore, wherein the term “side” means perpendicularly above or below the array axis of the nozzles as is seen in
The spacing away from the nozzle rim and the width of the side heaters along the direction perpendicular to the array of nozzles are important design parameters. Typically the inner edge of the side heater resistors is positioned approximately 1.5 microns to 0.5 microns away from the nozzle edge. The outer edge, hence width, of the side heater resistors is typically placed 1 micron to 3 microns from the inner edge of the side heater resistors.
One effect of pulsing side heaters 30 and 38 on a continuous stream of fluid 62 is illustrated in a side view in
In
In
a) illustrates power pulse sequences that may be applied to side one heater resistor 30 and side two heater resistor 38 to launch the dominant surface waves 70 depicted in
b) and 4(c) illustrate two pulse sequences that may be used to not only synchronize jet break-up but also to deflect a portion of the fluid in a sideward deflection. For example in
Alternatively,
For the purpose of understanding the present inventions it is necessary only to recognize that the application of asymmetric heat at the nozzle of a continuous jet can deflect the jet. Practically achievable deflection amounts are of the order of a few degrees. For the present inventions it is assumed that thermal deflection or deflection by other means to be discussed below, achieves deflections of 0.5 to 2.0 degrees away from the nominal, undeflected flight paths of undeflected drop streams.
Thermal pulse synchronization of the break-up of continuous liquid jets is also known to provide the capability of generating streams of drops of predetermined volumes wherein some drops may be formed having integer, m, multiple volumes, mV0, of a unit volume, V0. See for example U.S. Pat. No. 6,588,888 to Jeanmaire, et al. and assigned to the assignee of the present inventions.
In
The capability of producing drops in multiple units of the unit volume V0 may be used to advantage in differentiating between print and non-printing drops. As will be discussed below, drops may be deflected by entraining them in a cross air flow field. Larger drops have a smaller drag coefficient to mass ratio and so are deflected less than smaller volume drops in an air flow field. Thus an air deflection zone may be used to disperse drops of different volumes to different flight paths. In the present inventions, drops of a small volume are deflected the largest amount in an airflow plenum and are captured before they can impinge the liquid pattern receiving medium. The liquid pattern is formed by less-deflected, large volume drops. Large and small volume drops are produced by pulse sequences such as those illustrated in
An airflow plenum with extended sidewalls 90 according to the present inventions is added to the liquid pattern writing apparatus in the perspective view of
Airflow plenum 90 is illustrated as having a primary opening 98 over which the streams of drops of predetermined volumes travel. A source of negative pressure (not shown) is applied to the opposite end, the evacuation end 97 of the airflow plenum, creating an air flow in the direction “A”, generally along the negative x-direction. Airflow plenum 90 is bounded by upstream wall 160, downstream wall 170, first side wall 180 and second side wall 190. The terms “upstream” and “downstream” are used herein to convey the sense of drop motion from a printhead 10 located at the upstream end of the liquid travel to a receiver medium 300 located at the downstream end of liquid travel. Primary opening 98 is formed by the upstream wall end 162, downstream wall end 172, first side wall end 182 and second side wall end 192. Primary opening 98 is further defined by the inner edges of the impingement wall ends, that is, by upstream wall inner edge 164, downstream wall inner edge 174, first side wall inner edge 184 and second side wall inner edge 194.
For some preferred embodiments of the present inventions the side wall ends are extended above the upstream and downstream wall ends by first and second side wall extension lengths, L1sw, L2sw. The side walls are extended in this fashion to reduce undesirable deflection of end jet drops from in the y-direction, caused by air flow into the plenum over the side walls.
The airflow set up in airflow plenum 90 by a negative pressure source (not shown) applied to the airflow plenum evacuation end 97, entrains small volume drops 84 as well as large volume drops 85 as they travel over the primary opening. For the Reynolds number space involved, the drag of the airflow on the individual drops may be approximated by Stoke's Law. The aerodynamic drag force, Fa, on a drop of mass md and diameter Dd is approximately:
Fa=mdad=3πνADdVA, (1)
where ad is the drop acceleration in the direction of the air flow velocity VA and νA is the viscosity of the air. Substituting the drop volume and liquid density, ρ, into Equation 1 gives an expression for the drop acceleration in the air deflection direction as a function of the drop diameter, Dd:
From Equation 2 it may be appreciated that the acceleration of drops is inversely proportional to their diameter squared; smaller drops are accelerated by an air flow more than large volume drops.
The amount of spatial deflection that the drop acceleration creates depends on the time that the drop is impinged by the airflow. The time the air flow deflection force acts is estimated as the length of the interior of airflow plenum 90 along the z-direction near the nominal flight plane, Sdz, divided by the drop or fluid velocity, Vd. The amount of drop deflection in the air flow direction A (minus x-direction in
where the quantities are as previously defined. For example, the following parameters and deflection amounts are representative: νA=181 μpoise, ρ=1 g/cm3, Sdz=0.2 cm, VA=1500 cm/sec, Vd=1500 cm/sec. Equation 3 becomes:
Therefore, for Dd=17.8 μm (for 3 pL drops), xd≈137 μm and for Dd=30.6 μm (for 15 pL drops), xd=46.3 μm. For these example values, the air deflection system deflects the 3 pL drops by ˜91 μm more than it does the 15 pL drops.
It may be appreciated from Equation 3 that the dispersion of large and small drops into two separated flight paths using air flow deflection may be increased by the manipulation of several design factors. The dispersion increases with the square of the deflection zone length, Sddz, with the inverse square of the ratio of small drop diameter, Dds, to large drop diameter, Dd1, with the inverse square of the drop velocity, Vd, and linearly with the airflow velocity, VA. Note that because the drop diameter varies as the inverse cube of the drop volume, the dispersion of drop deflection will vary as the inverse ⅔ power of drop volume. In the above example, if the airflow deflection zone length, Sdz, were increased to 0.3 cm and the drop velocity, Vd, decreased to 1000 cm/sec, then all drops would be deflected by an increased factor of (1.5)4=5.05, so the dispersion between 3 pL and 15 pL drops would be also increased by this amount, i.e. to ˜460 μm.
In
An important object of the present inventions is to increase the effective or average deflection air flow velocity that drops are subjected to for a given amount of negative pressure applied to the evacuation end of the airflow plenum. Another object it to reduce drop placement errors due to air flows that develop along the y-direction near the end jets of an array.
Extended side wall airflow plenum 90 is illustrated in a cross sectional side view in
A computer calculation of airflow velocity vectors 200 has been superimposed on the apparatus elements. The computer calculation was done using a standard finite volume Computational Fluid Dynamics (CFD) approach. “Flow-3D” code available commercially from Flow Science Incorporated located in Santa Fe, N. Mex. was used. The airflow vectors 200 indicate both direction and velocity magnitude by their relative lengths. Air is drawn into the drop impingement end 98 of airflow plenum 90 from all directions. For the simple rectangular shape illustrated, the airflow has vector components along the z-direction that increase z-direction drop velocity at the upstream end and decrease z-direction velocity at the downstream end of the airflow plenum. To first order these z-direction acceleration affects on a drop cancel one another, leaving the primary affect acceleration in the minus-x-direction.
Small volume drops 84 are deflected along flight path 128, finally impacting the inner surface of downstream wall 170 at point 130. A captured drop recovery conduit 240 is provided to collect the non-print drops. The drop capture apparatus may have many well know forms. Drops may be captured in the airflow plenum interior 92, along the downstream wall inside surface, on the downstream wall end wall surface or even by a capture apparatus positioned beyond the downstream wall and in front of the receiver medium 300. A porous material 243 may also be included in the drop capture design to assist in wicking liquid rapidly away from the impact point to reduce potential splashing and mist generation. A liquid recovery connection 245 is indicated schematically. The liquid recovery subsystem may apply a separate vacuum to the liquid recovery conduit 240 or negative pressure from the negative pressure source 420 may be tapped for liquid recovery.
In keeping with the amount of deflection of small drops indicated in the calculations above, the point of small drop impact and collection 130 may be on the order of 100 to 700 microns away from the nominal flight plane. Large drops must be permitted to pass over the downstream wall end to reach receiver medium 300 so the closest surface of the downstream wall end must be positioned farther away than the large drop deflection amount, plus some margin for reliability.
Several additional features of the extended side wall airflow plenum 92 are illustrated in a schematic top view in
An enlarged view of the calculated vectors of air flow 200 over upstream wall 160 shown in
If the wall edges over which air flow is drawn into the airflow plenum are given an aerodynamic shape, the low velocity vortex can be reduced in size and drawn farther down into the airflow plenum, away from the nominal drop flight zone.
Deflected drops from an end jet located inwardly approximately 360 μm from the first side wall inner edge 184 land at point 342 at the media plane; drops from jets 600 μm and 830 μm inward land at points 344 and 346, respectively. The air flow deflection subsystem has deflected the large volume print drops in the minus x-direction by an amount δx1v≈46 μm. In the calculational simulation, small volume drops were deflected by significantly larger amounts and were captured before they reached the receiver medium plane 300.
Large volume print drops were also deflected in the y-direction, away from first side wall inner edge 184 by amounts that decrease with distance inward towards the interior of the airflow plenum. For the calculational example plotted, the y-deflection, δyej, for the end jet located 360 μm from the first side wall inner edge, is δyej≈7 μm. The y-deflection positions for drops 344, 346 emitted from the more inward jets, δy1 and δy2, are significantly smaller. A more significant y-direction deflection would be seen if the end jet were located within a side wall thickness of the first side wall edge 184, as may be appreciated by studying the air flow vectors plotted in
Side wall deflection effects may be reduced according the present inventions by airflow plenums that incorporate one or more of three design features. Firstly, the side walls may be positioned at least one wall thickness away from the nearest stream of print drops.
Airflow plenum designs according to the present inventions utilize the above discussed three design features, or combinations thereof, to reduce undesirable y-deflection of liquid pattern forming drops emitted from nozzles near the ends of the nozzle array, while maintaining compactness of the air flow deflection apparatus dimension along the nozzle array axis direction.
Some preferred embodiments of the present airflow deflection inventions may also be utilized in combination with a continuous drop emitter that uses mono-size drops and an initial deflection at the nozzle using a path selection element, as illustrated in
If the airflow pattern in airflow plenum 90 has a velocity magnitude gradient in the minus x-direction, then drops following the firstly deflected path 124 will be deflected more than drops following the nominal flight path 122. Contours of equal velocity magnitude from the same calculational example used for illustrative purposes in
Mono-size print drops emitted from nozzles near array ends will be more strongly affected by y-direction air flows than are the large volume drops used in two-volume-size printing systems. The preferred embodiments of side wall spacing, extension and aerodynamic shaping discussed above are also preferred for air plenums used with mono-sized drop printing.
An alternative air plenum design embodiment of the present inventions having extended upstream and downstream walls as well as side walls is illustrated in
It is not necessary for the practice of the present inventions for all of the walls of the slotted airflow plenum 91 to extend the same amount above the nominal flight plane. Each plenum wall may be designed to optimize and shape the deflection air flow field independently and in accordance with other surrounding printing system hardware. Also the downstream slot opening 230 need not be of equal height or position relative to the nominal flight plane as is the upstream slot opening 220. For example it may be advantageous for drop capture or for latitude for print drop clearance to position the first inner edge 232 of downstream slot 230 farther away in the minus x-direction from the nominal drop flight plane than the upstream spacing amount, Su.
For the simple rectangular shapes illustrated, the airflow has vector components along the z-direction that increase z-direction drop velocity at the upstream end and decrease z-direction velocity at the downstream end of the airflow plenum. To first order these z-direction acceleration affects on a drop cancel one another, leaving the primary affect acceleration in the minus-x-direction. Small volume and large volume drops are differentially deflected in the minus x-direction as was discussed above with respect to the extended side wall airflow plenum. The previous discussions of Stoke's Law acceleration and deflection magnitudes apply to the slotted airflow plenum embodiments in analogous fashion.
A first order benefit of the slotted airflow plenum design over the extended side wall plenum is an increase in average deflection air velocity over the nominal flight plane region within the airflow plenums.
The highest three velocity magnitude contours for the slotted airflow plenum 91 are re-plotted in
The slotted airflow plenum design may be further improved by forming the upstream and downstream slot first inner edges 222, 232 with an aerodynamically curved shape of increasing radius toward the interior of the plenum, as illustrated in
An optimum length for the extension of the slotted plenum was examined by calculating the flow rates through the upstream and downstream slot openings 220, 230 as compared to the flow rate through the primary opening 98. The performance of the slotted airflow plenum in terms of increased average deflection air flow velocity is optimized when the flow rate through the slot openings is minimized. A flow rate calculation was performed using the computational software noted above for a slotted airflow plenum having equal upstream and downstream slot opening heights, hus=500 μm and equal wall thicknesses, tuw=250 μm. The deflection zone length was Sdz=2000 μm. The negative pressure source was adjusted to produce a peak airflow velocity magnitude of 1700 cm/sec.
A plot of the total upstream and downstream slot opening airflow rate, Qus+Qds, versus equal upstream and downstream wall extension lengths, Luex, is plotted in
An additional benefit of the slotted airflow plenum design is a dampening of perturbing air currents that may be generated by a variety of system hardware components, and especially by the relative motion of a printhead and receiver media. The extended plenum walls shield the interior from some portion of air currents that are generated outside the plenum. An example was calculated using all of the previous calculational parameters and the addition of a 100 cm/sec exponentially decaying air velocity generated by, for example, a receiver media moved at 100 cm/sec in the positive x-direction past the printhead, VM=100 cm/sec, dragging along an air film.
Many methods of forming a liquid pattern using the deflection airflow plenum designs of the present inventions may be apparent from the forgoing discussion. One set of methods according to the present inventions is illustrated schematically in
Ambient air is drawn into the deflection airflow plenum by means of a negative pressure source connected to an evacuation end of the airflow plenum in step 806. The internal airflow created in the deflection air flow plenum deflects small volume drops significantly more than large volume drops, creating a spatial dispersion between small and large volume drops in the direction of airflow in the airflow plenum. Small volume drops are captured either within or on the deflection airflow plenum, or after passing through it, before reaching the receiving media in step 608. Large drops are permitted to pass through the airflow plenum region and travel to the receiver medium, thereby forming a desired liquid pattern on the receiver in final method step 810.
The inventions have 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 spirit and scope of the inventions.
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