Patent Application
20130033687
The present disclosure is related to marking and printing methods and systems, and more specifically to methods and systems for deposition of a dampening fluid directly onto the imaging member, without an intermediate member such as a form roller.
Offset lithography is a common method of printing today. (For the purposes hereof, the terms “printing” and “marking” are interchangeable.) In a typical lithographic process a printing plate, which may be a flat plate, the surface of a cylinder, or belt, etc., is formed to have “image regions” formed of hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. The image regions are regions corresponding to the areas on the final print (i.e., the target substrate) that are occupied by a printing or marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by said marking material. The hydrophilic regions accept and are readily wetted by a water-based fluid, commonly referred to as a dampening fluid or fountain fluid (typically consisting of water and a small amount of alcohol as well as other additives and/or surfactants to reduce surface tension). The hydrophobic regions repel dampening fluid and accept ink, whereas the dampening fluid formed over the hydrophilic regions forms a fluid “release layer” for rejecting ink. Therefore the hydrophilic regions of the printing plate correspond to unprinted areas, or “non-image areas”, of the final print.
The ink may be transferred directly to a substrate, such as paper, or may be applied to an intermediate surface, such as an offset (or blanket) cylinder in an offset printing system. The offset cylinder is covered with a conformable coating or sleeve with a surface that can conform to the texture of the substrate, which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the imaging plate. Also, the surface roughness of the offset blanket cylinder helps to deliver a more uniform layer of printing material to the substrate free of defects such as mottle. Sufficient pressure is used to transfer the image from the offset cylinder to the substrate. Pinching the substrate between the offset cylinder and an impression cylinder provides this pressure.
Typical lithographic and offset printing techniques utilize plates which are permanently patterned, and are therefore useful only when printing a large number of copies of the same image (long print runs), such as magazines, newspapers, and the like. However, they do not permit creating and printing a new pattern from one page to the next without removing and replacing the print cylinder and/or the imaging plate (i.e., the technique cannot accommodate true high speed variable data printing wherein the image changes from impression to impression, for example, as in the case of digital printing systems). Furthermore, the cost of the permanently patterned imaging plates or cylinders is amortized over the number of copies. The cost per printed copy is therefore higher for shorter print runs of the same image than for longer print runs of the same image, as opposed to prints from digital printing systems.
Accordingly, a lithographic technique, referred to as variable data lithography, has been developed which uses a non-patterned reimageable surface coated with dampening fluid. Regions of the dampening fluid are removed by exposure to a focused radiation source (e.g., a laser light source). A temporary pattern in the dampening fluid is thereby formed over the non-patterned reimageable surface. Ink applied thereover is retained in pockets formed by the removal of the dampening fluid. The inked surface is then brought into contact with a substrate, and the ink transfers from the pockets in the dampening fluid layer to the substrate. The dampening fluid may then be removed, a new, uniform layer of dampening fluid applied to the reimageable surface, and the process repeated.
In the aforementioned system it is very important to have an initial layer of dampening fluid that is of a uniform and desired thickness. To accomplish this, a form roller nip wetting system, which comprises a roller fed by a solution supply, is brought proximate the reimageable surface. Dampening fluid is then transferred from the form roller to the reimageable surface. However, such a system relies on the mechanical integrity of the form roller and the reimageable surface to obtain a uniform layer. Mechanical alignment errors, positional and rotational tolerances, and component wear each contribute to variation in the roller-surface spacing, resulting in deviation of the dampening fluid thickness from ideal.
Furthermore, an artifact known as ribbing instability in the roll-coating process leads to a non-uniform dampening solution layer thickness. This variable thickness manifests as streaks or continuous lines in a printed image.
Still further, while great efforts are taken to clean the roller after each printing pass, in some systems it is inevitable that contaminants (such as ink from prior passes) remain on the reimageable surface when a layer of dampening fluid is applied. The remaining contaminants can attach themselves to the form roller that deposits the dampening fluid. The roller may thereafter introduce image artifacts from the contaminants into subsequent prints, resulting in an unacceptable final print.
In addition, cavitation may occur on the form roller in the transfer nip due to Taylor Instabilities (see, e.g., “An Outline of Rheology in Printing” by W. H. Banks, in the journal Rheologica Acta, pp. 272-275 (1965)). To avoid these instabilities, systems have been designed with multiple rollers that move back and forth in the axial direction while also moving in rolling contact with the form roller, to break up the rib and streak formation. However, this roller mechanism adds delay in the “steadying out” of the dampening system so printing cannot start until the dampening fluid layer thickness has stabilized on all the roller surfaces. Also, on-the-fly dampening fluid flow control is not possible since the dampening fluid layer is at that point already built up on the form roller and the other dampening system rollers acts as a buffering mechanism.
Accordingly, efforts have been made to develop systems to deposit dampening fluid directly on the offset plate surface as opposed to on intermediate rollers or a form roller. One such system applies the dampening fluid onto the reimageable offset plate surface. See, e.g., U.S. Pat. No. 6,901,853 and U.S. Pat. No. 6,561,090. However, due to the fact that these dampening systems are used with conventional (pre-patterned) offset plates, the mechanism of transfer of the dampening fluid to the offset plate includes a ‘forming roller’ that is in rolling contact with the offset plate cylinder to transfer the FS to the plate surface in a pattern-wise fashion—since it is the nip action of contact rolling between the form roller and the patterned offset plate surface that squeezes out the fountain solution from the hydrophobic regions of the offset plate, allowing the subsequent ink transfer selectivity mechanism to work as desired.
While the spray dampening system provides the advantage of precisely metering out the desired flow rate of the dampening fluid through control of the spray system, as well as the ability to manipulate the dampening fluid layer thickness on-the-fly as needed, the requirement of using the dampening system form roller as the final means of transferring the dampening fluid to the plate surface reintroduces the disadvantages of thickness variation, roller contamination, roller cavitation, and so on.
Accordingly, the present disclosure is directed to systems and methods providing a dampening fluid directly to a reimageable surface of a variable data lithographic system that does not employ a dampening form roller. Systems and methods are disclosed for application of dampening fluid directly to a reimageable surface of an imaging member in such a system.
A system and corresponding methods are disclosed herein for applying a dampening fluid to a reimageable surface of an imaging member in a variable data lithography system, comprising a subsystem for converting a dampening fluid from a liquid phase to a fine droplet or vapor state (herein referred to as a dispersed fluid), a subsystem for directing flow of said dispersed fluid comprising the dampening fluid in droplet or vapor phase to the reimageable surface, whereby the dampening fluid reverts to a continuous liquid layer directly on, and is thereby deposited on, the reimageable surface to form a dampening fluid layer.
A number of alternative systems and methods may be used for converting the liquid dampening fluid to a dispersed fluid, such as: an ultrasonic-based subsystem, a nozzle-based nebulizer subsystem, an impeller-based subsystem, and a vapor chamber subsystem. A bias or ionic charging subsystem may optionally be provided for applying a charge to droplets of dampening fluid while the dampening fluid is in a dispersed fluid state, to thereby enable the droplets to repel each other and avoid recombination prior to deposition on the reimageable surface and to enhance deposition onto the reimageable surface.
Various feedback and control systems are provided to measure the thickness of the layer of dampening fluid applied to the reimageable surface, and control, dynamically or otherwise, aspects of the dampening fluid deposition process to obtain and maintain a desired layer thickness.
In an alternative dampening fluid deposition system and method, a continuous ribbon of dampening fluid may be applied directly to the reimageable surface. According to this alternative, a subsystem for applying a dampening fluid to a reimageable surface comprises: a body structure having formed therein a port, the port extending in a first direction substantially perpendicular to a direction of travel of the reimageable surface when in use, the port having a width at least equal to a width of the reimageable surface in the first direction, the port configured to deliver dampening fluid in a continuous fluid ribbon directly to the reimageable surface to thereby form a dampening fluid layer thereover; a mechanism, associated with the body structure, for disrupting an entrained air layer over the reimageable surface when the reimageable surface is in motion; a dampening fluid reservoir disposed to provide dampening fluid to the port; and a control mechanism for controlling the flow of dampening fluid from the reservoir to the port and from the port to the reimageable surface. The mechanism may be a vortex-generating surface formed in the body structure. The control mechanism may be a valve, and may form a part of a thickness sensor control mechanism.
The above is a summary of a number of the unique aspects, features, and advantages of the present disclosure. However, this summary is not exhaustive. Thus, these and other aspects, features, and advantages of the present disclosure will become more apparent from the following detailed description and the appended drawings, when considered in light of the claims provided herein.
In the drawings appended hereto like reference numerals denote like elements between the various drawings. While illustrative, the drawings are not drawn to scale. In the drawings:
We initially point out that description of well-known starting materials, processing techniques, components, equipment, and other well-known details are merely summarized or are omitted so as not to unnecessarily obscure the details of the present invention. Thus, where details are otherwise well known, we leave it to the application of the present invention to suggest or dictate choices relating to those details.
With reference to
The key requirement of dampening fluid subsystem 14 is to deliver a layer of dampening fluid having a uniform and controllable thickness over a reimageable surface layer over imaging member 12. In one embodiment this layer is in the range of 0.2 μm to 1.0 μm, and very uniform without pinholes. The dampening fluid must have the property that it wets and thus tends to spread out on contact with the reimageable surface. Depending on the surface free energy of the reimageable surface the dampening fluid itself may be composed mainly of water, optionally with small amounts of isopropyl alcohol or ethanol added to reduce its natural surface tension as well as lower the evaporation energy necessary for subsequent laser patterning. In addition, a suitable surfactant may be added in a small percentage by weight, which promotes a high amount of wetting to the reimageable surface layer. In one embodiment, this surfactant consists of silicone glycol copolymer families such as trisiloxane copolyol or dimethicone copolyol compounds which readily promote even spreading and surface tensions below 22 dynes/cm at a small percentage addition by weight. Other fluorosurfactants are also possible surface tension reducers. Optionally the dampening fluid may contain a radiation sensitive dye to partially absorb laser energy in the process of patterning. Optionally the dampening fluid may be non-aqueous consisting of, for example, polyfluorinated ether or fluorinated silicone fluid.
In the description of embodiments of a dampening fluid subsystem 14 that follow it will be appreciated that as there is no pre-formed hydrophilic-hydrophobic pattern on a printing plate in system 10, the need for a form roller to transfer the dampening fluid is obviated. As mentioned, a laser (or other radiation source) is used to form pockets in, and hence pattern, the dampening fluid. The characteristics of the pockets (such as depth and cross-sectional shape), which determine the quality of the ultimate printed image, are in large part a function of the effect that the laser has on the dampening fluid. This effect is to a large degree controlled by the thickness of the dampening fluid at the point of incidence of the laser. Therefore, to obtain a controlled and preferred pocket shape, it is important to control and make uniform the thickness of the dampening fluid layer, and to do so without introducing unwanted artifacts into the printed image.
Ultrasonic Spray Subsystem
Accordingly, with reference to
Many ultrasonic humidifier devices are known in the art, and such devices may be modified based on the present disclosure to perform the function described herein. A commercially available system on which such a system may be based is the KAZ 5520 ultrasonic humidifier manufactured by Honeywell. Other examples include the BNB and BNU Series Stulz-Ultrasonic™ Humidifier, by Stulz Air Technology Systems, Inc. Therefore, the specific embodiment shown in
In an alternative embodiment 31, shown in
In certain embodiments steps may be taken to ensure that the generated droplets do not re-combine in mid-air, so that a controlled layer of dampening fluid can be formed on the reimageable surface layer of imaging member 12. One method of achieving this objective is to electrically charge the droplets, to enable the droplets to repel each other and avoid recombination prior to deposition on the reimageable surface. This may be accomplished, for example, by a bias system 52, which applies a bias to nozzle 44 (
Nozzle-Based Nebulizer Spray Subsystem
Referring next to
Control over the carrier flow rates, carrier temperatures, and rate of dampening fluid introduction into tee-structure 66 provide control over the thickness of the layer 74 of dampening fluid deposited onto the reimageable surface layer of imaging member 12. A control subsystem incorporating thickness sensor subsystem 28 may accomplish this dampening fluid deposition control.
In an alternative embodiment 61, shown in
In certain embodiments steps may be taken to ensure that the generated droplets do not re-combine in mid-air, so that a controlled layer of dampening fluid can be formed on the reimageable surface layer of imaging member 12. One method of achieving this objective is to electrically charge the droplets exiting at nozzle 72, to enable the droplets to repel each other and avoid recombination prior to deposition on to the reimageable surface. This may be accomplished, for example, by a bias system 78, which applies a bias to nozzle 72, as shown in each of
Impeller-Based Spray Subsystem
Referring next to
In the exemplary subsystem 82, dampening fluid from reservoir 84 is introduced onto a disk or impeller 86, which is caused to rotate by motor 88. The dampening fluid briefly accumulates on impeller 86, but due to the centrifugal force induced by the rotation of impeller 86, droplets of the dampening fluid are accelerated in a direction away from the center of impeller 86 toward a diffuser 90 comprised of a mesh, screen, comb filter, etc. The droplets of the dampening fluid hit diffuser 90 at a relatively high velocity, and are thereby broken up into even finer droplets. Temperature of the fluid, impeller 86, and/or diffuser 90 may be controlled to enhance vapor production. A commercially available system that may form the basis for such an embodiment is the KAZ V400 impeller humidifier, manufactured by Honeywell. The vapor of dampening fluid is directed onto the reimageable surface layer of imaging member 12, where it accumulates as a layer 94 of dampening fluid.
In an alternative embodiment 81, shown in
In certain embodiments steps may be taken to ensure that the generated droplets do not re-combine in mid-air, so that a controlled layer of dampening fluid can be formed on the reimageable surface layer of imaging member 12. One method of achieving this objective is to electrically charge the droplets exiting at diffuser 90, to enable the droplets to repel each other and avoid recombination prior to deposition on to the reimageable surface. This may be accomplished, for example, by a bias system 98, which applies a bias to diffuser 90, as shown in each of
In each of the aforementioned embodiments there may be a desire to remove dampening fluid introduced into the environment but not deposited onto the reimageable surface layer of imaging member 12, referred to herein as overspray. Motivations to do so include reducing waste, ensuring that unsafe additives to the dampening fluid are not vented into the environment, etc. According to one embodiment 100 for capturing overspray illustrated in
Another embodiment 101 for preventing introduction of dampening fluid into the external environment is illustrated in
Solution-Extrusion Subsystem
With reference to
In the present case of depositing a relatively thin fluid layer over a rotating surface, surface effects must be considered in order to ensure uniform application of the dampening fluid over the reimageable surface. For various physical reasons, as imaging member 12 rotates, a layer of entrained air (or other ambient fluid) is formed at its surface. This entrained air layer may underlay a fluid layer deposited over the reimageable surface unless the entrained air layer is interrupted. To this aim, extruder 152 may be shaped or have attached thereto or associated therewith a structure for disrupting or evacuating the entrained air layer. According to one embodiment, a vortex generating wall 162 is formed in extruder 152. As imaging member 12 rotates, at least a portion of the boundary layer entrained air is directed into vortex generating wall 162. This produces a vortex, resulting in a slight negative pressure in the space between the nozzle and the plate cylinder. This negative pressure extracts the entrained air boundary layer and draws dampening fluid into surface contact with the reimageable surface of imaging member 12, resulting in more uniform coverage of the dampening fluid over the reimageable surface.
Vapor Chamber Deposition Subsystem
With reference next to
The dampening fluid vapor 204 is transmitted to a heated condensation chamber 210, by way of a heated or heat-conductive conduit 212. The surfaces of condensation chamber 210 may be heated by thermal conduction via conduit 212, or independently heated such as by a heating coil 214. By heating the surface of heated condensation chamber 210 a temperature differential is created between the interior of condensation chamber 210 and the relatively cooler reimageable surface of imaging member 12. If the ambient within condensation chamber 210 is well below the boiling point of the vapor, the vapor condenses in the ambient and forms droplets before coming into contact with the reimageable surface of the imaging member 12. If the interior surfaces of the vapor chamber are heated to near or above the boiling point then condensation occurs only, and preferably, on the reimageable surface.
In addition, in the case in which the heat flows between the vaporization chamber 202 and the condensation chamber 210, the heat flow into the vaporization chamber 202 determines the evaporation rate and thus the vapor flow rate. The flow rate of vapor 204 is set to equal the steady state condensation rate on the reimageable surface of imaging member 12 as that surface passes by the condensation chamber 210. The condensation rate is set to provide the desired thickness of a thus-formed dampening fluid layer 216.
When the vapor condenses on the reimageable surface, latent heat is produced. For low latent heat dampening fluids, the latent heat will typically be negligible. However, heating a portion of the reimageable surface of imaging member 12 proximate condensation chamber 210, such as by its proximity to heating coil 214 or by other mechanisms, before patterning by optical patterning subsystem 16 can provide a small assist by reducing the optical power needed for patterning. Furthermore, heating the reimageable surface before inking at inking subsystem 18 can assist with obtaining a desired rheology change between inking and transfer.
Blade Metering Subsystem
With reference next to
A dampening fluid source 234, such as a pressurized nozzle ejector, deposits dampening fluid in a region upstream (behind) blade 232 in the direction of rotation of imaging member 12 to form an accumulation 236 of dampening fluid. The rate of application of the dampening fluid is adjusted relative to the rate of rotation of imaging member 12 such that dampening fluid does not over-accumulate. The spacing and angle between blade 232 and the reimageable surface determines the thickness of layer 238 of dampening fluid over the reimageable surface. This spacing and angle may be adjustable by way of an optional mount 233.
Shown in
In another embodiment 250 shown in
The adjustment provided by two-part blade/contour member 252 may be locally variable, such as illustrated in
In another embodiment 300 shown in
With reference to
A number of different configurations for the tip of the aforementioned blade embodiments are contemplated herein. (While the term “tip” is used in the following, it will be appreciated that due to the blade extending into the page as illustrated in the following-described figures the tip is actually en edge of the blade.) The tip configuration will have a direct impact on the quality of the resulting metered layer of dampening fluid. For example, reduced “streaking” in the dampening fluid layer (and hence in the final image) may be achieved. In one embodiment, smoothness of the tip is an object. In others, a desired surface texture in the object.
With reference to
With reference to
With reference to
With reference to
In various of the above-described embodiments it may be desirable to supplement the dampening fluid deposition mechanisms with a blading metering system to further control the uniformity of the thin layer of dampening fluid applied over the reimageable surface of imaging member 12. Therefore, the blade metering system described above may be combined with other dampening fluid application embodiments described herein and operated in tandem.
No limitation in the description of the present disclosure or its claims can or should be read as absolute. The limitations of the claims are intended to define the boundaries of the present disclosure, up to and including those limitations. To further highlight this, the term “substantially” may occasionally be used herein in association with a claim limitation (although consideration for variations and imperfections is not restricted to only those limitations used with that term). While as difficult to precisely define as the limitations of the present disclosure themselves, we intend that this term be interpreted as “to a large extent”, “as nearly as practicable”, “within technical limitations”, and the like.
Furthermore, while a plurality of preferred exemplary embodiments have been presented in the foregoing detailed description, it should be understood that a vast number of variations exist, and these preferred exemplary embodiments are merely representative examples, and are not intended to limit the scope, applicability or configuration of the disclosure in any way. Various of the above-disclosed and other features and functions, or alternative thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications variations, or improvements therein or thereon may be subsequently made by those skilled in the art which are also intended to be encompassed by the claims, below.
Therefore, the foregoing description provides those of ordinary skill in the art with a convenient guide for implementation of the disclosure, and contemplates that various changes in the functions and arrangements of the described embodiments may be made without departing from the spirit and scope of the disclosure defined by the claims thereto.
The present disclosure is related to U.S. patent application titled “Variable Data Lithographic System”, Ser. No. 13/095,714, filed on Apr. 27, 2011, and assigned to the same assignee as the present application, and further which is incorporated herein by reference.
