Patent Grant
7063416
This invention relates to ink-jet printing.
In ink-jet printing, ink is ejected from a narrow orifice in the direction of a substrate. In one type of ink-jet printing, known as drop-on-demand printing, the ink is ejected in a series of drops. The drops may be produced and controlled using a piezoelectric ink-jet head that has a large number of orifices. Each orifice is separately controllable to selectively eject ink at desired locations, or pixels, of the image. For example, an ink-jet head may have 256 orifices that have spacing for a printing resolution of at least 100 pixels (dots) per inch (dpi) and sometimes far more than that. This dense array of orifices allows complex, highly accurate images to be produced. In high performance print heads, the nozzle openings typically have a diameter of 50 microns or less (e.g., around 25 microns), are separated at a pitch of 25–300 nozzles/inch, have a resolution of 100 to 3000 dpi or more, and provide drop sizes of about 1 to 70 picoliters (pl) or less. Drop ejection frequency is typically 10 kHz or more. A drop-on-demand piezoelectric print head is described in U.S. Pat. No. 4,825,227, the entire content of which is incorporated herein by reference.
“Cockle” or “cockling” refers to a morphological change (e.g., a dimensional change) in an area of a print substrate caused by the substrate's interaction with ink. Substrate cockle can be detrimental to image quality. One approach used in the office printer field to prevent image distortion effects related to cockle is to limit the coverage of ink disposed on the substrate so that any subsequent cockling minimally distorts the substrate. However, this approach can be limiting, especially in applications requiring high-resolution full-color images. Another approach to the problem of cockle distortion is to use a coated or treated substrate. These substrates typically include additives such as clay, silica, or other materials to produce a glossy paper and inhibits volume-changing interactions with ink, thereby preventing cockling. Coated papers are commonly used in commercial photo ink-jet printers, which produce high-resolution full-color images over, for example, a 6 inch×4 inch or larger area.
Commercial printing is commonly done on multi-color continuous web printing presses. The web, provided, e.g., as a roll of paper, is directed along a paper path that includes separate stations for each color. The web is then slit into sheets and stacked.
In general, in a first aspect, the invention features a method of printing, including providing a print zone and a substrate, the print zone including multiple print regions in which ink drops are sequentially deposited on the substrate, and moving the substrate and the print zone relative to one another while controlling the rate of relative motion such that subsequent drops are deposited before the substrate is substantially distorted by cockling.
Implementations of the method can include one or more of the following features and/or features of other aspects.
The method can further include moving the substrate and print zone relative to one another such that subsequent drops are deposited after previous drops have substantially wicked into the substrate. The print zone can include four print regions, and each print region can be configured to deposit different colored ink on the substrate. The substrate can be a plain paper substrate (e.g., newsprint paper). The ink can include a solvent (e.g., water or an organic solvent) and a pigment mixed in the solvent.
Drop placement errors due to distortion of the substrate by cockling can be less than about two pixels in length (e.g., less than about one pixel, 0.5 pixels). The maximum cockling magnitude in the print zone can be about 1 millimeter (e.g., less than about 1 millimeter, 500 microns, 300 microns, 200 microns). The rate of relative motion can be more than about one meter per second (e.g., more than about two, three, four, five meters per second). Ink coverage of an area of the substrate can be more than about 50 percent (e.g., more than about 100 percent, 150 percent, 200 percent, 250 percent). Subsequent drops can be deposited within about two seconds of initial drops being deposited (e.g., within about one second, 0.5 seconds, 0.3 seconds, 0.2 seconds).
Each print region can include one or more printhead and the rate of relative motion can be such that where the substrate is substantially distorted by cockling, the substrate does not contact any printhead.
In general, in a further aspect, the invention features a printing system, including a print zone having multiple print regions in which ink drops are sequentially deposited on a substrate as the substrate and the print zone move relative to one another, wherein the rate of relative movement satisfies the relationship ν≧L/τc, where L is the print zone length and τc is a cockling time constant.
Embodiments of the printing system can include one of more of the following features and/or features of other aspects.
τc can be such that the maximum cockling magnitude in the print zone is about 0.5 mm or less departure from substrate planarity at a coverage of about 30% or more. The ink drops can be formed of aqueous ink and the substrate can be plain paper. The substrate can be a continuous web and the printing regions can include print stations arranged sequentially along a web path. The ink drops can be generated by piezoelectric ink jet printheads. The rate of relative motion can also satisfy the relationship ν≦l/τw, where l is a distance between adjacent print regions and τw is a wicking time constant.
In general, in another aspect, the invention features a method of printing, including providing a print zone and a substrate, the print zone including multiple print regions in which ink drops are sequentially deposited on the substrate, and moving the substrate and the print zone relative to one another while controlling the rate of relative motion such that subsequent drops are deposited within a time characteristic of an interaction between the ink and the substrate, wherein ink deposited after the time would result in a distorted image.
Implementations of the method of printing can include one or more of the following features and/or features of other aspects.
The interaction can be a cockling of the substrate. The interaction can be a change in the surface energy of the substrate. A distorted image can have a dot placement error of more than about 0.5 pixels (e.g., more than about one pixel).
Embodiments of the invention may include one or more of the following advantages.
Embodiments can reduce image distortion due to substrate cockle when printing on absorbent substrates, for example, when sequentially printing aqueous inks onto untreated paper. This reduction in distortion can provide for high coverage printing of color images using absorbent substrates, e.g., aqueous inks on newsprint paper. Newsprint paper and aqueous inks can provide a cost savings compared to using treated papers. Moreover, newsprint provides an aesthetic appeal to consumers. In particular, newspaper readers are comfortable with the feel of newsprint. Aqueous ink chemistries are also desirable. For example, aqueous inks are widely available and can avoid environmental problems associated with solvent-based inks.
Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
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The print heads 30 can be of various types, including piezoelectric drop on demand ink-jet print heads with arrays of small, finely spaced nozzle openings. Piezoelectric ink-jet print heads are described in Hoisington U.S. Pat. No. 5,265,315, Fishbeck et al. U.S. Pat. No. 4,825,227 and Hine U.S. Pat. No. 4,937,598, the entire contents all of which are hereby incorporated by reference. Other types of print heads can be used, such as, for example, thermal ink-jet print heads in which heating of ink is used to effect ejection. Continuous ink-jet heads, that rely on deflection of a continuous stream of ink drops can also be used. In a typical arrangement, the stand off distance between the web path and the print bar is between about 0.5 and one millimeter.
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In some embodiments, the cockle time constant and web feed rate are such that the web is also clear of portions of print stations that would otherwise be impacted by the web due to cockling before the paper cockles. For example, where a printhead surface is positioned close to the web (e.g., less that 1 mm from the web), cockle that occurs after the web emerges from the print zone could cause the web to impact down-web portions of the printhead. Thus, the web feed rate and print station arrangement should be designed so that substantial cockle only occurs after the web clears the printhead portions that are close to the web.
In certain embodiments, where each print station jets an ink having a different interaction time constant with the paper, the print stations can be arranged so that the ink with longer time constant are jetted prior to inks with relatively shorter time constants.
This relationship between the web feed rate and the ink-substrate interaction time accommodates the negative effects of substrate cockle by running the web at a sufficient velocity and/or reducing the print zone length. However, printing on wet ink before it wicks into the paper can also cause negative printing effects (e.g., bleeding between different colors). A wick time constant, τw, can be associated with the time it takes for an ink drop to wick into the paper. The web velocity is selected to satisfy ν≦l/τw, where l is the distance between adjacent print stations. Thus, for a web-based printing line, such as illustrated in
The ink-substrate interaction time depends on the ink type, the substrate type and the ink coverage. The ink can be various types, including solvent-based, hot-melt or aqueous ink. An aqueous ink includes a pigment or a dye suspended in a carrier that includes a substantial amount of water (e.g., five percent by weight or more). More typically, the carrier in aqueous inks includes more than about 35 percent water, such as 80–90 percent or more. Often, aqueous ink carriers also include a substantial amount of glycol (e.g., more than about five percent by weight, such as 50 percent or more). Aqueous inks are desirable because of their low cost and they reduce or eliminate the use of organic solvents. The substrate type can be a coated or treated paper or uncoated paper. Uncoated or plain paper, such as newsprint, is substantially free of clay or silica additives, and is available at low cost.
Ink coverage refers to the fraction of ink provided by each print station with compared to the maximum amount the station could provide. For example, ink coverage of 50 percent corresponds to printing ink from one station on half the available pixels in the area for one side of an area of paper. Thus, for a four-station printing press, such as the press shown in
The type of ink and/or paper used can be selected according to their cockle time constant. Cockle time constant can be determined by observing the rate and magnitude of cockle distortion for a given ink coverage. The maximum acceptable cockling magnitude can be determined based on the desired image quality and other process characteristics. For example, in order to avoid contact between the web and the printhead that may damage the heads and cause subsequent image distortion, the maximum cockling magnitude in the print zone should not be greater than the stand off space between the substrate guide and the printhead nozzles. For high resolution ink jet printing using, e.g., piezoelectric printheads, the standoff distance is typically 1 mm or less, e.g., about 0.5 mm. The maximum cockling magnitude can be, for example, 50 percent, 20 percent, 10 percent or five percent or less of the standoff distance. The maximum cockling magnitude can also be determined on the basis of departure from web planarity. For example, the maximum departure from planarity can be about 0.7 mm, 0.5 mm, or 0.1 mm or less. The maximum cockling magnitude can also be delivered on the basis of image error. In embodiments, the cockle time constant can be more than about 0.1 seconds (e.g., 0.5 seconds, one second, two seconds, three seconds, or more). The maximum cockling magnitude can also be determined by the image error, which can be determined by visual examination or by quantitative dot placement error. Dot placement error refers to a distance of a jetted drop location from a target location on the substrate. Dot placement error can be measured in pixels. Typically, dot placement errors become apparent between about one and two pixels in length, although depending on the printing system and the image, errors can be apparent for dot placement errors as low as about 0.5 pixels in length. Drop placement error can be determined using microscopic inspection of images printed on a test target. Such inspection can be substantially automated, using commercial or customized image analysis techniques. Alternatively, or additionally, dot placement error can be determined using visual inspection of completed images. By completing printing before any substantial paper cockling, images with high ink coverage (e.g., more than about 50 percent, 75 percent, 100 percent, 150 percent, 200 percent) can be printed on absorbent substrates with drop placement errors of less than about two, such as 0.5 pixel lengths for errors due to cockling.
Alternatively, or additionally, the type of ink and/or paper used can be selected to have relatively short wick time constant with respect to the cockle time constant. For example, the ratio of the wick time constant to cockle time constant can be less than about 0.2 (e.g., less than about 0.1, 0.05). In some embodiments, the wick time constant can be less than about 0.5 seconds (e.g., 0.1 seconds, 0.05 seconds, 0.01 seconds or less). To further illustrate this velocity window, consider an example where L is 1.5 meters and l is 0.5 meters. Given a cockle time constant τc 0.5 seconds and a wick time constant τw
0.05 seconds, the velocity window is from 3 meters per second (ν≧1.5/0.5) to 10 meters per second (ν≦0.5/0.05).
In some embodiments, in order to complete printing between the ink drops wicking into the substrate and the substrate cockling, the web velocity can be between about one and five meters per second (e.g., between about two and three meters per second)
The velocity window for a printing line can be determined by measuring the time constants for the ink/paper combination to be used. Alternatively, or additionally, the velocity window can be determined empirically during a setup phase prior to a printing run. In order to determine an appropriate web velocity (or range of velocities), the line operator can run the line at several different velocities printing a test image having coverage corresponding to a maximum anticipated coverage for the printing run. Upon subsequent examination of the test images, the operator can select the web velocity corresponding to the best image.
Although the foregoing description addresses techniques for avoiding image distortion due to substrate cockle, the methods disclosed herein can be applied to other interactions between ink and the substrate as well, including chemical and physico-chemical interactions. In particular, the disclosed methods can be applied to interactions between the ink and substrate that have a characteristic interaction time and provide a time window during which additional ink can be deposited on the same area of the substrate without substantial image distortion. For example, an ink may interact with the substrate to alter the surface energy of the substrate. Altered surface energy may cause subsequent ink drops to wet the surface in a way that results in undesirable image distortion. Where such an interaction occurs with a characteristic time constant, substantial image distortion can be avoided by depositing the additional ink within the period defined by the time constant.
The following studies were performed using UPM Norm C 45 g/m2 newsprint paper obtained from Heidelberger Druckmaschinen AG (Heidelberg, Germany) and an aqueous fluid mixture of 65 wt. % 1,2-propanediol (from Acros Organics supplied by Fisher Scientific, Suwanee, Ga.), 0.25 wt. % BYK-333 surfactant (from BYK Chemie, Wallingford, Conn.) and 35 wt. % deionized water.
1. Newsprint paper samples were coated with an aqueous fluid using a drawdown coater (an RK Print-Coat Instrument, which was obtained from RK Print Coat Instruments Ltd., Herts, United Kingdom. A #0 bar was selected for coating, and the coater speed setting was set to 10. According to the coater manufacturer's calibration table, this bar and speed provided a coating thickness of approximately 6–8 microns. A small volume of fluid (e.g., 2–3 cm3) was pipetted onto the coating bar that was placed on top of a sheet of newsprint. Upon activation, the coater drew the wet coating bar across an area of the newsprint sheet, applying a coating of the fluid to the area. Visual observation of the coated newsprint revealed significant distortion of the paper within one second of the coating. This time was determined by first measuring the time for the bar to complete its coating cycle and then observing the paper at the end of the cycle.
Aqueous fluid was sprayed using an aerosol spray nozzle (model no. 1/4 JCO-SS-SV13A-SS obtained from Spraying Systems Co. (Wheaton, Ill.) onto a 4 inch by 5 inch surface area of newsprint paper samples. A plexiglass frame was used to mask off all but a 4×5 inch window of each sample. The air pressure and liquid pressure of the aerosol were adjusted to provide a coating thickness of approximately 10–12 microns, as determined by placing a Mettler Toledo PB303 scale (obtained from Fisher Scientific) in the same location as the paper and directly weighing an equivalent amount of sprayed liquid. Two fiber optic lamps (FiberLite Model PL800obtained from Cole Parmer, Vernon Hills, Ill.) were positioned approximately one inch above the paper surface, eight inches from the exposed paper, thereby illuminating the exposed paper portion at an oblique angle, with the two lights oriented approximately 90 degrees from each other. A video camera (from Sony) was positioned directly above the paper. During the aerosol coating, the exposed surface was illuminated with the fiber optic lamps while the video camera recorded an image of the exposed surface. The camera frame rate was approximately 30 Hz. A frame-by-frame visual analysis of the recorded video footage was performed, and changes in the paper morphology were determined by the extent of shadows formed due to paper distortions occluding light from the lamps.
The frame-by-frame analysis revealed substantially no change in the paper in the first 300 milliseconds after coating. A noticeable change occurred within 500 milliseconds, and pronounced change was observed by one second after coating.
Still further embodiments are within the following claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4825227 | Fischbeck et al. | Apr 1989 | A |
| 4937598 | Hine et al. | Jun 1990 | A |
| 5265315 | Hoisington et al. | Nov 1993 | A |
| 5365843 | House et al. | Nov 1994 | A |
| 6126273 | Van Vooren et al. | Oct 2000 | A |
| 6428159 | Roy et al. | Aug 2002 | B1 |
| 6554416 | Koto | Apr 2003 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040252149 A1 | Dec 2004 | US |
