Patent Application
20100224091
The present invention pertains to relief printing and in particular to improving the printability of solid relief features.
Flexography, which is one example of relief printing, produces an image on a substrate by transferring ink from the surface of a relief plate, representing the image, directly to the substrate. Relief features in a flexographic plate are typically formed by subjecting a plate precursor to a curing radiation (e.g. ultraviolet light) through an image-wise mask and then developing the precursor to wash away parts of the plate that have not received sufficient curing radiation. The resulting relief features typically comprise solid areas and halftone dots of varying sizes and/or quantities per area to represent a range of tones specified by the image data. For example, a highlight tone can be represented by an array of very small relief dots in an area, a shadow tone can be represented by an array of large dots in an area, and a full tone can be represented by a solid relief area.
The orientation of printing plate 5 on the plate cylinder 4 and the printing direction 3 of the press determine, for any given relief feature, the directionality of contact between printing plate 5 and both inking roller 7 and printed medium 2. In particular, for any given relief feature, one can consider that there are leading and trailing edges of contact. The leading edge is the one that first makes contact with the inking roller 7 or printing medium 2. Similarly, the trailing edge is the one that makes contact last. In the exemplary press of
One problem that can occur with such printing systems is the presence of an inking void near the trailing edge of a full tone relief feature making contact with the anilox roller. The inking void is typically recognized in the reproduced image on the printing medium as a substantial reduction in ink density in the void area relative to adjacent areas of the solid feature. In practice, an average reduction of 50% to nearly 100% is common in the void.
Interestingly, the problem seems to be restricted to full tone image features or nearly full tone image features (i.e. above 95% tonality). In other words, relief features (e.g. halftone dots) representing substantially less than full tone, typically will not show the presence of an inking void while full tone features on the same printing plate will. The inking void generally follows the outline of trailing edges of a full tone image feature. It generally does not appear on the leading edges or on edges that are substantially aligned with the printing direction.
Some prior art literature, such as the article “Increase Anilox pressure for trailing edge void problems”, by Cordes Porcher, in the October 2004 issue of Paperboard Packaging, discuss the problem and a solution involving controlling anilox roller pressure. Testing has confirmed that this can address the problem, but in general can result in decreased performance in reproduction of halftone (i.e. not full tone) relief features.
It is desirable, therefore, to find ways to reduce the occurrence of trailing edge inking voids in solid features without compromising the fidelity of halftone feature reproduction.
The present invention provides a system and method for reducing the occurrence of an inking void in the vicinity of the trailing edge of a full tone image feature of an inked relief plate. According to one embodiment of the invention, a system produces or obtains original halftone data for an image to be reproduced. The system modifies halftone data associated with the full tone feature to modify the topography of the finished relief plate in order reduce the occurrence of a trailing edge inking void.
According to one aspect of the invention, the system determines a printing direction with respect to the image data to identify, for an intended press configuration, which parts of an image relief feature correspond to a trailing edge of contact with either an inking roller of the press or the printing medium processed by the press.
According to another aspect of the invention, the system modifies halftone data corresponding to a trailing edge portion of a full tone feature. According to one embodiment of this aspect of the invention, the system modifies halftone data by disabling a pattern of halftone pixels near the trailing edge of a full tone feature. According to one preferred embodiment, the pattern comprises a pattern of slits, arranged with a regular spacing and with the slits substantially aligned with the printing direction.
These and other aspects of the present invention are illustrated in the detailed description of the invention.
Many relief features produced on flexible plate media exhibit an increased relief height near leading edge 12 and trailing edge 13. This generally occurs whether the plate was made by an analog or a digital process with the magnitude of the increase varying significantly depending on the process conditions used.
It was believed that edge height variations could contribute to the presence of trailing edge inking voids 14 and experiments were conducted to try and reduce their magnitude. The most promising approach discovered was to introduce a pattern of holes in the solid halftone data near trailing edge 13. In preferred embodiments, the pattern comprised a series of fine slits (or linear indentations) near and substantially perpendicular to the trailing edge(s) of a relief feature. The slits represent a reduced relief height that may or may not extend to the floor of the printing plate.
Experimentally, patterns having slits approximately 20-30 micron wide, 150-200 microns long and spaced 150-800 microns apart substantially reduced or eliminated the edge-void artifacts. Empirically it was observed that slit length to slit spacing ratios in the 0.25 to 1.00 range produced excellent results. In one preferred embodiment, slits have a regular spacing. In other embodiments, slits can have a randomized spacing.
Trailing edge 13 of the left hand line feature is indicated. To the left of trailing edge 13, is a medium-colored area indicating the desired presence of ink. To the left of the medium-colored area is a light-colored area corresponding to trailing edge inking void 14. To the left of trailing edge inking void 14 is another properly inked area. The left hand feature thus illustrates the typical problem of trailing edge inking voids present on an inked plate.
Leading edge 12 of the right hand line feature is also indicated. For comparative purposes, the right hand relief feature was made with a pattern 20 comprising slits 21 established with an approximate width of 20 microns, an approximate length of 150 microns and an approximate spacing of 300 microns. The coloring indicates that, with the exception of slits 21, all areas of the right hand feature carry the desired amount of ink.
Thus, original halftone data for area 22 would have included only “enabled” halftone data values. According to one embodiment of the present invention, pattern 20 has been created by disabling selected halftone data values near the trailing edge(s) of full tone image features.
For an imaging device, with pixel dimensions of approximately 10 microns per side, an exemplary slit width 35 comprises approximately 2 pixels, an exemplary slit length 36 comprises approximately 15 pixels, and an exemplary slit spacing 37 comprises approximately 30 pixels. For many plate making systems and plate precursors, a printing plate 5 made using halftone image data 32 will have relief profiles that are highly correlated with halftone image data 32.
In some embodiments, this can be accomplished by performing image processing to identify trailing edges of full tone features of image data 50. This may include first determining a printing direction with respect to image data 50. For example, the printing system may be configured to rotate one or more of the image data 50, printing plate 5, and printing medium 2 with respect to printing direction 3 throughout the image processing, plate making and printing processes.
Identifying a trailing edge of a full tone image feature can be performed through a variety of well known image processing techniques. Application of pattern 20 to disable halftone pixels can then be applied through masking or other techniques.
Modified halftone image data 52 can then used by plate maker 53 to produce relief printing plate 5. This can include, for example, production of an image mask followed by one of a variety of exposure and processing process.
Finally, relief printing plate 5 can be used by a prior art printing press 1 to produce one or more copies of a printing medium 2 carrying ink to represent the original image.
Embodiments of the present invention may comprise any medium which carries a set of computer-readable signals comprising instructions which, when executed by a computer processor, cause the computer processor to execute a method of the invention. Embodiments may be in any of a wide variety of forms. Embodiments may comprise, for example, physical media such as magnetic storage media including floppy diskettes, hard disk drives, optical data storage media including CD ROMs, DVDs, electronic data storage media including ROMs, flash RAM, or the like or transmission-type media such as digital or analog communication links. The instructions may optionally be compressed and/or encrypted on the medium.
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.
