EP-4741927-A1 - SYSTEM AND METHOD FOR MITIGATING TRAILING EDGE VOIDS IN FLEXO PRINTING

Abstract

An aspect relates to a method for creating a flexographic printing plate having a printing surface corresponding to information in an image file, the flexographic printing plate comprising a photopolymer cured through openings in a mask layer of the plate, the method comprising: storing in the image file first micro-screen information comprising a first pattern of "on" and "off" single pixels in at least one first region of the image file; storing in the image file second micro-screen information comprising a second pattern of "on" and "off" single pixels different than the first pattern in at least one second region of the image file, and detecting the first region and the second region with an imager, and forming first micro-screen openings in the mask layer corresponding to the first micro-screen information having a different opening size than second micro-screen openings corresponding to the second micro-screen information.

Inventors

• Wolterink, Jörg
• Hänsel, Thomas
• GREVE, Christian
• GREWLING, Frank
• STUBBE-THIERER, Karsten
• SIEVERS, WOLFGANG
• Morisse, Peter

Assignees

• Esko-Graphics Imaging GmbH

Dates

Publication Date

20260513

Application Date

20211130

Claims (15)

1. A method for creating a flexographic printing plate having a printing surface corresponding to information in an image file, the flexographic printing plate comprising a photopolymer cured through openings in a mask layer of the plate, the method comprising: storing in the image file first micro-screen information comprising a first pattern of "on" and "off" single pixels in at least one first region of the image file; storing in the image file second micro-screen information comprising a second pattern of "on" and "off" single pixels different than the first pattern in at least one second region of the image file, and detecting the first region and the second region with an imager, and forming first micro-screen openings in the mask layer corresponding to the first micro-screen information having a different opening size than second micro-screen openings corresponding to the second micro-screen information.
2. The method of claim 1, wherein the at least one first region of the image file corresponds to an edge of a solid rendition or linework image region and the at least one second region of the image file corresponds to a center of the solid rendition or linework image region.
3. The method of claim 1 or 2, wherein forming first and second micro-screen openings having a different opening size is performed by allocating a first laser light intensity to each "on" pixel in the first micro-screen, particularly by hardware or firmware of the imager, and a second laser light intensity to each "on" pixel in the second micro-screen, particularly by hardware or firmware of the imager.
4. The method of claim 3, wherein the first laser light intensity is less than the second laser light intensity.
5. The method of claim 3 or 4, further comprising: determining an optimum value for the second laser light intensity for the image file, and applying a predetermined ratio of the second laser light intensity to the first laser light intensity to determine a corresponding value for the first laser light intensity.
6. The method of claim 5, wherein the predetermined ratio is in a range of 0.65 - 0.85, in particular in a range of 0.77 - 0.8, in particular 0.785.
7. The method of any one of claims 3 to 6, further comprising: allocating a gradient of laser light intensities including the first laser light intensity, the second laser light intensity and one or more other laser light intensity different from the first laser light intensity and the second laser light, particularly in accordance with a function.
8. The method of any one of claims 3 to 7, further comprising: storing information corresponding to the first laser light intensity and the second laser light intensity in the image file.
9. The method of any one of the preceding claims, further comprising: aligning the second pattern to the first pattern to avoid a visible discontinuity, in particular positioning the respective patterns to avoid pixel clusters, particularly non-isolated pixels, at the transition between patterns.
10. The method of any one of the preceding claims, further comprising: switching "off" selected "on" pixels in the first region such that imaged areas of the mask corresponding to the first region form a pattern identical to the second pattern.
11. The method of any one of the preceding claims, further comprising: applying a filter that only images every second track of the first pattern.
12. The method of any one of the preceding claims, wherein the first pattern is a cluster dot pattern, in particular a groovy pattern, and the second pattern is an isolated dot pattern, in particular a single pixel or MCWSI pattern.
13. The method of any one of the preceding claims, wherein the step of detecting the first region and the second region is performed by the imager on a digital data stream corresponding to the image file; and/or wherein the information in the image file further includes at least one halftone image region including halftone screen information comprising a pattern of "on" and "off" groups of pixels, each group of pixels comprising two or more single pixels.
14. The method of any one of the preceding claims, further comprising: storing in the image file one or more further micro-screen information comprising a further pattern of "on" and "off" single pixels different than the first pattern and the second pattern in at least one second region of the image file, the one or more further pattern having a different boost factor, each resulting in different mask openings compared to the other patterns.
15. A system for forming openings in a mask layer of a flexographic printing plate, the system comprising: an imager; a controller for the imager; computer memory media programmed with a first set of machine-readable instructions corresponding to an image file, the image file storing first micro-screen information comprising a first pattern of "on" and "off" single pixels in at least one first region of the image file and storing second micro-screen information comprising a second pattern of "on" and "off" single pixels different than the first pattern in at least one second region of the image file; wherein the computer memory media is further programmed with a second set of machine-readable instructions for causing the controller for the imager to detect the first region and the second region, the instructions further comprising instructions for causing the imager to create first micro-screen openings in the mask layer corresponding to the first micro-screen information having a different opening size than second micro-screen openings corresponding to the second micro-screen information.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from U.S. Provisional Application Serial Number 63/119,988, filed December 1, 2020, titled SYSTEM AND METHOD FOR MITIGATING TRAILING EDGE VOIDS IN FLEXO PRINTING, incorporated herein by reference in its entirety. BACKGROUND OF THE INVENTION It is a known problem in flexographic printing that the trailing edges of solid printed surfaces on a flexographic printing plate often have a row of pinholes in the ink film shortly before the printing plate loses contact with the printing substrate. This effect is independent from the type of printing surface structure and occurs in conventional solid regions as well as in solid regions covered with micro screens. FIG. 1A illustrates the problem. FIG. 1A is a photograph of a printed region on a first substrate printed using a first plate formed using a first set of pixel boost parameters to form the corresponding mask, wherein the printed region exhibits voids on the trailing edge side. As depicted, the printing plate (not shown) rolls in direction of the arrow 102 over the printing substrate 104. The printed region 100, which comprises a line, also referred to herein as a "linework print region," has voids 101 (areas in which the relatively lighter substrate is visible through the relatively darker ink) on the trailing edge side 106 of the line. As used herein, the term "trailing edge side" refers to the side where the printing plate surface lifts off from the substrate. Numerous approaches have been tried for resolving this trailing edge void problem, including but not limited to approaches detailed in U.S. Pat. No. 10,150,319 as well as in U.S. Published Pat. App. Nos. US20180354288A1, US20100224091A1, and US20160221379A1. Nonetheless, there remains a need in the art for new approaches to avoid or reduce voids. Early laser imagers, such as the Cyrel Digital Imager (CDI) sold by Esko Graphic-Imaging GmbH of Itzehoe, Germany, completely remove corresponding portions of the mask from the polymer plate corresponding to all solid printed regions of the image by means of a laser beam modulated by an acousto-optical deflector. Later, after the discovery that micro-structures in solid regions provide better ink transfer, the use of "Pixel Boost" technology (e.g. boosting the laser power when ablating the portion of the mask corresponding to selected pixels relative to others, based on predetermined criteria) has become a standard technique employed by Esko in CDI imagers over the last decade. The boosting is accomplished by increasing the RF signal to the acousto-optical deflector accordingly. Pixel boosting allows creation of micro screens in the mask of a digital photopolymer printing plate (e.g. DuPont Cyrel DPR) by ablating only individual single pixels. These single pixels are arranged in patterns that form surface structures in regions of the plate intended to transfer ink to the print substrate. The capability of ink transfer of such surface structures is optimized by choosing an optimum size for the individual mask openings. The size of the mask openings can be altered by adjusting the energy of the laser pulse that creates the mask opening. When imaging a laser-ablateable mask in a CDI, the mask openings are cut by a multitude of beams simultaneously. This multitude of beams can be operated in two different modes: standard imaging mode or pulsed mode. In standard imaging mode, the laser power for each individual beam is selected in a way that the size of the mask opening just slightly overlaps with the mask openings cut by neighbouring beams. The beams are switched "on" continuously in solid rendition areas, according to the image information of the image to be printed, such that the opening in a mask for a solid rendition area extends across the entire solid rendition area. As used here, the term "solid rendition area" refers to an area of an image to which a halftone screen has not been applied, which image areas may also include "linework" - e.g. solid lines within a graphic image). This mode is used for standard imaging, and creates solid rendition screened areas, including linework, having no surface structures (e.g. while highlight areas may comprise a plurality of discrete and separated halftone dots formed on the plate, the surface of each solid rendition area is not textured). The laser power for each beam in this mode is nominally considered to be at 100% power. In pulsed mode, the beams are turned on for short periods. Not all beams are turned on at the same time. Instead, the beams are turned on in certain sequences, forming patterns of mask openings that build surface structures that are suitable for improved ink transfer. An exemplary method for forming such surface structures is disclosed in U.S. Published Patent Application No. US2019315141A1, filed by the same inventors as the present invention, assigned to an affiliate of the assignee of the present invention, and incorporate