FIELD OF INVENTION

The invention relates to algorithms used to create halftone screens for the flexographic printing process.

BACKGROUND OF THE DISCLOSURE

Flexographic printing is a process that uses polymer printing plates wherein the image to be printed is raised from the surface of the plate in order to make physical contact with the printing substrate, thereby transferring the ink. The flexographic printing process faces many challenges associated with the quality of the finished product because small elements are hard to capture within the physical structure of the plate material. The relatively recent introduction of flexographic computer to plate devices has improved or eliminated many of the imaging problems that were once associated with film imaging prior to manual exposure of a flexographic plate. However, there is still much that can be accomplished, particularly where “halftone” imagery is involved.

A halftone is an image (such as a photograph) that has been converted into a series of graphical elements (e.g., dots) that may vary in size and spacing. Halftones are used in all forms of printing because it is not possible to reproduce continuous tones in any form of lithography or flexography. The process of breaking up images into halftone graphical elements is also referred to as screening. In some known printing workflows, a job to be printed is composed in a software program known as a layout application or design application, and is then submitted to another software program called a raster image processor (“RIP”). The RIP analyzes the job composed in the layout application or design application and “screens” any image data it finds. As used herein, “screening” includes breaking the image into a series of halftone graphical elements according to instructions programmed into the RIP. The size of each halftone graphical element may be determined by the resolution of the computer to plate device (“CTP device”) that is used to create the printing plate. In at least one example, it may take fewer than 16 pixels aligned in a group to create the smallest halftone graphical element.

The RIP then sends the job data to a CTP device that uses a laser to expose the job onto a printing plate. In the case of a flexographic plate, the laser hardens the plate material wherever the exposure occurs. Once complete, the plate is put into a processing unit that washes away any un-exposed areas, leaving the exposed, hardened areas in relief.

Flexographic printing is increasingly popular and used in a variety of applications including labels, packaging, corrugated, and others where the substrate cannot be printed on an offset printing press (e.g., specially coated papers, plastics, foils, etc.). However, the challenges in flexographic printing make certain kinds of printing unusually difficult to do. Two problem areas that dominate flexographic printing are highlights, where there are very few halftone graphical elements in a given area, and shadows where many halftone graphical elements populate the area and form solids.

Highlight areas can be defined as image areas that have very little detail in the subject matter, such as clouds in the sky or light reflecting off of a shiny surface. Highlight areas are difficult to reproduce because of the small number and size of halftone graphical elements used to create the imagery. Once those graphical elements are reproduced on the plate, they frequently do not pick up enough ink to transfer to the substrate, which means the printed piece does not faithfully reproduce the artwork of the original job.

A shadow area is a part of an image that is very dark. In a halftone, these are typically represented by large graphical elements that are very close together, often creating a solid area where there is no space between the graphical elements. In flexographic printing the plate can often pick up too much ink, which can create a mottling or pooling effect in shadow areas.

As a result of these issues, known methods of flexographic printing are frequently unable to accurately render highlights and shadow areas. Accordingly, improved methods of flexographic printing are desired that will overcome these problems

BRIEF SUMMARY OF THE INVENTION

In one aspect, a method of improved screening to increase stability in flexographic printing is provided. In the example embodiment, the method is performed by a flexographic printing system. The flexographic printing system includes a computer system and a flexographic printing device. The computer system includes a memory device and a processor. The computer system is in communication with the flexographic printing device. The processor is configured to perform the method comprising receiving a first proposed graphical input of a printing job, analyzing the first proposed graphical input of the printing job to identify at least one highlight area containing a plurality of halftone graphical elements in the first proposed graphical input, determining an amount of white space between the plurality of the halftone graphical elements in the first proposed graphical input, assigning at least one graphical element to be placed in a spatial relationship with each of the plurality of the halftone graphical elements, generating a second proposed graphical input by placing the at least one graphical elements on the first proposed graphical input based on the respective assignment, and instructing the flexographic printing device to print the second proposed graphical input.

In another aspect, a method of improved screening to reduce mottling in flexographic printing performed by a flexographic printing system is provided. In the example embodiment, the method is performed by a flexographic printing system. The flexographic printing system includes a computer system and a flexographic printing device. The computer system includes a memory device and a processor. The computer system is in communication with the flexographic printing device. The processor is configured to perform the method comprising receiving a first proposed graphical input of a printing job, identifying at least one high density region in the first proposed graphical input, identify at least one candidate pixel within each of the at least one high density regions, removing each of the at least one candidate pixels from the first proposed graphical input to create a second proposed graphical input, and instructing the flexographic printing device to print the second proposed graphical input.

