Jonathan Sam

Abstract

The flexographic printing process has historically had difficulty printing vignettes and highlights. In comparison to offset lithography and gravure, flexography suffers from dark highlights, dirty colors, less "crispness" and an inability to print full-range vignettes. The predominant belief by the printers and manufacturers in the flexographic industry is that these difficulties arise from the excessive dot gain inherent to the process. One major cause of dot gain in flexography is believed to be the halftone dot structure on the printing plate. Many methods have been developed around modification of the traditional plate creation methods with the goal of obtaining an optimal dot structure for minimizing dot gain on press. These methods have included the use of alternate light sources, exposure time modifications, FM screening, direct-to-plate technology and others. One explanation that hasn't been explored is that the printed dot size is purely a function of the ability to produce a small dot on the printing plate. This work performed a series of pressruns using a selection of the popular halftone dot creation technologies to determine whether it is merely the ability to put the smallest dot on the plate that provides optimum print quality. Additionally, the performance of each of the methods was compared to determine which one (if any) produced the best results. The methods used included conventional and digital platemaking methods and the use of alternate light sources, exposure times, and screening methods. Tone reproduction was evaluated using a metric derived from colorimetric data known as %(delta)E*. Average values for this metric, average dot size on plate, average shoulder angle, and descriptor values for screen/film/exposure type were analyzed using linear xv regression to determine the variable that had the most significant effect. Tone stability and tone reproduction plots were utilized to assess overall tone reproduction for each of the conditions for several ranges of dot sizes. Lastly, a psychometric test was performed on the highlight vignettes to determine which had the smoothest appearance. Test results indicated that plate dot size was the most significant factor in the tone reproduction of the image tested. Of the plate creation methods tested, the screening technique that appeared to have the least noise as well as repeatability was the Hybrid screening. The film technique that showed the best stability over all conditions was the digital plate. The combination of techniques that exhibited the most stability was the digital plate/Hybrid Screening/normal light exposure combination. The ability to create and print the smallest dot possible on plate is essential for superior highlight quality. This conclusion in combination with the results of hypothesis number one (plate dot area is the only factor that affects tone reproduction) present the most significant findings of this study. The implication here is that printers should determine the best method at their disposal of creating the smallest dots possible to optimize high light printing. Tone reproduction plots indicate that for the highlight tones range (0-20% dot sizes) the best overall tone reproduction was that of digital plate/Hybrid screen/high expo sure and digital plate/Hybrid screen/normal exposure conditions. Tone reproduction plots indicate that for the extreme highlight tones range (0-5%dot sizes) the best tone reproduction was that of digital plate/AM screen/normal exposure. For vignette smoothness, the panel of observers selected the digital plate/AM screen/bank light normal exposure as the smoothest of the samples presented.

Library of Congress Subject Headings

Flexography; Printing plates, Photopolymer

Publication Date

4-1-1999

Document Type

Thesis

Department, Program, or Center

School of Print Media (CIAS)

Advisor

Lee, Barry

Comments

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works. Physical copy available through RIT's The Wallace Library at: Z252.5.F6 S36 1999

Campus

RIT – Main Campus

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