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FLEXO Magazine : January 2008
the printed dot. However, low reliefs also are more difficult to print and require better press controls. Further, low reliefs are most appropriate for smooth substrates and tonal work. Rough substrates and large solids may dictate more impression on press and will Uprint the floor" if low relief plates are made. For fine graphics on a smooth substrate, a narrow-web printer might shoot for a relief of around 0.020-in. Another important variable to ensure optimum plates is dot area, measured by a plate dot reader. These instruments A micrometer should be used to determine the outcome of an exposure. help to ensure repeatable re- same rate so you need to periodically check for consistent out- suIts by measuring a couple of patches on the plate, prior to put (no light integrator like a point light system). A radiometer printing. If I measure 10 plates and have consistency at 1 per- can measure the output or you can use a simple grid and mi- cent,S percent, 10 percent, 25 percent, 50 percent, 75 percent, crometer to determine the outcome of an exposure. 90 percent, 95 percent, and 99 percent, then I can be confident After the main exposure, the unpolymerized material is re- that I have a controlled process. Hopefully, I have an optimized moved by one of three methods: process too, but that depends on how those patches reproduce · Solvent washout on press. It is important to keep a log of key variables when · Water washout making plates. Measure, record, and correct when these vari- · Thermal processing abIes drift out of spec. Historically, solvent washout produced the finest plates. However, in recent years, both water and thermal plates have improved in resolution. Screen values of 175 lpi or more are possible with both water and thermal processing, with less en- vironmental impact. . . . . . ..... .... ",J , 1r @) " ")t:)'") ........ :::< ....... k . -, --- " MONITORING PLATE VARIABLES There has been a fair amount of discussion about plate dot shapes (volcano, pillow top, round top, flat top, etc.). The shape and surface characteristics of a dot is largely determined by the response of the material to the imaging and processing sys- tem. The debate will go on whether a flat dot surface is better than a round dot surface. The key is to ensure that you produce the sharpest, most consistent shape and size dot possible with your system. And the only way to do that is to have consistent and measurable processes. One of the most important variables to control and track is image relief. As noted earlier, this is the difference between the plate surface and the supporting floor-the measured physi- cal difference between image and non-image area on a relief plate. This difference is measured in inches. A digital plate mi- crometer is a must have for anyone making flexo plates. Some companies use only one relief spec while others may have two or more. The key is to know what relief is desired and to control your back exposure to hit that relief within the tolerance. Generally speaking, the lower the relief, the sharper - . SUMMARY Platemaking is a critical part of the printing system. It can be controllable and predictable with some standard best practices. Spend time analyzing, measuring, and improving your plate processes and you will be on your way to better printing. Your press room will love the results. ABOUT THE AUTHOR: Malcolm G. Keif is an associate pro- fessor in the Graphic Communication Department at Cal Poly State University, San Luis Obispo, CA. He oversees instruction inflexo- graphic plating and press operations, as well as teaching course work in quality management, cost estimating, web offset, and gravure printing. Keif is a frequent speaker at industry conferences and the au- thor of two books: Lean Printing: Pathway to Success, and Designer's Postpress Companion. -- "'- . .. ,. '1 ' .f;.. ,- := I , ; .i.:' . ! . !:! !:,i.., . . _ -"- ." / i,, .;:1; i i I": . II. I' . , -II ILl.. -.í ' ;.. ' .ff'. .jiiLl l lj: :,,: : : :... ,;,\ lii:: ',,:1,,':; i: ' ( .!; tr.I' JANUARY 2008 www. f I exog ra p hy.o rg FLEXO