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Mar 1, 2007 12:00 AM
When looking through a 100x glass or microscope, a double will show itself rather prominently in the 15 percent to 25 percent dots. It will show as a light dot beside or overlapping a strong dot.
Doubling can occur two ways on a sheetfed press. The first is premature contact with a blanket — normally on the first unit of the press — caused by stiff paper or board slapping the blanket. The second cause is unit-to-unit misregister caused by poor gripper timing or setting, worn grippers, dirty grippers, worn cam followers, wavy paper and so on. The net result is severe color variation even though density is being held to the best of the press and operator's abilities.
On web presses, the problem can be related to stock, roll condition, tilt boxes not working properly, and press out of tram, tension problems. Keep in mind: The longer the press and the heavier the stock, the more difficult it is to print without doubles.
The problem with doubling is that it is easy to state the problem, more difficult to identify the cause and expensive to eliminate. When purchasing a new or used press, test it thoroughly prior to installation.
Raymond J. Prince is a leading expert in pressroom technical and operational issues. He recently joined NAPL (Paramus, NJ) as vice president and senior consultant, operations management. During his tenure at PIA/GATF, Prince conducted nearly 1,000 Technical Plant Assessments (TPAs), in-plant analyses of a printer's production facility and capabilities. As a lecturer and trainer, he led numerous industry seminars on all phases of pressroom operations. Contact him at (605) 941-1492 or e-mail RaymondJPrince@aol.com.