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Aug 1, 2010 12:00 AM
There are many types of streaks: around the cylinder; across the cylinder; and radiating from the center of the gripper edge. Each may have many causes, and the press operator has the formidable job of determining this. Let's focus on the most common type, which is streaking across the cylinder.
The first step is to ensure that the press is at manufacturer's specification for roller durometer, ink form and dampening form roller pressure to the plate. The next item to check is bearer pressure, plate to blanket pressure and, finally, breakaway solids on each unit. When doing breakaway solids, I suggest that you document any streaks but do nothing about them at that time. Adjust the charge of ink so that the ink is as even as possible from the lead edge to the tail of the sheet.
The next step is to pull wet solids to show if there are any streaks being added by the introduction of the dampening system. At this point, you need to look at the horizontal streaks in case there are any that will match to the manufacturer's streak charts for your press model. Normally a great deal of information can be gleaned from these charts. This information will enable you to being making changes in setting and possibly improve the results.
If you have streaks in the wet solids, these must be addressed. Some are solvable; others might not be. Keep in mind that soft rollers streak less than hard rollers, clean rollers transfer in better, and roller stripes are a key point to watch. An old trick is to check vibration of the inker: Place a cup of coffee on the side frame cover of a unit and watch for vibration (rings) in the water. Too much vibration can cause a streak. Now, for a couple of quick fixes at this point:
The next test is to use a set of plates with a 30% screen tint, 150 line. On a 4-color press, use KCMY ink with plates at the proper angle. This is a diagnostic test to determine where the streaks are and how bad they are. On a modern press, you should see no streaks. If you do see a streak and wish to quantify it, then use the FOGRA test, which uses one plate at 30% screen tint, 150 line printing with cyan ink at a density of 1.30. FOGRA states that the Delta E should be no more than 0.9. Thus, you should not see the streak because the “just noticeable difference” usually quoted is 1.0.
A final word: Streaks have many causes, not just those listed above. Have fun tracking them down and eliminating them.
Raymond J. Prince, NAPL partner consultant, is a leading expert in pressroom technical and operational issues. Contact him at (605) 941-1492 or email@example.com.