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What's Your Streak Technique?

Mar 28, 2012 12:00 AM


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TECHTHOUGHT #66

By Raymond J. Prince
 
Here are 9 IDEAS for curing this offset press headache
 
New and old presses can both be subject to streaking. Identifying the cause can be time consuming if not impossible. Generally speaking, visual examination provides no clues. Most repair personnel carry a streak chart.
 
Prudent printers will have a mechanic examine the press for any mechanical issues prior to implementing streaking strategies. Two basic starting points: pack the press precisely and check roller durometer. Hard rollers streak far more often than soft rollers.
 
MORE SUGGESTIONS:

  1. Compressible packing can hide pressure streaks. This very specialized material is only available up to 64 inches, but it will work for many jobs. Vendors include SunShine Paper Co. (sunshinepaper.com).
  2. Shingling packing at the trailing edge of the blanket cylinder by 1/4 inches or 1/8 inches will help "bump streaks."
  3. Opaque inks for spot colors will make streaks less visually obvious. (Not perfect but helps!)
  4. Air curtains to evaporate fountain solution from ink on the dry side of the inker will reduce streaking and ghosting as well as improve ink lay and decrease ink emulsification.
  5. Try “dual durometer” rollers in the ink forms and ink ductor positions.
  6. For the units that will print heavy PMS, consider oscillating ink form rollers on the last two form rollers in each unit.
  7. Day International make a special blanket (No. 3610) I have used on older presses. This soft blanket eliminates streaks.
  8. Some pressmen like to use a soft packing. Rather than use .020-inch thick packing they prefer four .005- inch sheets.
  9. On some presses, lightening the last form roller to the plate setting to less than 2mm can help. To document your progress on a four-color press, make a set of 30% tint plates at the proper angles and print. This simple test really taxes a press and shows streaks well. An improvement will be readily apparent.

Streaking isn’t covered extensively in printing standards. FOGRA in Germany uses a screen tint and cyan ink and indicates a Delta E for acceptability. Wide streaks or very narrow streaks pose complications. A penciltip sharp streak may actually have a very small Delta E while a four-inch wide streak that gradually starts and ends may have a large (but acceptable) Delta E. Not an easy problem to assign a +/- tolerance.
 
THE EYES HAVE IT
Finally, take a good look. Our eyes are very sensitive to any density difference or surface disturbance. Many streaks that I could see were impossible to measure with a densitometer or spectrophotometer.
 
Raymond J. Prince, President, GreensheetBIZ, a division of OutputLinks Communications Group, is a leading technical expert in pressroom technical and operational issues. Email him at RP@GreensheetBIZ.com.