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Although often specific to paper, picking and loose contamination can sometimes be an interrelationship involving the entire printing process.

Definition

Although often specific to paper, picking and loose contamination can sometimes be an interrelationship involving the entire printing process. When these problems become pervasive rather than the exception, certain considerations may significantly broaden the operating window. Static electricity can cause loose contaminants to cling to the paper surface. This can be a problem when pressroom relative humidity is below 35% and when paper is not acclimated. The ideal range is 40-50% Rh.

 

Press Attributable
Excessive back cylinder pressure stressing paper surface;

  • Heavy plate moisture application breaking down paper or coated surface;
  • Hardened rollers do not set properly, impeding ink transfer and thorough wash-up — Check for proper roller durometer;
  • Synthetic fibers and contaminants from rags, hickey picker rollers, etc;
  • Spray powder contamination;

Ink Attributable
Tack of ink exceeding the cohesive strength of paper coating is causing stressful blanket release;

  • There is ink tack build on rollers from excessive downtime and long make-readies. Spray rollers with anti-skin inhibitor or similar retardant;
  • The setting speed of ink/paper is too fast causing subsequent ink tack build from unit to unit. Check slope of ink tack build relative to paper;
  • Dry ink particles;

Blanket Attributable
Stressful release causing pick or delamination (fast ink demands “quick release” compressibles);

  • Excessive blanket packing causing extreme back-cylinder pressure;
  • Blanket wash or solvent causing blanket tack or swelling on start-up;
  • Use edge-sealed blankets to minimize swelling from solvent and fountain solution contamination;
  • Maximum slip and clean release at the blanket/sheet impression nip is important in non-image areas as well as image areas. Fountain solution should offer good lubrication and release;

Paper Attributable
Pickoffs (Loose contamination);

  • Slitter dust/cut dust;
  • Coating particles;
  • Pickouts (Evidence of “crater” void at point of origin);
  • Poor coating adhesion;
  • Wood vessel segment fibers;
  • Basestock contaminants;

Pressroom Environment
Monitor relative humidity and temperature. The ambient temperature and humidity of the pressroom may be too high or too low, affecting ink tack and paper strength.

  • Ideal climate control is 45% (±5%) Rh and 72°(±5°)F.
  • Allow paper to acclimate to pressroom.
  • Paper acclimation time is relative to environmental extremes.
  • Properly conditioned paper runs with a broader operating window on press.
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