Doctor Streaks–Their Origin and Prevention

Basod upon the theory that excessive friction between doctor blade and roller surface is the primary cause of roltor damage and can provoke this trouble even in print pastes containing no trace of solid matter, the author reports on trials with a printing assembly linked with a friction‐measuring device. The entirely different behaviour of thickenings with high and others with low dry content gives evidence of their varying working properties. It is shown that the use of thickenings with high dry content, addition of lubricants to print pastes containing low dry content thickenings, and especially replacement of copper rollers by chromium‐plated ones, will in most cases eliminate the causes of roller damage. There is, however, an indication that changes of pH may completely alter the friction characteristics of thickenings and that with more and more ingredients present in the print pastes their performance becomes increasingly unpredictable and requires closer investigation.