Factors that control run-ahead on printing presses

Abstract

Run-ahead is a problem that occurs on printing presses where the paper rolls are braked by metallic bands applied to their periphery. Run-ahead occurs when the web in contact with the band decelerates faster than the roll, or when the web stops completely but the roll continues to rotate. This leads to a bubble, or bunching of paper layers, on the ingoing side of the bands. This breaks the web. A model has been developed to characterize run-ahead that takes into account press operating conditions, as well as paper and roll properties. The model shows that run-ahead depends on the press parameters of wrap angle, deceleration rate, and unwinding tension. It depends on paper parameters such as paper-to-paper coefficient of friction (COF), paper-to-band COF, wound-in tension, and the mass of the roll. We found that the paper-to-band COF increases significantly due to contamination of the band from the paper, and could be a determinant in the run-ahead problem. The probability of run-ahead can be reduced by increasing the wrap angle between the braking band and the paper roll, reducing the rate of deceleration, reducing the web tension on the press, or by increasing the wound-in tension. The key factors are the static paper-to-paper COF and the dynamic COF between the paper and the band material. Increasing the paper-to-paper COF will not be effective if there is also a proportional increase in the paper-to-band COF, so care must be taken when evaluating possible additives or treatments.