Web breaks in pressrooms have been modelled in terms of web strength variations and tension variations to identify principal factors controlling the pressroom performance. The web strength variations and their scaling law (the size dependence) have been formulated based on an extreme statistics approach, and the distribution parameters have been determined for a number of mechanical printing grades. By using the tension variation statistics data obtained from a pressroom trial, a parametric study has been conducted to examine the relative magnitude of the effects of key paper properties. The predictions have been compared with field study data. Among the conventional paper
properties, tensile strength and elastic stretch consistently predicted the break frequency. The strength uniformity parameter (Weibull exponent) was shown to have the highest impact on the break frequency based on the parametric study.