Metal Belt Calendering of Uncoated Fine Paper - Pilot and Mill Trial Results

摘要

Metal belt calendering produces higher stifihess for uncoated fine paper compared to hard or soft nip calendering. A pilot trial was performed at Metso's Technology Center and a mill trial with Sun Paper PM 23, where the first ValZone metal belt calender for fine paper was started up in April 2011. Increased stiffness enables raw material savings in uncoated paper making. The raw material saving for fine paper can be achieved in basically two ways: 1) decreasing the basis weight, or 2) increasing the amount of more economical raw materials, such as filler. In general, the price of filler (usually GCC) is significantly lower than the fiber price. The paper web temperature before calendering has a major impact on the calendering result, particularly in conventional hard and soft nip calendering. Due to the very short nip contact time, the paper web is not significantly heated in the nip contact, and the ingoing web temperature dominates the calendering temperature in the nip. In contrast, in the metal belt calender the contact time is about two orders of magnitude longer, and thus web heating is much more effective. Consequently, the paper web temperature before calendering has a much smaller effect on the calendering result. Pilot tests are typically run using stand-alone off-line equipment with a low ingoing web temperature (e.g. 15-25 °C) while in mill production the web temperature prior to calendering is typically 60-90 °C. It is therefore essential to recognize the effects of the ingoing web temperature when comparing different pilot-scale calendering concepts and the mill results. It was found that web preheating is necessary in order to obtain realistic pilot results in hard and soft nip pilot calendering. A new web pre-heating method was developed for the pilot machine.