Predictive 3D roll grinding method for reducing paper quality variations in coating machines

The predominant trend in paper machines is towards an increased running speed. At the same time, the paper produced must have a higher and more even quality. In printing papers the main end-use properties and quality components are runnability, printability, and print quality. These coexistent requirements create new demands for the behaviour of rolls under production conditions. High quality printing paper grades are coated. In blade coating the thickness of the coating film on the paper surface is found to be heavily dependent on the run-out of the backing roll which supports the paper web against the metering blade. The run-out tolerance of backing rolls at running speed has lately been 50 µm and should be substantially reduced in the future. The new tolerances can no longer be met by tightening the traditional roll manufacturing tolerances. A new predictive 3D grinding method has been developed to improve roll behaviour in the paper production environment. It consists of a measuring system, which can verify the rotational and geometry errors of the roll at running speed, and a 3D grinding system, which controls the grinding process according to the information gained by these measurements. In this study, the new method was applied to the backing rolls of a coating station. The experiments were carried out in a paper mill, on a medium-weight coated (MWC) paper production line. The paper was analysed before and after the predictive 3D grinding. The predictive 3D grinding reduced the machine direction (MD) ash variation caused by the backing rolls by an average of 76 %. Ash variation correlates well with coating variation. As a result of a more even coating film, MD gloss variation was reduced by 82 %. Reduced gloss variation improves the print quality of LWC paper. The variation in thickness caused by the backing rolls was reduced by 74 %. More even paper thickness reduces excitations and therefore improves runnability in calendering, winding, and printing. A new paradigm for roll grinding was set. The applications of the technology are not limited to high-speed paper machine rolls; the method can also be applied to different kinds of nips of rolls. The method can compensate for a systematic error causing nip force variation, such as uneven thermal expansion or the uneven flexural stiffness of a roll. The technology can be applied in different industries, such as the steel, plastics, or aluminium industries. With this method, it is also possible to use rolls in applications which have requirements too high to be met by traditional technology.reviewe