Poor cross-directional (CD) tension profiles of paper webs cause runnability problems in paper mills as well as in printing presses. This study explored the formation of the cross-directional web tension profile in paper machines. The research programme was implemented in a period of four years. Tension measurements were made during the production in both paper machines and printing machines.
The finite element method (FEM) was applied to explore the build-up mechanisms of the web tension profile. In FEM modelling, the web was constructed from continuum elements and the physical interactions of the elements were defined, for example, by the Hookean law. The FEM analysis has numerous advantages and it is important to investigate the influences of the mechanical conditions and the material properties of the paper web. The simulations made by FEM could quantitatively predict the shape of the tension profile.
The paper web is stretched in many stages through the drying section. Stretching in the machine direction (MD) causes a non-homogeneous stress field in the web because the paper is subjected to mechanical shrinkage which is defined by the Poisson’s ratio of the paper. This typically causes a situation where the edges of the web are slacker than the middle areas, the so-called crying shape. The uneven stress fields in the web also cause a lower MD and CD tensile stiffness because of lower frozen-in-stress in the paper sheet. Further on, the slacker edge areas are affected by higher CD drying shrinkage which further diminishes the tensile stiffness and also leads to higher relaxation in the machine direction.
Several trials were made to control the web tension profile. These included moisture profiling, jet-wire speed, edge flow, strain rate and nip trials. The most effective control was by moisture profiling with the steam box in the press section. The drier sections of the web became tenser and vice versa. This was because the moisture content of the paper has a strong effect on the formation of the tensile stiffness of the paper. The principal shape of the tension profile is convex. According to this study it is impossible to reach an even tension profile with the existing dryer section configurations. However, the tension profile can be adjusted within certain limits by controlling, for example, the development of the moisture profile. These controls play an important role in improving the runnability of the paper machine, the winder and the printing press.
Two different instruments were used to measure web tension. IQTension measures the pressure within an air film that is formed between the paper web and the curved metallic deflection plate. The curved deflection plate is perforated at intervals, so that the pressure sensors connected to the orifices in the plate measure the cross directional (CD) profile of the air pressure, which correlates with the web tension profile. The web tension is measured at a high sampling rate in several parallel positions across the whole web width. In addition, the tensile stiffness profile can be defined by measuring the tension at different draws.
Scanning by Tenscan was used, when there was only a limited space for the measurement, or there was a need for measuring simultaneously with IQTension. Tenscan uses a laser beam to measure the passing time of a propagating membrane wave in the web. Other measurements included normal online measurements in the paper machines. The TSO and shrinkage profile measurements were made in the laboratory.