Modeling the behavior of the paper web in printing presses

Abstract

The production efficiency of printing presses is limited by runnability problems such as web breaks, register errors, wrinkling and lateral instability of the paper web. To explore these phenomena, one must study the interaction mechanisms between the paper web and the printing press. In offset printing damping water is applied to the web affecting the rheology of the running paper web. In numerous printing applications many webs are printed simultaneously, split and run together in the folder. Different webs should ideally be combined in certain tension steps. In practice this goal is difficult to achieve as paper properties may change from reel to reel and the press conditions may change temporally.

Finite element modeling (FEM) was applied to study the interactions between the paper web and printing press. The tension changes along the press line, the nip load and the dimensional changes of the paper were modeled by FEM. The paper was modeled as in a plane stress state using membrane elements. Measurements were carried out in laboratory, pilot and production scale. Measurements included nip load distribution, tension profile and web dimension measurements. Material parameters for modeling were measured and obtained from earlier work.

Nip load and its distribution affect the web's dimensional changes like dry widening and presumably the rate of moisture absorption to the paper web. The effect of printing nips on the pressure and nip length were modeled and the results were verified with the measurements. Results showed that the structure of the printing blanket has a considerable effect on nip pressure and nip length. Nip conditions may affect the magnitude of the web widening (fan-out) through the delayed time of the paper in a nip and through the magnitude of the nip pressure.

The formation of the paper web's tension profile in a newspaper offset press was studied. The effect of damping water to the tension was modeled and the results obtained by FEM correlated well with the measurements.

It was found that FEM can be successfully applied in order to model and simulate the interactions between the printing press and the paper web. The effects of printing nips were modeled as well as the changes in the web tension profile in the press. The modeling results correlated well with the measurements. The benefits of modeling are obvious as the productivity of printing presses is highly dependent on the tension behavior of the web and the actions taken in the press. This means that better productivity can be achieved by increasing the knowledge of the interaction mechanisms.

In this paper we will also discuss the requirements of FEM in the printing process. Future work should include more precise material models as various parameters, such as the creep / relaxation phenomena of the paper web, were not included in this work.

Steady state analysis was performed for a moving paper web. The modeling results gave a better understanding of web transportation and FEM appears to be a promising tool for analyzing paper web behavior in different web handling systems.