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In the present work we propose a particle approach, which is designed to treat complex mechanics and dynamics of the open-draw sections that are still present in many of today's paper machines. First, known steady-state continuous solutions are successfully reproduced. However, it is shown that since the boundary conditions depend on the solution itself the solutions for web strain and web path in the open-draw section are generally time-dependent. With a certain set of system parameters, the nonsteady solutions are common. A temporal fluctuation of Young's modulus, for example, destabilizes the system irreversibly, resulting in the continuous growth of web strain, i.e., break. Finally we exemplify with some strategic draw countermeasures how to prevent a dangerous evolution in the web strain. [DOI: 10.1115/1.3197425] Keywords: paper machine, web breaks, open-draw, runnability, transient boundary conditions, release point, particle simulation, mechanics

Particle-level simulations are performed to study semidilute suspensions of monodispersed non-Brownian fibers in shear flow, with a Newtonian fluid medium. The incompressible three-dimensional Navier-Stokes equations are used to describe the motion of the medium, while fibers are modeled as chains of fiber segments, interacting with the fluid through viscous drag forces. The two-way coupling between the solids and the fluid phase is taken into account by enforcing momentum conservation. The model includes long-range and short-range hydrodynamic fiber-fiber interactions, as well as mechanical interactions. The simulations rendered the time-dependent fiber orientation distribution, whose time average was found to agree with experimental data in the literature. The viscosity and first normal stress difference was calculated from the orientation distribution using the slender body theory of Batchelor [J. Fluid Mech. 46, 813 (1971)], with corrections for the finite fiber aspect ratios. The viscosity was also obtained from direct computation of the shear stresses of the suspension for comparison. These two types of predictions compared well in the semidilute regime. At higher concentrations, however, a discrepancy was seen, most likely due to mechanical interactions, which are only accounted for in the direct computation method. The simulated viscosity determined directly from shear stresses was in fair agreement with experimental data found in the literature. The first normal stress difference was found to be proportional to the square of the volume concentration of fibers in the semidilute regime. As concentrations approached the concentrated regime, the first normal stress difference became proportional to volume concentration. It was also found that the coefficient of friction has a strong influence on the tendency for flocculation as well as the apparent viscosity of the suspension in the semidilute regime. [ABSTRACT FROM AUTHOR]

Nonuniformities in the coating layer, such as porosityvariations and binder distributions, are known to affect print uniformityand barrier properties. However, in the literature the results arerather scarce or sometimes conflicting. We acquired high-quality imagesof coated paper cross sections using field emission scanning electronmicroscopy in combination with a new argon-ion-beam milling techniqueto directly observe and analyze the coating microstructures in relationto underlying base sheet structures. The results showed that coatingporosity varied with mass density of the underlying base sheet forthe relatively bulky clay/GCC coating, whereas for the more compactclay coating, the effect was small. Areas with more fibers in thebase sheet were more compressed by calendering, resulting in a decreasedcoating porosity. A unique binder enriched layer of less than 500nm thickness was found at the coating surface as well as at the coating/basesheet interface. [ABSTRACT FROM AUTHOR]