A Laboratory Investigation on the Origin of Machine Direction Microstriations

Abstract

This paper is based on a laboratory-scale experimental study of machine direction microstriations (MDM) on board surfaces. We developed a pressing and drying simulator in which we can replicate some of the phenomena which are believed to be the origin of MDM: density stratification in wet pressing as well as in-plane restraining conditions during drying. Our laboratory experiments showed that we could generate surface features, visually similar to those classified as MDM in industrial paper production. In particular we could replicate the elongated appearance, the characteristic wavelength interval (1–4 mm) and the occurrence on one surface only.

The most important parameter in respect to the absolute amount of surface roughness was the in-plane restraining conditions during drying. Biaxial restraining resulted in much lower surface roughness and prevented the occurrence of MDM. MDM started to appear as soon as uniaxial shrinkage was permitted. Interestingly, however, shrinkage perpendicular to the main direction of fibre orientation in an oriented paper sheet caused a less pronounced occurrence of MDM in spite of a larger absolute value of shrinkage. The surface presented topography features parallel to the restraining direction. The press felt surface, in our investigation the coarseness of the batt fibres, influenced the surface roughness of paper, however at a characteristic length that was much smaller than that of MDM.