Tutorial: Fibre Orientation Angle Profiles - Process Principles and Cross Machine Control, 1998 Process Control, Electrical & Info. Conference Proceedings

This tutorial is intended as a general introduction to fibre orientation profiles and related processes, especially techniques for cross-machine control of fibre orientation. The tutorial is broadly divided into sections on paper properties, measurement technology, papermaking processes, control strategies, and case studies. Some paper grades have specific requirements for uniformity in their fibre orientation angle profiles. In particular, sheets used in laser printing, photocopiers, and similar devices need fibre orientation angles near zero, to prevent distortion of reproduced images. The importance of this requirement rivals those for uniformity of basis weight and caliper.

Increasing sheet handling speed and increasing reprographic resolution, together with increasing usage of multi-pass colour processes are leading to more demanding specifications for paper fibre orientation angle profiles. Moreover, differences in fibre orientation angle profiles between layers of the sheet lead to twisting phenomena in all grades. This is especially problematic for multi-ply board grades, causing stacking and structural problems.

On-machine fibre orientation angle measurements are not common in papermaking today. Instead, a variety of laboratory instruments are used. The measurements may be of the distribution of fibre orientation angles, or of a proxy property such as tensile stiffness orientation. A survey of current measurement methods is presented.

The wet end processes leading to non-uniform fibre orientation angle profiles are described. In particular, the headbox slice lip profile influences both the orientation angle profile and the basis weight profile. The jet velocity field and the difference between jet and wire speeds are critical determinants of the orientation angle profile. The vertical distribution of fibre orientation angles depends additionally on the type of forming unit, as unidirectional and bidirectional dewatering patterns have different effects on the layered structural anisotropy of the sheet. Pressing and drying have minor effects on fibre orientation, but modify the relations between fibre orientation and properties such as strength, tension, curl, and twist. Control strategies are presented for the fibre orientation angle profile, suitable for use with either conventional headboxes or dilution headboxes. Case studies with both manual and automatic adjustment show how the fibre orientation angle profile can be effectively controlled in office grades made with conventional headboxes.