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Abstract:
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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 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. -Two control strategies and corresponding optimization algorithms are presented for simulation and control of basis weight and fibre orientation angle profiles. The first strategy employs an algorithm for simultaneous optimization of weight and orientation angle profiles using the slice lip directly. This strategy is also suitable for regulating orientation angle profiles with a dilution headbox, where the basis weight profile can be independently regulated using the dilution profile. The second strategy is appropriate for conventional headboxes with an existing basis weight profile regulator controlling the slice lip. In this case, the optimization algorithm is implemented as a supervisory layer which controls the setpoint profile for the basis weight regulator. -An implementation of the second control strategy and supervisory optimization algorithm is described. Results are presented for a large fine paper machine with a conventional headbox making primarily laser grades. At jet to wire ratios near unity, a modest change to the basis weight profile can effectively eliminate large fibre orientation angle errors. The required change in basis weight is a suitably chosen smooth shape, which depends on the shrinkage nonlinearity and jet to wire ratio. -Performance is very good on all laser grades, and exceeded the customer's expectations. The controller has been in use for more than three years without significant retuning or other modification. Thus, the process models in use have been confirmed and the efficacy of the control strategy has been demonstrated. -Possible future developments in fibre orientation measurements and controls are briefly discussed. |