In the prior art, no reliable method has been available for on-line measurement of fiber orientation. However, fiber orientation ratio and orientation angle are critical parameters of a paper web. Fiber orientation affects the strength properties of paper and paperboard: the tensile strength of the web is higher for tension in parallel with the fibers than in a direction perpendicular to the fibers. Also the runnability of the paper in a printing machine or copier is dependent on the fiber orientation. A paper grade having an unfavourable fiber orientation angle and orientation ratio tends to curl and shrink in an undesirable manner.
In a paper machine, fiber orientation in web formation is most crucially affected by the difference between the wire speed and the jet exit velocity from the headbox slice. If this speed difference, or the draw, is greater than zero, the fibers tend to orient themselves predominantly in the machine direction, whereby the tensile strength of the web increases in the machine direction as is desirable in most cases. If the jet velocity at the headbox slice contains a cross-machine component, the main orientation direction of fibers may deviate from the machine direction, whereby the fiber orientation angle becomes different from zero. By definition, fiber orientation ratio is the difference between orientations in the cross-machine and machine directions. When measured as the tensile strength in these two main directions, the ratio generally is from 2 to 3.
Fiber orientation can be measured off-line using a conventional method based on laser light. In this method, the paper web surface is illuminated with a circular pencil beam of laser light. On the paper surface, the laser light spot assumes an elliptical shape, because the fibers that are principally oriented in the machine direction of the formed web have different optical properties along the fibers from those measured orthogonally to the fibers. The length ratio of the major axes of the ellipse is proportional to the fiber orientation ratio and the angle of rotation of the major axis of the ellipse with regard to the machine direction is proportional to the orientation angle. While the method has found use in off-line laboratory measurements, it has not been reliably adapted to on-line measurements. Furthermore, under on-line conditions the method is hampered, among other things, by the sensitivity of the optics to soiling, the small size of the laser light spot and the difficulty in focusing the optics onto a fluttering web. Moreover, the method is applicable only to thin-caliper paper grades that are light-transmissive.
Also known in the art is a method in which the Young's modulus of the paper web is measured by means of ultrasound in both the machine and cross-machine directions. The method is based on the dependence of the acoustic wave propagation speed in different directions along the web on the orientation of fibers. This method has not been adapted to on-line measurements, and under laboratory conditions the results are dependent, besides fiber orientation, also on the internal stresses of the bonded fibers. On-line application of this method is complicated by the rapid pressure variations of the ambient air that occur on the surface of a fast-moving web. Furthermore, since the method requires physical contact with the web, its suitability to on-line measurement is severely limited.
Fiber orientation is also determined by means of a conventional method based on the measurement of tensile strength in the machine and cross-machine directions using elongation test equipment. This method is applicable to off-line measurement of paper samples only.
Still another method of orientation determination is based on imaging the surface of the paper web after staining of a portion of the superficial fibers. This method is clumsy and reveals the orientation of the superficial fibers only.