New Insight Into Fiber Orientation Streaks, 2001 Papermakers Conference Proceedings

Ari Kiviranta, Pekka Pakarinen--Fiber orientation streaks, also knows as "tiger stripes" or galvanization or water streaks, represent a well-known quality defect for many paper and paperboard grades. The problem is especially pronounced when low headbox consistencies are used, like in linerboard manufacture. Also, fiber orientation streakiness is critical for coated paper grades. After calendering of coated grades, gloss variations and wavy flatness deviations of a sheet together with a full paper roll surface corrugation are found to be most visible characteristics of orientation streaks.

The development of new paper structure measuring methods has significantly increased the understanding of the phenomenon. A method to measure fiber orientation in various fiber layers in z-direction of paper was developed by VTT and Valmet. Using tape-stripping methods, paper samples can be divided into several layers. Orientation streakiness of each individual layer can be analyzed using image analysis techniques.

Several paper grades have been tested using the method described above. In fourdrinier and hybrid forming, streaks are found only on the wire side, i.e., in the first formed layers. The depth of the fiber orientation streaks is usually only two or three fiber layers on the wire side surface. In gap forming, fiber orientation streaks can be found on both sides of the sheet.

It is obvious that the streaks are caused by flow defects like boundary layer instabilities in the converging slice channel of the headbox. These instabilities along with large-scale vortices in the headbox jet can be greatly reduced by using vanes in the headbox slice channel. The vane design can also have an effect on streakiness. Another efficient tool to reduce streakiness is to include a small step in the slice channel relatively close to the slice opening. These techniques will be discussed in detail.