Layer Mixing During Three-layer Stratified Forming; The Role of Vane Length and Mix-Wire Speed Difference, 1996 Engineering Conference Proceedings

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Stratified forming, or the simultaneous forming of a multi-layer sheet from a single headbox, can suffer from significant mixing of the different layers. This is holding back the introduction of this technique for the manufacture of printing and writing grades papers. To investigate this mixing, we produced 70 g/m 2 woodfree paper in three layers on an experimental paper machine. The centre layer of the pulp was dyed blue to distinguish it and to help ascertain the level of mixing. Layer mixing caused the resulting paper to have a pattern of blue and white flocs on its surface. The variation of this within the 0.3 to 30 mm wavelength range, defined as “Surface Ply Variation” (SPV), showed that the heterogeneity of the surface ply was affected by the length of the flexible vanes used to separate the different layers in the headbox, and by the mix-wire speed difference during dewatering. Longer vanes in a contracting nozzle meant higher flow velocities, both at the vane tips and in the flow channels, and this was thought to produce more intense and smaller scale turbulence. These tended to mix the layers more at the fibre level rather than at the floc level. The total formation of the sheets was however independent of vane length.