LAYER MIXING DURING 3-LAYER STRATIFIED FORMING - THE ROLE OF VANE LENGTH AND MIX-WIRE SPEED DIFFERENCE

Stratified forming, or the simultaneous forming of multilayer sheet fr om a single headbox, can suffer from significant mixing of the differe nt layers. This is holding back the introduction of this technique for the manufacture of printing and writing grade papers. To investigate this mixing, we produced 70 g/m(2) woodfree paper in three layers on a n experimental paper machine. The center layer of the pulp was dyed bl ue 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 the blue content of the surface within the 0.3-30 mm wavelength range, defined as surface ply variati on (SPV), shaved that the heterogeneity of the surface ply was affecte d by the length of the flexible vanes used to separate the different l ayers in the headbox, and by the mix-wire speed difference during dewa tering. Longer vanes in a contracting nozzle meant higher flow velocit ies, both at the vane tips and in the flow channels, and this was thou ght to produce more intense and smaller-scale turbulence. This type of turbulence tended to mix the layers more at the fiber level rather th an at the flee level. The total formation of the sheets, however, was independent of vane length.