Fluid Dynamics
Numerical comparison of the velocity of the stock slurry leaving the slice lip to the speed of the forming fabric defines the primary mechanism for sheet structure control on a paper machine. The fourdrinier jet-to-wire ratio determines whether the fibres are deposited in a highly aligned or a randomly dispersed manner. If the jet speed is identical to the wire speed, the ratio is one, which typically minimizes fibre orientation.
Fibre Orientation
Operating either above or below a ratio of one introduces shear forces that align the fibres in the direction of machine travel. This fourdrinier jet-to-wire ratio can be adjusted to run drag where the wire is faster than the jet, or rush where the jet is faster than the wire. Both conditions create an anisotropic sheet with higher strength in the machine direction.
Paper machines producing sack kraft run with specific drag to increase tear resistance in the cross direction, while copy paper mills run closer to unity to limit thermal curling.
Formation Quality
Visual uniformity and tensile ratios of the sheet are direct consequences of this hydrodynamic adjustment. Small deviations in the fourdrinier jet-to-wire ratio can cause flocking or uneven basis weight profiles across the web width. Maintaining the correct setting prevents poor printability and weak bonding.
