Abstract
Paper structure characterisation has been extended to include 2-D formation and periodic marks as well as wavelet techniques. Local fibre orientation anisotropy and misalignment angle analysis, in
combination with tape-splitting layering and image analysis have improved the understanding of the relationship between paper structure and properties as well as increased the possibilities to
trace the forming history from final paper structure.
The Crowding Factor has been introduced, to describe the tendency of fibre suspensions to form fibre flocs. It gives an improved predictability in comparison with fibre concentration, by also taking fibre slenderness into account. The dynamic characterisation of de-flocculating and re-flocculating mechanisms is getting increased attention, partly due to the potential of CCD-cameras and image analysis techniques. It is the opinion of the authors that floc stretching is a more promising way of de-flocculating than turbulent shear. It is doubtful whether turbulence in simple fluids can aim as a model for the flow irregularities in fibre suspensions, due to the damping effects of fibres and flocs.
For headbox CD-profile control, dilution water injection has been introduced. Since this makes slice lip deformations un-needed, it has the potential to uncouple control of grammage and misalignment angle profiles and also to improve CD-control resolution.
The importance of headbox nozzle design for fibre orientation anisotropy is now appreciated. A high nozzle contraction ratio will increase anisotropy and also introduce some deflocculation through floc stretching. Vane insertion helps to decrease fibre orientation anisotropy and is also applied for multi-layer forming. Although applied in tissue forming and in linerboard forming, further improvements have to be made for printing paper applications.
In twin-wire forming, the roll-blade principle has been accepted by the main machine manufacturers. Since its introduction in the STFI-Former in 1991, the blade section design with stationary blades on one side and loadable blades on the opposite side is the main design for printing paper applications, and also increasingly used for different board qualities.
Download PDF