Fibre flocculation during twin-wire roll forming

Abstract: At the division of Paper Technology a forming unitconsisting of headbox, forming roll unit and reservoir systemfor suspension recirculation has been constructed, with themain purpose to experimentally study the paper forming event byvisually following the fibre flow phenomena in the formingzone.The effect of various running parameters upon the pressuredistribution, which is in correlation to the flow phenomenaoccuring during forming, has been studied with a flush mountedpressure sensor. Some combinations of running parametersresulted in highly oscillating pressure pulses, which wereevaluated by their wavelength.The visualisation was made with a high-speed video cameraand a pulsed laser light source. The forming zone was observedvia a mirror mounted on the forming roll, thus following aspecific small area of the forming zone. This made it possibleto follow floc motions relative to the forming roll/wire. Theflocs were found to move towards the wire due to the dewateringflow wereupon the bottom part of the floc was pinned to thewire. If the suspensionspeed is different from the wire speedthe floc will be stretched out, because the upper part of thefloc has a slightly different speed in comparison to the bottompart. If this speed difference is high enough, the flocs maysplit and thus contribute to fragmentation.The floc-floc interaction has been studied in a Couetteapparatus. It has been observed that voids in the suspensionplay a central role for the floc break-up process. The reasonis belived to be that voids induce movement inside thesuspension, which is a prerequisite for floc break-up. Thefloc-floc relative movements have been found to obey simplelaws of rack-and-pinion principles.