Flow mechanism in twin-wire blade forming. Part I

In a one-sided twin-wire model gap former, the KTH-former, the effects of blade load and wire permeability upon a blade forming pressure pulse, flow velocity and roll-to-wire gap has been studied. A forming blade was applied against the Outer wire of the forming roll and the blade pressure was measured with a pressure sensor mounted in the roll surface. A camcorder was used to evaluate the roll-to-wire gap. Increased blade load gave higher pressure peaks upstream of the blade tip. Tracer particles (fibre floc fragments, confetti) were injected into the headbox and recorded with a high speed video camera when passing the blade in the forming zone. Velocity measurements of the tracer particles showed how the velocity was slowed down upstream of the blade, and then was accelerated when the blade was approached. Tracer particles were pinned to the permeable forming wire and their velocity eventually reached the wire velocity. Increased blade load generated a higher pressure peak and was accompanied by a lower velocity and larger gap upstream of the blade tip.