With fourdriniers, the jet lands at a distinct but small angle to the horizontal fabric sufficiently close to the leading edge of the forming board to avoid excessive initial drainage. The angle and exact location of the landing require careful optimization. Ideally after impact, the velocity of the fluid remaining on the fabric should be exactly horizontal. Small velocity pulsations in the jet caused by pressure variations in the headbox can cause serious disturbances during this initial dewatering, either by creating standing waves on the free surface or by moving the landing location (3). Thereafter, drainage by gravity is augmented by the various suction dewatering elements under the fabric, usually foils, vacuum-assisted foils, vacuum boxes (Figure 9.1) and finally the suction boxes in the couch roll.
