The effect of press felt non-uniformity on sheet smoothness and dewatering

Non-uniformity exists in a press felt at three different scales. Micro-scale non-uniformity has the size of a batt fibre (10 to 80 μm) and is created by the coarseness of batt fibre. Laboratory two-roll press testing has indicated that with increasing micro-scale uniformity (decreasing batt fibre size), sheet solids content goes up, and sheets become smoother. Medium-scale non-uniformity has a size of 0.5 mm to 3mm. This type of non-uniformity is mainly associated with the topography of the felt (e.g., the needling track) and the felt base fabric. After felt break-in, the felt surface becomes smoother and the voids (or pockets) on the felt surface become smaller and shallower. This reduction in void size leads to reduced rewetting, providing a sheet with higher solids. With more break-in cycles, the felt becomes smoother, leading to a smoother sheet. If the base is coarse and there is insufficient batt to cover the base yarns, the base fabric creates non-uniform pressure application during wet pressing and leads to a lower solids content. Large-scale non-uniformity has a size of 3 mm to 15 mm. This non-uniformity can result from non-uniform batt distribution, interference between the top and bottom layers of the base fabrics, or non-uniform shedding of batt fibres. Laboratory testing has indicated that there is a significant difference in dewatering between the 'floc' and 'void' areas of the felt. The local pressure in the 'floc' areas can be twice as high as in the 'void' areas.