An Investigation of Z-direction density profile development during wet pressing.

by Burns, James Robert

The dynamic compression and expansion behavior of the wet fiber mat is a key factor in understanding the mechanisms of wet pressing and density development. At present, this behavior is poorly understood. This thesis presents the results of an experimental program which clarifies the complex process of unidirectional, pressure-driven fluid flow through a highly deformable porous web. The objectives are 1) to develop a technique for illustration of dynamic density gradient development in handsheets pressed in a platen press, 2) to apply the technique to measurement of the influence of process variables on density development, and 3) to develop a density profile database for future wet pressing model development. The major experiment carried out in this research was the careful investigation of the rapid stress-deformation characteristics (wet pressing) of 150 g/m 2 mats of softwood and hardwood pulp fibers. This included investigation of the deformation and recovery behavior of three zones within handsheets under the influence of several process variables (nip residence time, moisture ratio, freeness, and furnish). A laboratory press simulator, built by Burton at IPST, was modified to supply the applied pressure profile using a " rock dropper " and a servo-hydraulic press. This modified simulator permits controlled pressing within the wide speed ranges encountered in paper machine press sections. The simulator is instrumented to measure the applied mechanical load, the height of the moving platen above the stationary platen, and the height of copper targets embedded in the sheet at various levels during the forming process. In simulated wet pressing, these targets move with the fibers as the sheet is compressed indicating both web -2and zonal thicknesses. The instantaneous apparent density of the zones within the web is calculated from the target separation distances and the known zonal basis weights. From the measurements described above and others, the indications are that increasing moisture ratios in high freeness softwood handsheets produce no significant increase in the densification gradient development. In addition, increasing the moisture level in the same softwood furnish at a lower freeness still appears to have no significant effect on density gradient development. However, the net effect of lowering the freeness appears to be an overall decrease in the rate of densification of the zones. This indicates that as freeness decreases the fiber network resistance to fluid flow increases. -3-