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Influence on sheet anisotropy, formation, Z-toughness and tensile stiffness of reduced feed area to a headbox nozzle

The feed area of a hydraulic headbox nozzle was decreased by a reduction from seven to three tube rows in the turbulence generator. This meant a lower strength of the elongational field in the nozzle, and, in addition, the effects of the turbulence were expected change. The influence of the headbox geometry was studied in a twin-wire roll former of a pilot paper machine at different levels of oriented shear, introduced by a speed difference between mix and wires. A TMP and a chemical pulp mixture, 50% softwood and 50% hardwood, were used. It was found that the reduction in the nozzle feed area resulted in a lower MD/CD-ratio (tensile stiffness) for both the TMP and the chemical pulp, the largest effect being observed at the speed difference corresponding to the minimum MD/CD-ratio. The effect was larger for the TMP than for the chemical pulp, with a decrease in the minimum MD/CDratio from 2.4 to 1.6 for the TMP, compared to a decrease from 2.2 to 1.7 for the chemical pulp. The formation for the TMP deteriorated at speed differences around the minimum MD/CD-ratio when the feed area was reduced, but it was unchanged for the chemical pulp. Z-toughness for both pulps was enhanced over the whole speed difference range investigated. The tensile stiffness (given as the geometric mean of the machine and the cross direction values) for both pulps was also enhanced by a reduced nozzle feed area. For both sizes of the nozzle feed area, and for both the TMP and the chemical pulp, maximum Z-toughness and tensile stiffness were achieved at a speed difference corresponding to the minimum MD/CD-ratio.