Effects of pulp type and headbox design on anisotropy and other sheet properties in twin-wire roll forming

摘要

The combined effect of pulp type and headbox design was investigated on a pilot machine with a twin-wire roll former and with the speed difference between fiber suspension and wire used as a secondary variable. A hardwood furnish and a softwood furnish were used in combination with two headbox designs comprising four and ten tube rows, respectively, feeding the headbox nozzle. These designs were primarily chosen to give different fiber orientation anisotropy levels. The softwood furnish gave higher anisotropy than the hardwood furnish at the point of minimum shear with ten tube rows, whereas the softwood furnish gave an anisotropy close to that of the hardwood furnish with four tube rows. The anisotropy response to the speed difference was larger for the softwood than the hardwood furnish, that is, a higher maximum level was reached for the softwood than the hardwood furnish in the speed difference range investigated. For both furnishes, the difference in anisotropy due to the headbox design decreased with increasing speed difference magnitude. The headbox design had no significant effect on formation, neither for the hardwood nor for the softwood furnish. In spite of a substantial difference in formation level, the hardwood and the softwood furnish gave similar tensile strength in terms of the machine-to-handsheet ratio. The softwood furnish gave higher Z-toughness (machine-to-handsheet ratio) than did the hardwood furnish, whereas the headbox design with four or ten tube rows had no significant effect.