Controlling web anistropy in a roll and blade twin-wire gap former

摘要

A roll and blade gap former for a paper machine having first and second wires guided in a respective loop and defining a twin-wire forming zone, a forming gap in which the first and second wires converge before the twin-wire zone, a headbox including a slice channel having a slice opening through which a stock suspension jet is fed into the forming gap to form a web between the wires, and drainage and forming elements arranged in the twin-wire zone for removing water from the web. To provide an improvement in the control of the Z-direction properties of the web, the former includes turbulence generating vanes arranged in the slice channel in the headbox to cause turbulence in the stock suspension jet upon its discharge from the slice opening into the forming gap and a first forming roll arranged in the twin-wire zone and which constitutes the first drainage and forming elements in the twin-wire zone after the forming gap. A run of the twin-wire zone after the forming gap is curved over a wrap angle sector of the first forming roll which is less than about 25 DEG . The forming and draining elements produce a pulsating pressure effect on the web after the curved run of the twin-wire zone over the wrap angle sector of the first forming roll. A method for controlling the anisotropy of web formed in a roll and blade former is also described.