Sheet Structure
Orientation of cellulose fibers describes the predominant direction in which the pulp fibers lie on the paper machine wire. This fiber alignment is determined during the drainage and forming phase of the paper manufacturing process. Modern paper machines use pressurized headboxes to control the slurry jet speed relative to the wire speed, which directly influences how the fibers align.
Dimensional Stability
Variations in paper moisture cause the sheet to expand or contract differently across different directions. When the fiber alignment is highly directional, the sheet exhibits high anisotropy, meaning its physical properties vary depending on the angle of measurement. This differential expansion can cause curling and twisting in laser printing paper, where the intense heat of the fuser roll drives out moisture unevenly.
Tensile Strength
Mechanical strength is highest in the direction parallel to the majority of the fibers, which is typically the machine direction. The fiber alignment dictates the ratio of tensile strength between the machine direction and the cross direction, a critical factor for high-speed web tension in commercial printing presses. In bag paper and heavy-duty wrapping grades, a more random alignment is preferred to distribute strength evenly, which prevents tearing when the filled sack is dropped or subjected to rough handling.
Converting lines use ultrasonic testing devices to continuously measure the ratio across the width of the moving web to ensure uniformity before the rolls are wound and shipped to the customer.
