Evaluating Pulp Yield Losses from Extrusion Coated Board in Industrial Hydro-Pulpers
Polyethylene-coated board yields drop 8 to 22 percent in industrial pulpers as unseparated fiber bundles exit screens bound to rejected plastic film.
Fiber Flake
Unpulped Bundle
Aggregated clusters of paper fibers that remain bonded together after initial wetting and agitation constitute a common reject material in recycling mills. A fiber flake typically originates from wet strength papers or heavily sized boards that resist water penetration during the pulping cycle. These bundles are too large to pass through fine screen slots and require further mechanical treatment to become useful pulp.
Their presence in the raw stock indicates incomplete hydration or insufficient shear force in the pulper. Because they are denser than individual fibers, they tend to settle in low velocity areas of the piping, potentially causing blockages that disrupt the steady flow of the slurry.
Breakdown Mechanism
Secondary deflaking units apply intense hydraulic shear and impact to separate these clusters into individual filaments. While the initial pulper handles the bulk of the material, the fiber flake survives until it encounters the close tolerances of a high speed rotor and stator. The gap between these components determines the level of reduction achieved in a single pass.
If the gap is too wide, the bundles remain intact and return to the beginning of the cleaning circuit.
Impact Limit
Visual defects in the finished sheet often stem from these clusters surviving the entire stock preparation process. A fiber flake that reaches the headbox appears as a translucent or dark spot in the paper web, weakening the local structure and disrupting the surface smoothness. Such defects interfere with ink absorption during printing and can cause breaks on a high speed press.
Controlling the volume of these bundles ensures a uniform formation across the width of the machine.