Refining
Also: refining, beating
A stock preparation step where pulp passes between rotating discs with bars, mechanically working the fibers to increase bonding and strength.
Refining (also called beating, historically) passes the pulp slurry through the narrow gap between rotating and stationary discs with surface bars. The mechanical action fibrillates the fiber surface (exposing fine fibrils) and collapses the fiber lumen. Both effects increase bond strength in the final sheet.
Over-refining cuts fibers and kills tear strength. Under-refining leaves weak bonds. Every paper grade has an optimum refining level, usually measured by the freeness drop from unrefined pulp.
Related
- Freeness (CSF). A measure of how readily a pulp dewaters, reflecting the degree of refining. Lower freeness means more refined, slower-draining pulp.