Paper formation refers to the uniformity of fiber distribution within the paper sheet structure. It is generally inspected by transmitted‑light viewing. Uniform fiber distribution indicates good fiber dispersion. Poor fiber dispersion manifests as clumpy formation, disjointed formation, wavy fiber texture, stock streaks, and cloud formation.
Rules of fiber dispersion and flocculation: During sheet forming, longer fibers tend to flocculate more readily. Higher stock consistency promotes flocculation, while lower shear force also favors flocculation. Conversely, these conditions facilitate fiber dispersion.
Clumpy formation: Fibers flocculate into irregular lumps scattered across the paper. Under transmitted light, the lumpy areas have thicker paper layers, whereas regions between lumps are thinner, resulting in uneven light transmission across the web.Causes: Insufficient beating; improper headbox consistency; poor performance of flow‑straightening elements; inadequate dewatering control on the wire; excessive or poorly dispersed retention aid; improper application of wet‑end chemicals.
Disjointed formation: Poor interweaving of fibers in the machine direction, shown by numerous transverse gaps of varying lengths distributed over the paper web.Cause: Inappropriate stock‑to‑wire speed ratio, where stock velocity is substantially lower than wire speed.
Wavy fiber texture: Numerous transverse stock marks in wavy patterns spread across the paper web.Cause: Inappropriate stock‑to‑wire speed ratio, where stock velocity is significantly higher than wire speed. The incoming stock impinges upon fibers already deposited on the wire.
Stock streaks: Longitudinal strip‑like stock marks visible under transmitted light on the paper web. Their positions are not fixed; even when relatively stable, they vary within a certain width range, with lengths ranging from tens to hundreds of millimeters. These strips are essentially thickened stock layers.Causes: Excessive dilution water; defective lip plate or flow‑straightening components leading to unstable stock flow and uneven free‑surface profile of the stock. Longitudinal raised stock streams form across the wire width. In severe cases, collision and merging of two stock streams solidify into thick stock streaks.
Cloud formation: Aggregated fiber clusters scattered like clouds throughout the sheet, causing uneven basis weight and irregular light transmission. Large‑scale cloud formation usually arises from excessive water volume and excessively rapid initial forming. After partial fiber deposition on the wire, substantial surface water surges and disturbs the top layer of the pre‑formed fiber mat, producing cloud defects.
Moderate beating: Beating reduces fiber length and improves fiber uniformity, which is essential for uniform sheet formation. However, over‑beating shall be avoided. Excessively high beating degree impedes wire‑section dewatering and prolongs setting time, which triggers re‑flocculation of fibers on the wire. The optimal beating degree shall be determined according to actual paper‑machine conditions and paper grades.
Application of chemical additives: Dispersants are adopted for long‑fiber papermaking to improve fiber dispersion. For general papermaking, appropriate dosage of retention aids induces micro‑flocculation of fine fibers. It enhances fine‑fiber retention and dewatering performance, thereby improving final paper formation.
