Sheet Uniformity
Structural variation across a paper web determines how light scatters during high-speed printing. The formation index measures the micro-scale distribution of cellulose fibers within a finished sheet, quantifying the degree of local grammage fluctuation. Optical sensors illuminate the moving web to record transmitted light intensity variations, mapping out clump formations and voids with high spatial resolution.
Low numerical values indicate severe clumping that causes print mottle on coated art papers, whereas high values denote homogeneous fiber dispersion required for fine halftone reproduction. Papermakers control this attribute through headbox turbulence generation and slice lip adjustments, balancing stock velocity against wire speed to prevent flocculation before water drainage locks the network in place.
Visual Quality
Optical density variations on a printed sheet often originate from underlying formation defects rather than ink transfer inconsistencies. Uneven fiber distribution creates microscopic density gradients that distort specular highlights on gloss substrates and produce objectionable mottle in solid ink areas. Converting plants reject stock falling below established formation thresholds when running high-end packaging jobs that demand flawless halftone dot transfer.
Printers compensate for poor base sheets by increasing dampening water volume or applying heavier coating layers, measures that reduce printing speeds and elevate production costs.
Basis Weight
Localized grammage variations invalidate standard burst and tensile strength measurements when tested specimens contain unrepresentative fiber clumps or voids. The formation index sets strict boundaries for mechanical testing validity, ensuring that small sample punches accurately reflect average sheet properties without skewing data through localized anomalies. Standardized test methods require specific illumination geometry to calculate the coefficient of variation in light transmission, separating true structural formation from surface roughness artifacts.
Production specifications establish minimum acceptable indices for specific end uses, ranging from lightweight tissue grades to heavy containerboard where structural integrity matters more than aesthetic perfection.
