Substrate Holdout and Pinhole Formation in Inline Flexographic Coating
Substrate holdout and pinhole control require matching anilox coat weight delivery to sheet absorbency while regulating dynamic wetting and surface skinning.
Cobb Absorption Value
Water Absorption
Calculation of the mass of water absorbed by a one square meter area of paper or board within a specified exposure duration defines the liquid resistance of a sized fiber structure. This metric, known as the cobb absorption value, is determined using a standard metal cylinder filled with a specific volume of water. The resulting dry-to-wet weight differential reveals how well the material resists liquid penetration during storage and shipping.
Testing Protocol
The procedure requires a clamped sample of paper to be exposed to a column of water for a set period, frequently sixty or one hundred and twenty seconds. After this exposure, the excess water is removed using a heavy metal roller and standardized blotting paper before the sample is weighed again. A low measurement indicates a highly sized, water-resistant sheet that is suitable for outdoor packaging and moist environments.
High values indicate a highly absorbent substrate that may lose its structural integrity when exposed to condensation or spillages. Corrugated boxes designed for fresh produce often require low values to ensure they do not collapse in refrigerated transport.
Structural Performance
Internal sizing with chemicals like alkyl ketene dimer provides the primary defense against water absorption. This sizing hinders the capillary movement of liquid through the paperboard.