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.
Sheffield Roughness
Surface Contour
Evaluation of the surface texture of paper by measuring the rate of air leakage under a standardized glass-plate seal determines the printability of a paper sheet. This air-leak measurement, called sheffield roughness, is expressed in Sheffield units that correspond directly to the volume of escaping air. It indicates the micro-contour of the paper sheet and helps predict how well inks will transfer during printing.
Leakage Mechanism
The testing instrument places a circular steel head of specific mass and area onto the paper sample. Compressed air is supplied to the center of the head at a constant pressure of ninety-seven kilopascals, and the flowmeter measures the air that escapes between the paper surface and the metal land. Smooth paper allows very little air to escape, producing a low Sheffield value.
Rough sheets with deep surface valleys let air escape easily, resulting in a high measurement. These values are used to adjust the pressure of the printing press, as rougher sheets require higher impression forces to ensure full ink coverage.
Printing Limitation
High roughness values in lightweight paper stocks can cause broken half-tone dots and loss of fine detail. This condition requires operators to select more compliant blankets or thicker ink films.