ISO 12647 2

Dot Expansion

Paper surface structure dictates how wet ink droplets migrate into the porous matrix and spread under the pressure of the blanket cylinder. Mechanical dot gain occurs when ink spreads physically during the contact phase between the cylinder and the substrate. Optical dot gain arises from light scattering within the paper fibres, which creates a perception of darker tones than the physical surface area suggests.

Since uncoated stocks possess greater porosity than coated grades, the standard specifies higher allowances for dot expansion on these materials to prevent the shadows from filling in prematurely. Press operators calculate the transfer function of the lithographic process to adjust plate curves before the start of the job. Matching the dot gain to these prescribed curves ensures that the printed mid-tones match the digital proofs generated in the prepress department.

Tolerance Limits

Acceptable deviation from the target values relies on the delta E calculation that quantifies the difference between measured colour coordinates and the reference specification. Laboratories and press rooms use these figures to determine if a print run remains compliant with the quality agreement between the converter and the brand owner. Differences beyond the defined thresholds indicate a shift in ink chemistry, fountain solution pH or blanket tension that demands a recalibration of the press unit.

Consistency across long production runs depends on holding these variances within a tight corridor. Professional print quality remains a function of objective measurement rather than subjective interpretation of a printed sheet.