Physical Dimension
Sheet caliper measures the perpendicular distance between two parallel faces of a paper or paperboard substrate, expressed in micrometres or thousandths of an inch. Micrometer anvils exert a standardized static pressure of fifty kilopascals against the sample during the measurement cycle, eliminating operator bias from the reading. Papermakers control this property through refining intensity, wet press loading, and calendering nip pressure before the stock reaches the finishing winders.
Substrates varying beyond strict thickness tolerances cause registration errors on multi-color offset presses and jamming on high-speed folding carton packaging lines.
Calendering Effect
Mechanical compression applied in the calender stack reduces sheet caliper by rearranging fiber networks and crushing internal voids. High nip pressure increases surface smoothness and gloss while simultaneously decreasing air permeability and tearing resistance. Converting plants monitor the resulting density gradients across the web because uneven compaction creates differential stretch during subsequent printing passes.
Standard Pressure
Testing protocols mandate specific load conditions and contact area dimensions to ensure reproducible thickness values across different laboratories. Soft compressible boards require lower contact pressures than dense securities papers to prevent fiber crushing under the measuring foot. Ambient humidity and temperature conditioning prior to testing remain mandatory because moisture absorption swells cellulose fibers and alters the measured dimension.
