Pass Order Decides Which Effects Remain Achievable
Pass order determines surface energy, registration accuracy, recyclability, and unit scrap rates across multi-step print converting runs.
Sheet Stretch
Substrate Strain Distortion
Dimensional elongation occurring in individual paper or paperboard sheets under mechanical web tension, printing nip pressure, and moisture absorption alters sheet geometry during converting. Processing equipment exerting mechanical pull combined with aqueous ink deposition causes sheet stretch across both the machine direction and cross-machine direction. Cellulose fibers absorb water from fountain solutions, adhesives, and water-based inks, expanding significantly in their diameter and inducing overall physical distortion.
Print units running in series encounter progressive image misregistration, plate-fit mismatch, and die-cutting misalignment as the distorted sheet passes through successive print stations. Press operators and converting engineers adjust cylinder packing, sheet tension, and trapping parameters to compensate for this continuous physical expansion.
Hygroexpansion and Tension Dynamics
Unconstrained paper fibers expand five to ten times more in diameter than in length when exposed to relative humidity shifts or fountain solutions during printing passes. Mechanical nip pressure between offset blanket cylinders and steel impression cylinders pulls the softened, damp paperboard web, stretching its mechanical structure along the print direction. In multi-unit offset presses, the trailing edge of a large sheet format expands outward, generating a characteristic fan-out distortion pattern where corner colors pull out of alignment.
Paperboards with low internal bond strength or poor dimensional stability stretch more under high-tack inks, requiring lower press speeds and reduced blanket grip pressures. Adjusting plate clamping mechanisms and scaling digital artwork separations horizontally or vertically provides pre-compensation for known substrate stretch rates.
Mechanical Boundary Limits
Permanent dimensional changes caused by mechanical folding, score-line cracking, or die-rule creasing fall outside the scope of elastic and hygroexpansive sheet strain. Purely thermal shrinkage occurring in plastic film substrates inside high-temperature drying tunnels represents thermal plastic contraction rather than cellulosic sheet stretch. Instantaneous elastic recovery that snaps a dry web back to its exact initial dimensions immediately upon releasing roll unwind tension does not generate persistent stretch defects.
Sheet stretch measurements evaluate only the physical elongation and hygroexpansive spreading of discrete paper or paperboard sheets during active press processing.