Environmental Variance
Permitted fluctuation in the moisture content of the air within a storage or printing hall defines the operational limits for stable paper handling. This relative humidity tolerance determines how much the environmental moisture can depart from standard values before paper begins to deform or misfeed. Maintainers of printing presses and converting lines monitor this range to prevent misregistration and sheet fed feeding errors.
Physical Bounds
The paper structure expands and contracts as it reacts to changes in ambient humidity. Typical tolerances are set within a narrow range of plus or minus five percent around the target relative humidity. If the room exceeds these limits, paper curl and wavy edges can arise because the moisture distribution across the sheet becomes uneven.
Tight edges happen when a dry stack of paper absorbs moisture only at its outer borders, causing the edges to swell. Conversely, dry air draws moisture out of the sheet edges, causing them to shrink and creating a dished center. Maintaining these tolerances within the specified limits prevents the misregistration of colors during multi-pass offset printing.
Mechanical Adjustment
Specialized HVAC systems equipped with steam humidifiers and industrial dehumidifiers maintain the atmosphere of the pressroom within the designated boundaries. This continuous control prevents paper deformation during long production schedules.
