Unheated Warehouse Storage Impact on Multi Ply Boxboard Moisture Equilibrium
Unheated storage alters multi-ply boxboard moisture equilibrium, swelling sheet edges, reducing internal bond strength, and causing severe converting curl.
Edge Wave Deformation
Dimensional Flaw
Undulating patterns appearing along the perimeter of a paper reel or sheet stack indicate a localized moisture imbalance within the substrate. Edge wave deformation occurs when the outer edges of the paper absorb moisture from the surrounding air while the center remains at its original dry state. Moisture absorption at the margins causes the paper to ripple because the fixed dimensions of the core material cannot accommodate the increased surface area.
The condition is most prevalent in environments where the relative humidity is substantially higher than the equilibrium moisture content of the paper.
Maintaining a Climate
Maintaining a controlled climate in the pressroom and warehouse provides the primary defense against this physical distortion. Paper should remain in its moisture-proof wrapping until it reaches the ambient temperature of the processing area to prevent condensation. Once the fibers have expanded, the resulting ripples often interfere with the feeding mechanisms of printing presses or sheeters.
Tension adjustments on the web can sometimes mitigate the visual effect, but the underlying physical change to the fiber structure is irreversible. If the waves are severe, the paper may strike the printing blankets or cause misregistration across the width of the sheet.
Inspection of Stack
Inspection of the stack profile before loading the feeder identifies batches that require additional acclimatization time. Digital sensors can detect the height variation across the pallet to quantify the severity of the distortion. Proper storage prevents the loss of expensive stock due to environmental exposure.