Paper is hygroscopic — its fiber structure absorbs and releases moisture with every shift in ambient humidity. When moisture content changes unevenly across a sheet, the sheet deforms. And when a deformed sheet hits the press, three defects follow: wrinkling, registration drift, and ghosting. The root cause is almost always moisture imbalance before the paper ever reaches the feeder.
Tight Edges: Shrunken Periphery, Buckled Center
When the four edges of a sheet lose moisture faster than the center — typically during storage in a dry environment — the edges shrink while the center retains its original dimensions. The sheet develops a tight border surrounding a relatively loose middle. Under impression pressure, the center buckles into wrinkles.
The fix is simple: add moisture to the edges, not remove it from the center. Splash water onto the floor near the paper stacks to raise local humidity and let the edges absorb moisture back to equilibrium. Better: use a paper conditioning machine to hang and aerate the sheets in a controlled-humidity environment. Combined with floor water, this produces the fastest recovery.
Edge Curl: One Side Drier Than the Other
When the felt side and wire side of the sheet have different moisture content, the drier side contracts and the sheet curls toward it. On press, the curled edge misfeeds — it catches on guides, folds at the gripper, and produces wrinkles that appear as irregular, diagonal creases without a fixed position. The same misfeed causes registration shift and contact ghosting.
The fix: mechanically work the curled edge back to flatness — bending and flexing it in the opposite direction until the sheet lies straight and stiff. This is a press-room countermeasure, not a permanent solution; the root cause is moisture differential between the two sides, which traces back to storage conditions or one-sided exposure.
Wavy Edges: The Most Common Culprit
Wavy (baggy) edges form when the sheet perimeter absorbs moisture and expands while the center stays dry. The edge is now longer than the center. Under the impression nip, the excess edge material has nowhere to go — it compresses into wrinkles, most commonly at the tail edge of the printed sheet.
The fix is the same conditioning approach: hang the paper to equalize moisture content throughout the sheet until the edges and center reach equilibrium with the pressroom environment.
Prevention: Climate Control at the Source
All three defects share a single prevention strategy:
- Don’t print fresh deliveries. Let newly arrived paper acclimate — the longer it sits in the pressroom environment before printing, the more dimensionally stable it becomes.
- Match climate zones. The warehouse, the cutting room, and the pressroom must maintain the same temperature and humidity. A paper stack conditioned in a humid warehouse will deform the moment it enters a dry pressroom — and vice versa. The wider the gap between these environments, the worse the deformation.
Paper conditioning isn’t a secondary consideration — it’s the difference between a press that runs and a press that stops every few hundred sheets to clear a wrinkle. For multi-color work where registration tolerances are tight, conditioned paper is the cheapest quality investment available.
References
- Wikipedia: Paper: Overview of paper composition including cellulose fiber hygroscopicity, moisture absorption mechanisms, and the relationship between moisture content and dimensional stability.
- Wikipedia: Hygroscopy: Fundamentals of hygroscopic material behavior including equilibrium moisture content, humidity-driven expansion and contraction, and conditioning requirements.
- Wikipedia: Printing Registration: Principles of color-to-color alignment including the effects of substrate dimensional change on registration accuracy in sheet-fed printing.
- ISO 12647-4:2014 — Gravure Process Control: International standard for print process control including substrate conditioning requirements and their impact on registration stability.
- Flexible Packaging Association (FPA): Industry resource covering substrate handling, pressroom environmental control, and defect prevention best practices.