The percentage of water contained in paper or paperboard relative to its oven-dry mass. It is a key factor affecting dimensional stability, curl/warp, flatness, strength, and runnability during printing and converting.
Formula
Moisture Content (%) = (WetMass−Oven−DryMass)÷Oven−DryMass(Wet Mass − Oven-Dry Mass) ÷ Oven-Dry Mass(WetMass−Oven−DryMass)÷Oven−DryMass × 100
Typical Range
Most packaging and printing grades are shipped at ≈5–8% moisture, but the actual value varies with climate, grade, and storage conditions.
Measurement Methods
Oven-dry (gravimetric) method: Lab reference—dry at ~105 °C to constant mass, then calculate.
Electrical moisture meters (resistance/capacitance):Â Fast, convenient shop-floor checks.
Conditioned equilibrium: Condition samples at 23 ± 1 °C, 50 ± 2% RH before testing for comparable results.
Effects
Too low:Â Brittle edges, cracking at folds, increased static, upward curl/warp.
Too high:Â Soft sheet, lower strength, wrinkling, unstable print/cut dimensions, blocking.
Control & Best Practices
Match incoming paper to plant humidity; acclimate rolls/sheets before use.
Store in protective wrap; avoid direct heat, drafts, and damp floors.
Maintain stable pressroom conditions (commonly ~45–55% RH, ~20–24 °C).
For shipments across climates or seasonal changes, allow adequate acclimation time prior to converting to minimize curl and dimensional drift.