Thermal Defect
Moisture vapor expansion inside coated paperboard causes web blistering during high heat setting stages in offset printing presses. Trapped liquid converts rapidly into steam when subjected to sudden infrared dryer exposure, exceeding the internal bond strength of compressed cellulose networks. Coated stocks possessing dense impermeable clay layers present the highest vulnerability because escaping gas finds no immediate micro-porous escape route.
Press operators minimize this damage by lowering dryer intensities or running stock through multi-stage pre-heating zones to drive off excess water gradually.
Moisture Equilibrium
Internal hydration levels within incoming substrate lots dictate subsequent susceptibility to explosive steam pockets during commercial runs. Paperboard mills control ambient humidity during final winding to lock moisture percentages inside a tight band between four and six percent. Excessive ambient moisture absorption during shipping shifts this equilibrium upward, creating unstable hydraulic pressures upon sudden contact with high temperature printing hardware.
Converting plants monitor relative humidity strictly across storage floors to prevent localized water gradients from forming inside large paper rolls.
Substrate Porosity
Coating weight formulations and pigment particle sizing determine how efficiently gas escapes under thermal stress. Calendering pressure closes surface voids to achieve smooth printability, yet over-calendering inadvertently seals the sheet against normal vapor transmission. Dual-layer pigment applications require balanced binder ratios to maintain sufficient air permeability while delivering high gloss finish requirements for luxury packaging markets.
Sheet structure integrity ultimately depends on refining degrees and filler selections established during wet end formation on the paper machine.
