Structural Failure
Heat-induced delamination in coated paperboard arises when rapid moisture vaporisation exceeds the internal bond strength of the fibrous matrix. A blister defect typically appears during the high-temperature drying of double-sided clay-coated grades or within the heat-seal station of a liquid packaging line. When internal moisture cannot escape through the impermeable polymer barrier or dense coating layer, it builds localized pressure that forces the paperboard plies or coating layer apart.
This phenomenon causes a raised dome-like bubble on the surface, which disrupts the uniformity of the sheet and ruins the print finish. The separation occurs at the weakest interface, which is usually between the clay coating and the top liner.
Prevention Method
Control of pre-press moisture levels prevents the formation of these surface disruptions. In printing and converting plants, maintaining the board at a relative humidity between forty and fifty percent limits the amount of free water available to vaporise. Lowering the surface temperature of the drying cylinders or reducing the dwell time in the heat sealer also assists.
The internal bond strength of the substrate must remain high enough to withstand the mechanical stress of residual vapor pressure.
Physical Assessment
Visual inspection combined with manual compression identifies the compromised zones across the printed web. If the blister defect is present, the dome collapses under light thumb pressure, revealing a hollow core between the separated layers. Laboratory cross-sections of the damaged area show distinct voids within the fibre network or at the boundary where the coating meets the base sheet.
Measuring the diameter of these bubbles helps technicians adjust the process tension.
