Substrate Core Thermal Expansion Limits in High Speed Sheetfed Converting Operations
Substrate core thermal expansion limits dictate sheet dimensional stability, requiring precise thermal control to prevent misregister and bimetallic warp.
Polymer Core Strain
Internal Tension
Tensile stress residing within the plastic layer of a multi-ply laminated board affects its flatness and structural integrity. Paperboard co-extruded with a polymer layer can experience polymer core strain if the plastic is stretched or cooled too quickly during the coating process. This stored stress can later cause the finished sheets to curl or warp, making them difficult to print or die-cut.
Cooling Stress
Molten polymer applied to the paper web shrinks as it cools, but is held in place by the more stable paper layers. If the cooling is not carefully managed, the polymer layer solidifies under tension, creating a continuous pull on the sheet. When the web is later sheeted or die-cut, the release of tension allows the polymer to contract, pulling the board into a curl.
This unbalancing of forces is especially troublesome during high-speed printing because the curled sheets cannot feed smoothly through the press, resulting in frequent feeding errors and machine stops.
Stress Mitigation
Adjusting the temperature of the chill rolls and the speed of the extrusion line helps balance the tension within the composite sheet. Mills test the curl of the board under different environmental conditions to ensure the polymer and paper layers remain in balance. This careful processing ensures that the board remains flat and stable during subsequent conversion steps.