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.
Cross-Machine Direction Expansion
Transverse Growth
Dimensional movement across the width of the paper machine wire represents the largest moisture-driven change in a finished paper sheet. Paper displays cross-machine direction expansion when it absorbs moisture, as the majority of the fibres are oriented parallel to the direction of web travel and thus expand outwards. This transverse movement is often five to ten times greater than the growth along the machine direction.
Moisture Response
Fibres in the paper web align with the direction of the flow as the pulp is deposited on the forming wire. During drying, the web is held under tension lengthwise, but can shrink across its width, leaving the cross-machine direction highly susceptible to subsequent moisture changes. When the paper later encounters water-based inks or high humidity, the fibres swell sideways, which causes the sheet to widen and can disrupt the alignment of subsequent print colours.
This expansion can create a scalloped edge on the roll, which makes feeding difficult and causes web tension to fluctuate wildly. In extreme cases, the lateral sheet growth is so pronounced that it exceeds the registration tolerances of modern high speed multi colour presses.
Press Management
Layout designers arrange the printed pieces on the sheet to ensure the grain runs parallel to the cylinder axis. This alignment means the greater expansion occurs in the direction of sheet travel, where the press operator can compensate for sheet growth by adjusting the cylinder packing. Ensuring consistent humidity in the paper storage area prevents the edges of the sheets from absorbing moisture and developing wavy edges before they reach the feeder.