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.
Hydrogen Bond Thermal Stretching
Molecular Expansion
Microscopic physical changes within the fiber network of a paper sheet occur when heat is applied, causing the molecular connections to expand and slightly shift. Hydrogen bond thermal stretching describes the minute elongation of the cellulose structure as moisture evaporates and temperature rises. This movement can lead to dimensional instability in the sheet during multi-color printing or heat-sealing.
The effect is most pronounced in papers with high moisture content or low density.
Dimensional Change
Moisture loss during the heating process allows the individual fibers to contract, but the initial thermal energy causes the bonds between them to stretch. This hydrogen bond thermal stretching happens most rapidly in the machine direction of the paper. It results in a change in sheet size that can throw off the registration between different ink colors.
Accurate temperature control in the dryer section is needed to minimize this movement. The sheet usually returns to its original size once it reaches ambient temperature and moisture.
Printing Accuracy
Excessive heat in the dryer section triggers these dimensional inaccuracies. Problems with fit and registration are common when the web temperature exceeds the stable operating range. Careful management of the heat profile prevents these registration errors.