KEEPING WEBS WRINKLE-FREE AND DIMENSIONALLY STABLE AFTER HEATING

摘要

Web dimensions are affected by tension, temperature and humidity. This paper will summarise the understanding of viscoelasticity, thermal and hygroscopic expansion, shrinkage, and their anisotropy, using polyester as an example. Thermomechanical and Dynamic Mechanical Analysis data are particularly useful to understand and predict substrate behaviour. Many web processes involve heating to cure or dry a coating and subsequent cooling; but problems may arise as a result of the thermal behaviour. Wrinkles may form in long unsupported spans and be set-in as temperature falls. Tension whilst hot can increase the residual shrinkage, hence reducing the dimensional stability during subsequent processing and in the final product. Predictions of web width changes during heating and cooling, together with the tendency of the web edges to travel straight, shows which phenomena are responsible for generating wrinkles. This analysis can show whether there is scope for optimising the temperature profile or placing a spreading device to reduce wrinkles.