HEAT LOAD VARIATIONS ON WEB SUBSTRATES AS SEEN BY DEPOSITION MODELLING

摘要

Wrinkling is one of the most common problems that can occur during the vacuum deposition of coatingsonto web substrates. It is useful to be able to predict which of the many possible deposition conditions mightcause it. Where the roll to roll vacuum system is expected to coat a variety of different substrates ofdifferent thicknesses without previous experience it is impossible to know if each new combination ofmaterials will run well or will have problems. Being able to predict which of the webs might be moresusceptible to wrinkling would enable the process to be adjusted without the need to waste materials inprocess trials.The heat transfer coefficient, HTC, is a critical, largely unknown factor in the deposition process. The HTCdetermines the rate at which energy arriving on the web by radiation from the sources and by deposition ofthe coating flows into the cooling drum. It can be estimated providing there is suitable knowledge of otherparameters. However, the HTC is not a constant throughout the process. It has three components,conduction, convection and radiation. The contribution of each of these can vary with tension on the web orthe amount of gas present between the web and deposition drum. The lower the HTC, the less the heatflows into the drum so the higher the temperature of the web which then expands and changes the tensionand dimensions of the web which in turn (generally) reduces the heat transfer coefficient further, leading tothe possibility of runaway temperature rise, large expansion and serious wrinkling.Our earlier work [Refs 2, 3] calculated the tension distribution of a web around a drum for constant HTC andestimated the risks of wrinkling. This paper extends that work, to deal with the more general case where theHTC varies with both the tension and the temperature.We now have a robust algorithm that can accommodate a wide range of speeds, heat load, web thicknessesand properties to deliver the interacting tension and temperature distributions round a coating drum.Experiments with this computational tool to explore the sensitivity of the tension and temperature profiles tovarious forms of the dependency of HTC on temperature and pressure are described. From the tension andtemperature profiles the risk of wrinkling can be assessed.