The Use of Numerical Simulations in the Development of Inerting Systems For Electron Beam Processing on High Speed Converting Lines

Several important advances in EB system technology have been driven by the curing
requirements of EB silicone release coatings. The residual oxygen must be reduced to a
level that does not inhibit the curing and/or affect the functional performance
characteristics. For silicone release coatings this needs to be less than 50 ppm (parts per
million), whereas for many' other EB coatings, the residual oxygen level can be a factor of
5-10 higher. In addition, production economics are continually pushing converters in the
direction of higher running speeds, for example up to 800 rn/mm. (2624 ft/mm.). It has
therefore become increasingly critical that the inerting system function efficiently as well
as effectively, since the cost of the inert gas (typically nitrogen) is the largest component
of the hourly cost of operating a production EB processor.

A high speed web carries with it a boundary layer of air which must be removed from the
web and replaced with nitrogen. Nitrogen knives (linear nozzles) can be used to direct a
continuous high velocity flow of nitrogen across the web to strip off the boundary layer.
High velocity flow is created by using high pressure to force the nitrogen through the
narrow linear gap of the knife. The mechanical design must include a method for
accurately maintaining the dimension of the gap for the full length of the knife and also
provide for its overall strength and rigidity so that the flow impinging on the web will be
uniform. As the web speed increases the required nitrogen flow velocity increases
exponentially, thereby further increasing the demands on the mechanical design of the
nitrogen knife system, especially for wide webs.

Quantifying the performance of an inerting system is difficult. There is not a convenient
direct method for measuring oxygen concentration at the surface of the web. Oxygen
analyzers are useful in obtaining a reading of the average oxygen concentration at
locations inside the reaction chamber (i.e. curing region) but the reading thus obtained
cannot be relied upon to indicate conditions within the boundary layer, nor can it beused
quantitatively to compare the inerting effectiveness of one machine to another. The
measure of success which has been used in most inerting system development work is the
ability to cure silicone release coatings, which roughly equates to achieving an indicated
oxygen concentration of less than 50 ppm.