In the Nordson select system it is necessary to clean out the nozzles and
tubing very well.  If not, it builds a thin laminar coating on the sidewalls
and in the nozzle aperture which tends to constrict the coating going thru.
 The coating is then meeting with a greater resistance to laminar flow then
originally specified.  This in turn can cause or produce bubbles due to what
is an apparent increase in the elastic  portion of the viscosity curve. You
won't see these bubbles until they coalesce in the film. By then its too late
to get them out if you heat too fast. 

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