A lot of work on shoreline drying was performed in 1973-74 when ECN-2 came along and a manufacturer of processing machines took our specifications at face value and designed a machine with only 3 minutes of drying time with impingement drying, including recirculation of the air at a level of about 50%. In order to dry the film they needed to use very high temperatures around 140 F, when the RH went down to a very low level. This was particularly evident in a lab in Sweden where the outside RH was already low in the winter weather. Lots of shoreline was seen.
The key was to control the rate of moisture transfer from the interior of the emulsion pack so that the emulsion dried down uniformly without forming a skin of dried gel on the surface, necessitating even more aggressive drying conditions to remove the remaining moisture. Since moisture extraction is also taking place through the cut edges of perforations and the slit edge, as well as from the film surface, this effect is greater at these areas, hence giving shoreline. We found that the whole answer to avoiding shoreline drying is to reliably control the RH of the air which is impinging the film. It is a complete and utter waste of time to hang a hygrometer in the dry cabinet; it must be fitted into the plenum which supplies the drying air. Hanging vaguely in the corner of the cabinet serves only to measure the RH of the air after it has done its job. What we found was that there is no need to use recirculation so we shut off the recirculation damper, fitted a carefully calibrated hygrometer to the dryer plenum and then ran film to load up the cabinet with a moisture load until equilibrium was reached. At this time the temperature was high enough to dry the film. We then progressively reduced the head input and allowed the RH to rise until the film was drying at the traditional two thirds of the way through the drybox.
The corresponding temperature for a 3 minute dry time was down around 75 F; this value will depend on the air flow, of course, and no shoreline . As a result, we found that for ECN at that time, the critical RH which allows the removal of moisture without causing shoreline to be around 30%. This value has been reliably used by labs all over the world. The value of other films needs to be determined by experiment.
It can not be emphasized enough the need to calibrate the RH measuring instrument. Most of them are way off. Use a sling psychrometer to cross check at the RH range you want to measure and adjust the RH meter to match.
Equilibration to 50% RH once the film has dried is the final stage. Often, room air is around this value and the film acclimates on the wind up elevator.