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— —GAP 0005* MORE
THAN FILM THICKNESS
Fig. 4. Squeegee.
the drying of film rewetted in this manner with film which was dried on the equipment immediately after processing showed that there was no measurable difference in the required drying times. It is extremely fortunate that this was the case because the complication of having to process the film as it was being used would have made an extensive survey impractical.
In making an experiment the air temperature and pressure were adjusted and the machine allowed to run until it had reached constant conditions. Film was then attached to the leader, threaded through the machine — the supply roll still being submerged in the tank of water — and the machine drive started. It was allowed to run at the chosen film speed until steady conditions prevailed. Samples of the dry film were cut from the film as it emerged from the end of the dry cabinet and placed in stoppered bottles. Wet film samples were then cut from the film just beyond the holdback sprocket after it had emerged from the squeegee and before it entered the drying cabinet. The moisture content of the film was determined by accurately weighing the samples in the bottles before and after being subjected to an evacuation procedure. The difference between these
two readings accurately determined the moisture loss of the film passing through the drier. For each set of air conditions investigated, the effect of drying time was explored by making experiments at a< series of film speeds, thereby obtaining different drying times.
In Fig. 5 is shown the performance of the machine on Eastman Fine Grain Release Positive Safety Film Type 5302. In this plot the ordinate is the moisture content of the film and the abscissa is the drying time. The impingement velocities used were 2,000 and 4,000 fpm and at each of these, temperatures of 125, 150 and 200 F were used. The dry point, indicated as a horizontal line on the graph at the 2.5% moisture line, was arbitrarily chosen and is the point at which the film has the same moisture content as it would have if it were in equilibrium with air at 70 F and 50% R.H. To illustrate the margin of safety, the 60% and 30% R.H. levels are also indicated. Under the most severe drying conditions used it can be seen that required drying times of 10 sec were obtained and the longest required drying time was somewhat more than 42 sec at 125 F and 2,000 fpm air velocity. The moisture absorption characteristics of emulsion vary to a considerable extent. This emulsion is quite hard and as a result the film absorbs only about 15 to 19% moisture. The performance of the drier on films of relatively high moisture capacity is shown in Fig. 6. This is Eastman Plus X Panchromatic Negative Safety Film, Type 5231, which absorbs approximately 35% moisture. With this film the minimum required drying time was 16 sec and the maximum was more than 1 min.
General Curve Shape
Interpreting these curves in terms of the classical constant and falling-rate phase of drying, it might be said that the early part of the drying approximates the constant-rate phase, then
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February 1953 Journal of the SMPTE Vol. 60