c. Since the first two types of squeegees come into direct contact with
the film, it is imperative that they be kept absolutely clean.
A chemical
deposit or an abrasion on the squeegee will seriously damage the film. The air
squeegee, although it never touches the film, must also be used with caution.
The compressed air must be clean and free of oil used in many types of
d. Washing the film is done in the last two tanks of the processor and the
movement of the film in water provides adequate agitation.
7.
Drying Systems.
a. The drying of film is a complex, two-fold operation. So that moisture
may be removed from the thickness of an emulsion, it must first migrate to the
surface and then evaporate in the air.
The rate at which this takes place
depends upon the thickness of the emulsion, the amount of hardening it
received, the density of the developed image, the temperature, relative
humidity, and movement of ambient air reaching the film.
b. The air is heated to expand it, to lower its relative humidity, and to
accelerate the evaporation of moisture.
These factors are directly
proportional to the temperature. The temperature must be held within certain
limits. If it is too low, evaporation is slowed and drying marks result, and
if it is too high, the emulsion may become brittle and crack. The exact limits
are governed by the kind of material being dried, how it was processed, and the
type of dryer being used.
c. The reason air should be kept moving is that air at the surface of the
wet emulsion soon becomes saturated and will not take up additional moisture.
The saturated air must be replaced by unsaturated air if drying is to continue.
Of course, even still air is in constant motion, and an emulsion will dry
eventually, but the drying will most likely be more rapid at the edges than in
the center.
Thus, any considerable change in the drying conditions that
persists for comparatively long periods of time will leave visible lines at the
wet-dry borders. This condition is apt to occur in certain types of dryers.
d. Parallel-flow dryers utilize a series of reverse-bend ducts to conduct
warm dry air across the film as it moves through the drying cabinet. Figure
1-8 shows schematically a typical arrangement.
(1) In order to prevent the drying marks mentioned above, parallel-flow
dryers should be operated at rather high air velocities.
The temperature
should not be excessive, and the relative humidity of the air should not be too
low.
(2) The exact conditions must be adjusted to the climate in which the
dryer is being used. Different settings would be required for coastal areas
than, for example, the desert.
e. One of the most common methods of drying is called impingement drying.
In this method, jets of air are applied at right angles to the
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