Wash water is an important processing consideration. Not only is the water temperature important but also the flow rate of the water. Two factors that must be considered are as follows: sufficient flow to ensure complete washing of the material and to control or eliminate waste.

If the wash-water temperature is allowed to drop to 65F (18C) or below, emulsion staining may result. As the temperature decreases, less emulsion swelling occurs, reducing the effective penetration of fresh water supplied to the emulsion. When the emulsion does not swell, the chemical-laden water does not get out through the emulsion surface. These retained chemicals can cause stains.

The wash water flow rate is another important factor to consider. This rate must be high enough to wash the material, but no more. When insufficient water flow is supplied to the machine, crystallized chemicals may be seen on the material, and additional staining can result. You must not adjust the water flow rate higher than is needed. A few extra gallons-per-minute flow rate may not seem important; however, over time this effect can be extremely costly, particularly aboard ship.

SOLUTION TEMPERATURE

In machine processing, the temperature may vary, depending on the machine and the kind of processing being performed. High-speed processing machines operate at higher solution temperatures. Temperature control is critical and must be maintained to produce correct results. Although this may be considered a variable factor, the temperature is controlled automatically by processing machines. In some machines, the solution tanks are immersed in a temperature-controlled water jacket. By controlling the water temperature within the water jacket, you can control the temperature of the solutions inside the tanks. In other machines, the solution temperature is directly controlled by separate heaters or heat exchange control units in the recirculation system. A temperature probe in the solution tank monitors and controls the temperature control unit.

SOLUTION LEVELS

The solution levels of a processor must be checked before processing material. If the solution level is too low, stains, improper tracking, and roller marks may affect the film or paper. When the machines are shut down for a period of time, some evaporation occurs. Since only the water from the solution evaporates, you must top off the solution tanks with water before processing material. There is a certain amount of carry-over of solution from one tank to another within the machine. Usually, chemical carry-over is minimized with roller squeegees. When the replenishment rate of the processor is set properly, this carry-over is compensated by supplying fresh chemistry to the solution tanks.

REPLENISHMENT

Most processing machines use relatively large quantities of solutions to carry out the process properly. However, even considering the large quantities involved, certain chemical components within a given solution are used up at varying rates. In addition, there are certain reactions that form by-products that build up in the tank of the processor; for example, bromide (a restrainer) gradually builds up in the developing solution. Also, there is a certain amount of carry-over of solutions from one tank to another. This causes a continuous change in solution strength and solution purity. The replenisher solution replaces the used chemicals, dilutes the excess chemicals or by-products that have built up, and replaces the solution lost by carry-over and evaporation.

The replenishment system used in machine processing is called the bleed method. When the bleed system is used, a calculated amount of replenisher solution is added and forces some of the used solution out through an overflow drain in the solution tank. You must check the established replenishment rates as well as the replenisher holding tanks before and during processing. Inconsistent results occur when the process is not replenished correctly.

DRYER TEMPERATURE

After the material is processed and washed, it continues through the machine into the drying cabinet where moisture is removed. The drying cabinet is more than a heated compartment for the processed material. In a majority of machines, both the temperature and the humidity of the cabinet are carefully controlled. Too little drying causes the emulsion to be tacky, whereas too much drying may produce excessive curl and brittleness. Brittleness, once it occurs, cannot be eliminated; so it must be prevented. Both the temperature and the relative humidity must be adjusted for the speed of the machine and the type of material being dried.

Under ideal conditions, the drying cycle should yield a stable 50 percent relative air humidity. To lower the relative humidity of air, you must heat the air; this accelerates the evaporation of moisture. The rate of evaporation and the relative humidity are directly proportional to the temperature. When the temperature is too low, evaporation is slowed down. When it is too high, the emulsion may be damaged.

Roller transport processors provide very consistent processing results and can be converted easily to a new process. The disadvantages of roller transport processors are: they can leave scratches and scuffs from dirty rollers touching the film, they require a high amount of maintenance due to the large number of moving parts, and oxidation can be a problem due to the churning action of the rollers in the chemistry.