The following is a description of the film processing steps from the automated film processor equipment where the film is chemically processed to produce an image that will be chemically transferred to a printing plate. This process step is used in offset lithographic, flexographic, screen and gravure pre-press printing processes.
Light-sensitive film is used to transfer the artwork that will be printed onto a printing plate through the subtractive process. The film has several layers of light sensitive subtractive coating (i.e. silver halide (AgX)). The subtractive coating is insoluble in water and diffusion-resistant.
Images are first exposed or captured in the sensitized silver halide emulsion layers on the film. Then the film is developed in the automated film processor machine.
The process of film developing involves removing the layers of silver halide emulsion from the film only in areas where the light sensitive compound or has been exposed to ultraviolet light (UV). The images form on the film during the development process where couplers react with the oxidized development agent, CD4 or paraphenylendiamine, in the developer bath during processing.
During film developing process, the oxidation product of the developer solution reacts ("couples") with the couplers dispersed in the emulsion layers to form the emulsion. After development the silver is bleached (converted into soluble silver salt) and dissolved out of the emulsion in the fixer.
Together with the chemical couplers, in each layer there are crystals of silver halides sensitive to the light that registers the image. The crystals of silver halides and the color couplers are distributed at random in the layers. All are suspended in jelly (just as it is sometimes the fruit inside an eatable jelly). The color couplers surround each one of the crystals of silver halides.
The developing process consist of three chemical steps, developer, bleach, fixer and stabilizer.
The chemical reducing action of the developer solution converts the exposed silver halide into metallic silver (silver image). The activity of the developer depends on temperature, time, replenishment rate, and replenisher concentration. Time, temperature, and agitation determine the diffusion rate.
The Developer transforms into metallic silver the crystals of exposed silver halides. The non-exposed crystals remain unaffected. During the transformation of the silver halides into metallic silver, the silver halides are oxidized.
After the developer follows the step of bleach. The bleach stops the action of the developer and it transforms the metallic silver another time into crystals of silver halides. The silver should be in form of crystals of silver halides, so that the fixer can eliminate it. The bleach processes the metallic silver to transform it into crystals of silver halides again. The bleach affects the metallic silver, never to the crystals of non exposed silver halides.
The bleach stops the action of the developer, and its oxidizing agents convert metallic silver into silver halide. The silver halide is later dissolved in the fixer. The amount of metallic silver converted to silver halide depends on the bleach concentration and the rate of bleach diffusion through the emulsion. Bleach time, agitation, and temperature affect the rate of diffusion. Bleach concentration is affected by replenishment rate, mixing procedures, and aeration efficiency. Aeration reconverts the reduced bleaching agent to an active form.
The bleach-fix has several purposes. First, it stops the action of the developer. Its oxidizing agents convert metallic silver into silver halide. The silver halide is also dissolved in this solution. Oxygen taken into the bleach-fix during normal processing aerates the bleach-fix to reconvert the reduced bleaching agent to an active form. Secondly, it acts as a bleach to convert the silver in the emulsion to an ionic form. And thirdly, it acts as a fixer to convert the ionic silver to a soluble form so that it can be removed from the emulsion.
The amount of metallic silver converted to silver halide depends on the bleach-fix concentration and the rate of bleach-fix diffusion through the emulsion, Bleach-fix concentration is affected by replenishment rate, mixing procedures, and aeration efficiency, Bleach-fix time, agitation, and temperature affect the rate of diffusion. Using an abnormal bleach-fix can affect the amount of dye formed.
Insufficient bleach-fixing (BF) can cause retained silver when some of the metallic silver is not converted to silver halide. An increase in the BF densities of your control plot may indicate that there is a retained-silver problem. The effect of retained silver is greatest at the highest exposure levels, where there is more silver to convert. Retained silver adversely affects image quality.
The fixer is the following step in the process. The function of the fixer is to convert all the crystals of silver halides into complex of soluble silver. The fixer eliminates the soluble complexes of the negative
The fixer solution converts all the silver halide in the film into soluble silver complexes. Most of these silver complexes remain in the fixer solution where they can be recovered by silver-recovery methods. The amount of silver halide converted to soluble silver complexes depends on fixer activity and the diffusion rate into the emulsion. Temperature, replenisher concentration, and replenishment rate affect fixer activity. Time and agitation affect the diffusion rate.
A fixer solution that is near exhaustion will not completely remove the silver halide. A milky appearance in the D-min areas of the film also indicates insufficient fixing. Temperature has very little effect on fixing rate if the other fixer conditions are within limits. Agitation primarily provides uniform fixing.
Washing removes the chemicals from the film after processing. Effective washing requires enough circulation to keep fresh water in contact with the emulsion. The water temperature must be warm enough to swell the gelatin so that the water moves freely into the emulsion to remove the chemicals, but not so warm that the gelatin melts or is permanently distorted.