Types and Technical Characteristics
of Processing Machines
Processing machines can be categorised according to their method of
loading, their drive system and their process, and the following features are
important in their design.
- Darkroom Loading
The construction of this type of machine is such that the section of the machine that has to be in darkness or in safelight is in an area that can be light-proofed. The rolls of film are loaded directly on to the machine without using a magazine in the dark or under a safelight. When a safelight is in use it is possible to observe the film running through the dark areas and spot any problems that might occur. It is also possible to rectify some problems without fogging the film as would occur with a daylight or magazine loading machine in the event of serious trouble. - Magazine Loading
This type of machine is entirely in the light. The dark sections of the machine are covered with a lid and the film loaded on to the machine by means of a light tight magazine. This has the advantage that maintenance can be done on the machine in white light without affecting any other machines that might be in the same area. It is not possible to rectify problems in the dark section without fogging the film.
A major problem with all film, although least with polyester base, is that the wetting of the base, sometimes combined with a higher temperature than the ambient air causes film to stretch by a small amount and to shrink back to it's original dimensions as it dries. The effect is quite small but sufficient to create slack in the film path in the wet section and a dangerous tightening in the drier. Uncorrected the film would not be driven as it comes away from the rollers and in the drier will stretch or even break. Over the years, many inventions have been directed at providing a slip free and even transport and several successful designs exist today.
The manufacture of the various components of the processing machines must be from materials that are suitable for the purpose but also that will not affect the solutions. Stainless steel is used extensively for the high mechanical strength and where there is risk of corrosion. However, some stainless steels are attacked readily and it is usual to use steel of the US standard AISI 316. Some bleaches and stop baths can only be stored or used if Titanium is used for metal parts. Some manufacturers choose to use plastics such as rigid PVC in this case
If film passes through a developer without any form of agitation uneven development results.
Additionally chemical products of processing, principally sodium iodide and bromide reduce the activity of the developer and local exhaustion of the developer occurs in areas of high density, if the solution is not continuously mixed.
Several methods are in use to overcome this, sometimes in combination.
All solutions are usually recirculated by being extracted from the tank, passed through some form of temperature control system and a filter and returned to the tank. The pump required for this purpose also provides local agitation. The need for agitation is greatest in the developer.
Squeegees remove solution from the film as it passes from one solution to the next to prevent contamination by "carry over" of the solutions, and before drying, in order to prevent drying marks from uneven drying.
Squeegees were originally just rubber wipers rather like windscreen rubbers but developed into a technology of their own as machine speeds increased and the risks of contamination increased.
Washing has two purposes in a processing machine.
First, to prevent carryover of chemicals from one bath to the next and second to remove the water soluble thiosulphates and argentothiosulphates produced by the fixing bath and any other chemicals that are present before drying the film. Sometimes the wash is followed by a solution treatment with stabilisers, fungicides or waxes.
Both temperature and humidity of the drying cabinet is controlled so that the film dries at a reasonable rate and retains the enough moisture to prevent excessive curl and brittleness of the film.
The airflow is controlled so that there is a mix of fresh and recirculated air to dry adequately. It is always recommended to adjust the temperature so that the film emulsion dries about two-thirds of the way through the drying cabinet. The point at which the film dries is the point at which the film changes from having a convex curl [with the emulsion on the inside] to having a concave curl.
The remainder of the dryer time is used to even out the dryness. Moisture removed from the film increases the humidity in the cabinet and the temperature also affects the final relative humidity.
Control of the humidity in the cabinet is by varying the ratio of fresh outside air to the amount of air re-circulating within the cabinet. The humidity level in the cabinet will be more or less equal to the internal humidity of the final dry film.
A relative humidity of about 50% RH is the usual objective, about the same as ambient air.
During processing, the active chemicals such as the developing agents in the developer are utilised and changed to inactive decomposition products. In addition, in development the emulsion releases sodium bromide that builds up in solution, in time slows the development action, and increases contrast.
In other solutions similar effects also occur.
Carry-over of solution from one tank to the next tank on the surface of the film occurs in all processes, the amount depending on the efficiency of the squeegees.
To maintain a process with consistent activity it is therefore necessary to replenish the solutions in order to keep the chemical components constant.
A replenisher is a solution devised to replace some of the used process solution and, in doing so, bring the concentration of active chemicals back to the same level as the original fresh solution.
A replenisher solution is usually formulated by the manufacturer of the film and the volume replaced is dependant on the film processed. Replenishment may be continuous or batch.
Backing is the name for the anti-halation coating on the base side of colour film.
A photograph of a bright light has a halo around the image of the light due to the reflection of the light from the film base. Backing is coated on the film base preventing high levels of light transmitted through the emulsion being reflected back to the emulsion, and causing a halation image.
The backing layer on most films is a water or alkali soluble layer of dye that is removed in the developer solution. The backing on many colour negative, positive and intermediate films consists of a light absorbing material called rem [i.e. removable] jet - a dispersion of carbon particles in an alkali-soluble layer.
Black and white films do not have rem-jet backing and therefore do not need the additional processing stages of backing removal. Additional advantages of backing are the protection of the base from scratches and the prevention of static because the backing is conductive
All solutions collect particulate matter, and this is removed by filtration.
Dust particles can cause scratches due to build up on the squeegee lips and on the rollers.
Other causes of scratches could be rollers not rotating or by film touching parts of the machine it should not touch.
Scratches tests
It is good laboratory practice to run a scratch test before beginning to process film each day. You examine a piece of film in the light to ensure that it is free from scratches, then you run it through the normal process and examine it again. You must ascertain the causes of any scratches present before processing work.
Static Damages
Unprocessed film, handled in a rough manner, rewound too fast or used on equipment that is not earthed, can all cause the film to become electrostatically charged.
These charges discharge either by the film touching a better conductor [like a human being] or the film touching an earthed item. The problem is caused by the spark produced that will give an image on the film. They usually look like lightening although they can have other forms such as spots or lines, or at worst like images of trees!
This problem is most common in the feed-on elevator of processing machines, and is usually eliminated by earthing the equipment or by filling the elevator cabinet with already positively charged air that counteracts the charges on the film.
Colour Film Soundtrack
Applications
Black and white print films are exposed to the sound track in the printer on the same or a separate head to the picture, and the process is the same for both picture and sound.
Photocells employed in projectors are sensitive to infrared radiation. Silver has a high density to infrared giving a good signal to noise ratio.
Colour film sound tracks require a special processing stage involving a specialised Sound Track Developer Applicator in order to redevelop silver as the sound track.
Many of the subtractive dyes do not have high infrared densities.