When broken down into simple common denominators, all diazo duplication requires two ingredients -- exposure and development. As a starting point, let us begin with the basics.
Since most diazo films are sensitive to violet light (410 nm) all duplicators use lamps which radiate in the violet region. Today the most popular exposure source is an additive type, mercury vapor (metal halide) lamp. Fluorescent lamps, Xenon flash and standard mercury vapor lamps have all been used over the years. Additive type vapor lamps have become the most popular exposure source because of their emission characteristics, high wattage potential and availability. While many lamps produce some violet energy, the additive type mercury lamp has been tuned, through the inclusion (called "doping") of a chemical inside the lamp, to concentrate more energy in the violet range. When using additive type mercury vapor lamps the following operational procedures should be kept in mind:
1. Because of vagaries in the manufacturing process lamps will vary in their spectral emission characteristic. This means that two lamps of the same wattage may not radiate the same kind of energy spectrum. The lamp that puts out more of its light at 410 nm (violet light) will allow your duplicator to operate at higher speeds.
2. As additive lamps age, they will lose some of their exposure energy. The exact fall-off will vary from lamp to lamp, but a good guideline would be 5% loss per hundred hours of operation.
3. Do not leave fingerprints on the lamps’ surface. The high operating temperatures will cause the fingerprint to discolor and turn brown. This brown stain will block some of the lamps’ radiation.
4. All short wave radiation (< 350 nm) is potentially harmful for humans, especially the eyes. Mercury vapor lamps do radiate strongly in the ultra violet. All modern duplicators come with interlocks and protective shields to protect the operator. These safety devices should not be interfered with.
Ammonia and heat develop all standard diazo products.
Ammonia is used to neutralize the acid stabilizer contained within the light sensitive coating. Once the stabilizer has been neutralized, the dye couplers and diazo salts will combine to form a dye. If only ammonia vapors were used for development, the development (coupling) process would be inadequate with a green image being the final product. All diazo developing units rely on heat to accelerate the coupling process. Most processors operate in the 150° to 200° F range.
The type of ammonia employed for development will also affect development rate. The water contained in aqueous ammonia creates a high-humidity environment and will act as a plasticizer that helps the ammonia penetrate the film coating and improve the development rate. If anhydrous ammonia is used as the developing agent, high gas pressure can be used to force the ammonia into the image layer. A number of developer units utilize this technique using between 20 psi and 80 psi in the developer chamber.
In addition to aqueous developers and anhydrous developers, there is a third category that is a combination of the two. Many 16/35 mm duplicators use anhydrous ammonia as the developing agent, though not in a pressurized chamber. The gas usually flows into a large developer chamber where it immediately drops to ambient pressure. Since high pressure is not present within the chamber to accelerate development, water is fed into an evaporator and produces a high humidity environment. The ammonia gas and moisture combine to produce an "aqueous ammonia" environment with the water vapor acting as a plasticizer to speed up development.
MAKING THE COPY
All diazo duplicators require the master and diazo film to be brought together, "emulsion to emulsion," and exposed to the printing lamp. Both density and resolution are affected by the exposure step.
The resolution of the copy film is a function of many factors that would include:
1. Copy film acutance
2. Master film resolution
4. Exposure collimation
5. Nitrogen outgassing
COPY FILM ACUTANCE is a function of the duplicating media. Since diazo film is a molecular coating versus a granular structure, its resolution capabilities are very high -- in excess of 1000 lines/mm.
MASTER FILM RESOLUTION cannot be improved upon during duplication. Depending on the duplicator, some loss in resolution is inevitable. Every effort should be made to create a master film with high resolution.
CONTACT between the master film and the copy film plays an important role in duplication. Poor contact will cause the exposure energy to diffuse between the two emulsions causing a loss in contrast and image sharpness. Proper contact is a function of the duplicator. Fiche duplicators rely on a pressure pad or platen to assure contact. Both sufficient pressure and even pressure are necessary. A test master with resolution targets in all four corners and the center can be used as an evaluation tool. A regular check of the duplicator's exposure station (using a test target) is a good practice.
16/35mm duplicators use a combination of tension and/or moving belts to maintain contact. Because 16/35 duplicators print while both the master and copy film are moving, they require care in set-up and maintenance. The machines should be kept clean, wind tensions should be checked regularly, and all idler rollers in the film path should be free turning. A reel of master film containing targets should be run regularly to check imaging performance.
EXPOSURE COLLIMATION, assuming good contact, is the second most important property concerning resolution. Collimation refers to the design of the exposure system in terms of how the light is directed to the exposure plane.
Ideally, we want all the exposure energy to hit the film plane perpendicularly. This is the characteristic of a highly collimated system. Energy that strikes the film plane at a less than 90° angle tends to undercut the image and soften edges. Since collimation is a function of the machine design, it is not something you can do much about once you own the duplicator. Most duplicators provide adequate collimation. If you plan to auto-generate (make a 3rd generation copy from a 2nd generation copy) or have high reduction ratios on source documents (higher than 40x), you may wish to evaluate your duplicator for imaging performance before purchase. We would recommend you use a high-resolution test target on film for duplicator evaluation.
NITROGEN OUTGASSING is a normal by-product of the exposure process. The amount of gas generated is a function of the particular diazo product, the image appearance of the original, the amount of exposure and the size of the format. In most instances, the gas bleeds off to the atmosphere with no harm being done. Under certain circumstances the gas can cause separations of the two film planes, causing de-focus or worse still – slippage.
Imaging problems due to out-gassing are seldom encountered on stationary flat bed printers. Dynamic printers where master and copy film are moved past the exposure source are vulnerable. Roll-to-roll printers, especially those using a printing drum as part of the transport system, have intrinsic tension problems. As the copy film and the master film pass around the printing drum, they must be moving in synchronization, yet because of the curved surface the outer film has a greater distance to traverse. In other words, if the two films were wrapped around a 10' cylinder, the inner film would have a 31.410" path (~D). The outer film would have a 10.005" cylinder to circumnavigate (10" drum plus a .005" film) resulting in a circumference of 31.425". Since the travel difference between the two films is small, a high contact pressure will prevent the tendency to re-adjust (slip). High contact, film-to-film pressure is maintained either through film tension or in some cases film tension and pressure belts that hold the films in close contact.
Imagine what will happen to our copy and master film if at the moment of duplication gas boils off and forms a "gas bearing" between the two films. (Remember that gas release is a normal part of the diazo salts decomposition.) Under various circumstances the gas will either:
1. Diffuse out through the edge of the coating,
2. Cause the master film to separate slightly from the copy films (de-focus),
3. Allow the film to correct for the natural speed imbalance as they go around a drum and
Slippage and loss of image resolution is a very complicated problem for dynamic printers. There are no simple answers. The following guidelines may be of some assistance if you encounter imaging problems on your duplicator:
Your duplicator should be set to factory specifications for tensions on your unwind, rewind, brakes and clutches. All idlers in both the diazo and master film paths should be free turning. The drive system should be running smoothly with no pulsing.
Make sure your lamp is properly cooled. Excessive heat at the exposure station will cause the gas to diffuse out more quickly.
No lubrication should be used in the film path or on the film itself. Silicone in particular can cause slippage problems. Make sure all drive and brake rollers are cleaned frequently with isopropyl alcohol.
If all else fails, there are anti-slip diazo films available which can improve the situation. These products delay the release of nitrogen long enough for the films to finish the print cycle without the gas acting as a "gas bearing." Anti-slip films will not solve all problems: a duplicator with mechanical difficulties may continue to slip even with special diazo products. Positive appearing imagery can cause problems because more salts are decomposed on the diazo print producing more gas. Also wide films -- 35mm/105mm -- are more prone to problems because of their larger gas producing surface area and the old 105mm roll to roll machines can be the worst.
Since diazo films are positive working media (the image sign of the copy is the same as the original) exposure reduces density. Furthermore the maximum density is a function of the emulsion thickness and development. For these reasons evaluating the minimum density (clear areas) on your copy assesses proper exposure. Experience indicates that proper exposure on negative appearing imagery should produce a minimum density of .05 - .07. Positive appearing imagery can benefit from slightly higher minimum densities, .07 - .09. For negative appearing imagery, over-exposure will cause a fattening of the characters and, in the extreme, a loss in maximum density. Under-exposure will reduce character density. For positive appearing imagery, overexposure will cause the characters to fill in while under-exposure will cause the background density to increase.
The best technique for assuring proper exposure is to run step tests with a representative master film. The results should be evaluated in your reader.
The development of diazo films requires only sufficient heat (150° - 200°F), sufficient ammonia (either under pressure, or in the presence of water vapors) and sufficient time to produce full development.
For blue and blue/black products, a wide range of temperatures will produce satisfactory results -- 150° 200°F. Higher temperatures will tend to speed up the reaction. On ammonia starved duplicators, higher processing temperatures can compensate, to some degree, for lack of ammonia. Very high temperatures, in excess of 200° F, should be avoided because the duplicator seal life will be reduced and because title stripes may begin to pick-off.
Black diazo films are temperature sensitive with most products working best in the 150° - 175°F range. Black products contain two dye couplers, a blue and a sepia. The blue dye prefers higher temperatures while the sepia dye favors lower temperatures. Functionally, this means that the image color can be shifted between brown and blue by lowering or raising development temperature. Each film will have its best temperature where the color is most neutral. To find the ideal on your duplicator, run step tests between 150° and 180° F and select the product that looks best. We prefer a light box having a color temperature of 5000°K (Kelvin) for this evaluation, though you may prefer a reader. Remember that the viewing source will have a strong influence on the appearance of black diazo films.
Finally, be wary of the dial settings on the temperature control system. The dials are frequently out of calibration. It is wise to have your service representative recalibrate.
You cannot overdevelop diazo film, thus you cannot have too much ammonia. More likely the opposite is true, not enough ammonia. This is especially true for some non-vented duplicators where the ammonia has been cut back to reduce odors. The best way to determine if your ammonia level is high enough is to run a development test. Run a sample of film through your developer under standard conditions and check the density. Next, run the same sample through the developer a second time and re-check the density. If the first and second reading are within 5% of each other you have sufficient development. If you are running the ammonia level much higher than recommended or there is a strong odor about the, machine, you may have a hardware problem.
One final word on ammonia concentration. With anhydrous ammonia, concentration can never be an issue because it is pure ammonia. Aqueous ammonia is an ammonia and water mixture that comes in various concentrations. Most duplicators use 20-24° Baume ammonia. Concentrations below 20° may not provide full development while concentrations above 24° may cause vapor lock in the ammonia supply system. If you are experiencing unexplainable problems with your aqueous developer unit, have your ammonia concentrations checked. Often the contents of an ammonia supply bottle are far below their stated concentrations.
The length of time the film remains in the developer chamber is referred to as dwell. Some duplicators have a fixed dwell period while other machines have a variable dwell depending on printing speed. For a given film, ammonia concentration and heat setting, there is a certain minimum dwell time required. Again, since you cannot overdevelop diazo film, longer than necessary dwell times cannot cause problems. Most fixed dwell duplicators have selected a long enough dwell so as to ensure full development. On roll-to-roll 16/35mm duplicators the dwell time will be reduced as the printing speeds are increased.
Duplicators with large developers will provide enough dwell for a wide range of operational speeds. Duplicators with small developer chambers may run out of sufficient dwell at higher operating speeds. This problem will be exacerbated when using high-speed diazo films because the dwell will be further reduced by the faster than normal print speeds. If you encounter a situation where you are not providing adequate dwell because of high operating speeds, you must slow the duplicator down. To do this without over-exposing the film requires that you either use a slower photo speed diazo film or replace your printing lamp with an aged lamp that is putting out less energy or reduce the size of the exposure aperture.
In closing, since there is a wide variety of duplicator and film combinations available we would recommend you setup your duplicator to specification and then run development tests. Once you are assured of proper development you should then conduct exposure step tests for proper exposure. We would recommend a re-check of both proper development and exposure on a regular basis.