Dampening System

In offset lithographic printing, the system that transfers a water-based fountain solution to the printing plate as a means of making non-image areas ink repellent. Offset lithography is based on the principle that oil and water do not mix readily, thus the water-based fountain solution ensures that the oil-based ink does not collect in undesirable regions of the printing plate. (See Offset Lithography.)

In addition to water, the fountain solution contains a variety of other additives to enhance the performance of the fountain solution, and to prevent press problems and plate damage. The concentrated fountain solution (called fountain concentrate) comprises a variety of different substances depending on the composition of the ink used, but most solutions include, in addition to water, an acid—such as phosphoric acid, citric acid, or lactic acid—or a base, so as to control the pH of the solution; a gum (either a natural gum such as gum arabic or a synthetic gum) to desensitize the non-image regions of the plate; a corrosion inhibitor which prevents oxidation or other chemical reaction between the dampening solution and the metallic plate and also contributes to pH adjustment; an alcohol-based wetting agent which allows the solution to flow more easily by reducing the surface tension of the fountain solution; a fungicide which prevents the growth of fungi, mildew, or bacteria in the fountain; and an antifoaming agent which prevents the generation of foam in the solution, which can have deleterious effects on fountain solution distribution through the press. The fountain solution may also contain a drying stimulator, which acts to enhance the performance of the ink drier, typically used when ink isn't drying fast enough. Most suppliers provide premixed fountain concentrates, the printer only needing to add water and, perhaps, suggested quantities of alcohol and/or gum. Adjusting several properties of the dampening solution, particularly pH and electrical conductivity, may also be necessary. Refrigeration of fountain solution also helps prevent various press problems, such as scumming and tinting. (See Fountain Solution.)

There are two fundamental varieties of offset press dampening system: an intermittent-flow dampening system, and a continuous-flow dampening system.

'Intermittent-Flow System'. Also known as a conventional dampening system, an intermittent-flow system closely resembles the press's inking system. However, since the fountain solution does not need to be worked to the extent that the ink does, much fewer rollers are involved. At the head of the dampening system is a water pan (also known as a fountain pan), the reservoir containing the solution. Turning in the water pan is a chrome- or aluminum-plated fountain pan roller, which picks up a film of solution as it turns. The rotation of the fountain pan roller is controlled by its own motor or, if connected to the main drive of the press, at a speed greatly slower than that of the press (produced via gear reduction), so as to prevent the spraying of fountain solution all over the pressroom. On some presses, the fountain pan roller is covered with an absorbent material, so as to increase the amount of fountain solution transferred to the rest of the system. A fabric-covered ductor roller alternately contacts the fountain pan roller and an oscillator, transferring the dampening solution at set intervals. Adjusting the length of time the ductor roller contacts the fountain pan roller (and, therefore, the amount of fountain solution it picks up) controls the amount of solution ultimately transferred to the plate. The chrome- or aluminum-plated oscillator, as its name indicates, not only transfers fountain solution from the ductor roller to the form rollers, but also oscillates back and forth parallel to the ductor roller, so as to even out fountain solution distribution across the dampening system and ensure the transfer of solution uniformly across the surface of the plate. The rotational speed of the oscillator is commonly the same as that of the plate cylinder. The final set of dampening rollers are the form rollers, of which most presses contain two. These rollers contact the plate before the inking form rollers, and desensitize the non-image areas of the plate before the application of ink.

Web presses utilize other configurations of dampening rollers. One common system uses a flap roller, or flapper, as a fountain roller. The flapper, as its name implies, is covered with canvas flaps which transfer fountain solution to the adjacent oscillator. Adjusting the speed of rotation of the flaps controls the amount of solution sent toward the plate. A prime advantage to this system is its elimination of the ductor roller and the consequent ductor shock. Another system uses a roller covered with stiff bristles which flex when under pressure with the fountain roller. The bristles pick up fountain solution and, as the roller rotates, the bristles "flick" the solution at the adjacent oscillator (with which it is not in contact). A similiar configuration uses adjustable flicker blades to flex and release the bristles. In this case, the bristle-covered brush roller replaces the fountain roller.

As with ink rollers, the proper setting of dampening rollers is important; the pressure of the various rollers against their neighbors can be adjusted, and correct pressure ensures the proper transfer of fountain solution. If the pressure between rollers is not strong enough, not enough solution will be transferred, and the form rollers, which are driven solely by pressure from the neighboring oscillator, will not turn at their proper speed. If the pressure is too strong, however, the dampening solution will be squeezed out the sides of the rollers, again preventing sufficient solution transfer. Also of consideration is the pressure between the form rollers and the plate, which should be less than that between the form roller and the adjacent oscillator. The oscillator should be perfectly parallel to the form roller, the fountain pan roller, and the plate cylinder. Various tests that gauge drag (such as the three-strip method) can ensure that the setting of the rollers is correct and consistent across the length of the rollers. It is also important that the form roller not bump when passing over the plate cylinder gap. As for regulating the amount of dampening solution transferred to the plate, there are three primary means of adjustment. The first involves changing the rate of rotation of the fountain pan roller; the faster it turns, the more solution it picks up and the more surface area it brings into contact with the ductor roller. The second involves controlling the length of time the ductor roller contacts the fountain pan roller, which also regulates the amount of dampening solution transferred to the surface of the ductor. A third means of regulating the fountain solution is with attachments called water stops, which are metal or plastic tabs, squeegees, or rollers that press against the fountain pan roller with varying pressures. These act to squeeze or "scrape" water from the surface of the fountain pan roller before it contacts the ductor, removing a preset, desired amount of solution. An advantage of water stops is the ability to loosen or tighten selected stops across the length of the fountain roller, depending on which portions of the plate require more or less fountain solution. Running dampening rollers bareback, or without fabric covers, also makes the rollers more sensitive to adjustments elsewhere in the dampening system. Rollers that run bareback commonly need wetting agents added to the solution, so as to decrease surface tension and allow the comparatively nonabsorbent roller to pick up more fluid.

This conventional dampening system is called an intermittent-flow system because the movement of the ductor roller prevents the continuous flow of solution from the fountain to the plate. This set-up causes problems when fountain solution surges through the system, and these surges are difficult to control. (A similar problem can exist in inking systems which also utilize an alternating ductor roller, but there the increased number of rollers mitigates against surge problems.)

'Continuous-Flow System'. Of primary advantage with continuous-flow systems is their quick response to dampening system adjustments; there is no need to wait for the ductor roller to cycle to the fountain, then back to the oscillator before seeing changes take place. There are two varieties of continuous-flow system, which can exist separately or in some combination.

'Inker-Feed'. An inker-feed dampening system does away with the ductor roller and uses the first inking form roller as a dampening form roller. Fountain solution is transferred from the water pan to a resilient, rubber-covered metering roller, which contacts a hard, chrome-plated transfer roller, the nip between these two rollers—called the metering nip—regulating the thickness of the film that is transferred. The transfer roller then contacts the first inking form roller, the nip between these latter two rollers—called the slip nip—also metering the solution film ultimately transferred to the plate. The speed of the metering and transfer rollers as well as the size of the metering nip controls the amount of dampening solution transferred to the plate. An obvious consideration in this arrangement is ensuring that a uniform film of dampening solution will sit on top of the already-inked form roller. Using a fountain solution that is 5:10% alcohol helps. The form roller used as both an inking and dampening roller must be set harder to the plate than would a traditional dampening form roller.

'Plate-Feed'. A plate-feed system is set up almost identically to an inker-feed system—a hard transfer roller contacts a soft metering roller at a metering nip, which regulates the thickness of the fountain solution film—but a plate-feed system has its own dampening form roller which contacts the plate. Unlike other systems, the metering and transfer rollers can be skewed at various angles to adjust the thickness of the solution film across the rollers.

Combinations of inker-feed and plate-feed dampening systems exist in a variety of configurations, all of which typically feature an oscillating or vibrating bridge roller contacting both the dampening form roller and the first inking form roller. In one system, the difference in rotational speeds between the form roller, the bridge roller, and the plate cylinder results in a kind of hickey-removal system. Other configurations of combination continuous-flow systems use either skewed rollers or water stops in combination with the bridge roller to ensure an accurate and effective ink-water balance.

Some configurations of continuous-flow systems have a metering roller and a transfer roller that rotate in the same direction, producing a reverse slip nip where they meet. This means that the rollers are travelling in opposite directions at the point of contact, keeping liquid from flowing through the nip, the point being to reduce or eliminate the excess dampening solution returning from the plate from interacting with the "fresh" metered solution from the fountain. This helps to more closely control the flow rate of the fountain solution to the plate, as the incoming solution doesn't interfere with the outgoing solution. An interesting variation on the continuous-flow system used on some web presses is a spray-bar dampening system, which sprays jets of fountain solution onto the rollers of the inking system using a row of nozzles. An advantage is that each nozzle in the row can be adjusted, allowing for flexibility in the lateral distribution of fountain solution. And since this system does not recirculate the fountain solution as other systems do, there is no need to worry about contamination or subtle changes in chemistry and/or pH. However, substances in the fountain solution itself (especially gum) can clog and/or contaminate the nozzles.

Dampening roller maintenance has some of the same considerations as ink roller maintenance (for example, rollers should never be used if they become out-of-round, or rotate with a detectable bump). However, one of the primary dampening roller considerations is the roller cover, which over time can pick up ink, and become greasy or worn, all of which reduce their ability to properly pick up dampening solution. A covered roller that is removed must be replaced so that it rotates in the same direction as it did before, otherwise damage to the cover will result, causing bagginess, linting, and creeping. Metal uncovered rollers should be periodically treated with gum etch to properly desensitize them and restore their water receptivity, as they can become greasy over time. A broken or beaded-up film of dampening solution on a metal roller's surface is an indication that the roller needs to be degreased. The growth of fungi or other organisms can be prevented by using fungicidal additives, or by frequent cleaning of the dampening system. A 1:9 bleach-water solution can be flushed through the system to kill anything that may be growing, but the bleach should be thoroughly flushed out before replacing the fountain solution.

Common printing problems, such as catch-up (the appearance of ink in non-image areas of the plate), and undesirable color strengthening and halftone thickening, are caused by uneven or inadequate dampening of the plate, commonly the result of improper setting of the form rollers to both the plate and oscillator, or the ductor roller to the oscillator. Dampening system effectiveness can also be affected by changes in atmospheric conditions, in particular relative humidity, and by changes in the desensitizing film on the plate caused by prolonged print runs. Problems can also be caused by too much dampening solution, which excessively emulsifies the ink, causing poor ink transfer. (Increasing the flow of ink only aggravates the problem.) Too much fountain solution on the plate eventually transfers it back into the inking system, where it can desensitize the ink roller train, causing problems such as roller stripping, or inability of the ink rollers to accept ink. Another type of appearance of ink in non-image plate regions—tinting—can be caused by an excessive amount of dampening-solution emulsification by the ink. Too much fountain solution also impedes the drying characteristics of the ink, causing smearing and ink setoff. Increased absorption of the fountain solution by the paper itself can cause varieties of drying and appearance problems, in particular expansion of the paper in the cross-grain direction as it absorbs water, which can cause misregister on multi-color jobs, or other jobs involving more than one pass through the press. Another problem is bleeding, or a fundamental incompatibility between the ink and the fountain solution, which causes ink pigment particles to be transferred to and dissolved in the dampening solution, which then results in an undesirable colored tone to be printed.

A clean, well-kept dampening system with properly adjusted rollers should help minimize any problems likely to be caused by the fountain solution.

All text and images are licensed under a Creative Commons License

PrintWiki – the Free Encyclopedia of Print
About    Hosted by WhatTheyThink