Inking and dampening systems are of major importance to printing quality. It often occurs that they are not reset until a problem arises, a roller has to be changed or if the entry for the Color Quality Club is scheduled to be printed the next day! The consequence of such behavior is that, for the rest of the time, it is impossible to state what condition these press elements are in. In addition, the working tolerances of the process cannot be controlled.

The definition and selection of the rollers as well as their exact identification (size, hardness, etc.) will allow a reduction of the working tolerance of the process. For example, the use of a color code for each type of roller may eliminate the possibility of mistakenly mounting the wrong roller.

A representation of the impressions left on the rollers at the points of contact.
Photo courtesy of Ifra

A system for monitoring the hardness of rollers during their useful life should be introduced. The Shore hardness of a roller will determine the setting to the extent that, with an identical setting (in terms of ink stripe), a harder roller will result in a much greater force being exerted on the ink film.

The choice of cleaning products will be made on the basis of the standard in effect, and when the cleaning products and settings have been determined, it will be possible to express the changes in roller hardness in terms of useful life and draw up a time schedule for their replacement.

The settings and frequency of these settings should be considered in terms of the use of the rollers for printing and the observed drift: It is essential that the roller settings should be well defined, reliable, and carried out systematically. But before talking about defining setting values, it must be defined how to measure them.

There are three general approaches towards doing this. The first two techniques call on the sensitivity of the operator who, by pulling on the plastic foil (or worse, metal foil), judges the resistance that the roller nip exerts on the traction of the foil. While it is perhaps possible to partially move away from the friction coefficient between the foil and the roller, it will never be possible to get away from the said “sensitivity” of the operator other than by equipping him with a scale, something that is not realistic in many cases.

Good old ink stripe method still only realistic one

There have been attempts to develop instruments that, when inserted between the rollers, give the value in millimeters of the size of impression. The idea is not bad, but has never been realized. The force exerted by the rubber on the sensor is neither sufficient nor stable enough to give a reliable value. The only realistic method continues to be the “good old ink stripe” that in addition offers the advantage of not having to enter the printing unit nor disassemble half of the rollers. This method consists of directly establishing the size of the impression left by the contact between two rollers that is, again according to Hertz, an image of the force exerted between the two rollers. In order to establish this impression, a distinction is made between two cases:

• Between two rollers.

• Between the roller and the plate.

Between two rollers, especially the transfer rollers, the most reliable method is to establish the size of the impression left by the contact of the rollers on the ink film. If the rollers are coated with a layer of ink, then the same rollers are rotated several times at a consistent speed, stopping the rotation for several seconds will leave an impression that is identical to the contact between one roller and another.

This allows a precise measurement of inking. This method is all the easier to implement because the rollers are often already covered by a layer of ink. It is possible to start training the operators by using a small amount of yellow ink at each end and in the middle, which facilitates the reading of the impression. The same approach can be applied with the dampening rollers, even though in this case it is better to schedule the cleaning of these rollers before press start-up. The reading of this impression is described in the figure below.

When the operators are sufficiently practiced, they can avoid adding the ink for a fast confirmation, provided that they are given a small torch allowing them to make out the tracks formed by the impression. It is recommended also to equip the operators with a transparent film indicating the lines for the width corresponding to the settings to be achieved in order to facilitate measuring and eliminate the possibility of misinterpretation.

It is difficult to imagine the same procedure with regard to the contact between the form rollers and the plate. The time and amount of ink required to cover the plate with a uniform coating are prohibitive. Therefore, a different method is used, of an equivalent sensitivity, that consists of establishing the impression left by the roller at the time of impression throw-on and throw-off of the rollers. Special attention must be paid to the speed at which these rollers are brought into contact with the plate.

The mechanisms for throwing-on pressure to form rollers are usually comprised of a jack that acts on an arm moving around an axis and that rests on an adjustable device. If pressure is applied too quickly, for a brief moment there will be a backlash and the roller inertia will make it bend in the same direction. The thus-obtained impression will not reflect the actual setting when the roller is under constant pressure, but instead a much larger impression.

Consequently, care must be taken to throw-on pressure gradually, though not too slowly in order to avoid delaying the start-up sequence. However, it is also possible to adjust the sequence of throwing-on roller pressure to suit the new system reaction time. The simplest method to soften the application of pressure to the form rollers is to equip the air supply with flow reducer jacks. Besides being inexpensive, they offer the advantage of being adjustable and not changing the final pressure of the jack.

Once this minor adjustment has been realized, this method can be relied upon to establish the impression of the rollers on the plate, taking due care when applying this method that the cylinders are under pressure in order to ensure that it accurately reflects the production situation.

As far as defining the setting, the values specified by the manufacturer are usually sufficiently reliable as they have proven their success in practice. In any case, it is possible to exclude all ridiculously low values (e.g. 2 mm) that could not be guaranteed on an industrial scale. The measurement precision, combined with the effects of temperature and other possible missettings, will rapidly cancel out a contact that is too weak, or the opposite. If a situation exists where the system only works with such a value, it will be necessary to consider changing the system.

The most common reason for this problem is the quantity of ink required on the first roller of the roller train for a given ink load cannot be transferred through the nip of the first two rollers. The solution is to calculate this quantity by examining the separation of the ink film in the inking system and then re-sizing the rollers limiting the transfer.

Dampening solution and consumables

It is astonishing to discover that the vast majority of newspaper companies spend enormous amounts of money every year on consumables, most of which are ordered without specification. Even without going into the details of producing a schedule of specifications for consumables, it is important to define exact criteria for every consumable. This should be dealt with in a special project involving all the people concerned (printers, buyers, supply managers, etc.). The outcome of this project must be that a switch from one supplier to another should have no influence on the process, or that any possible influence can be reliably foreseen in advance.

Returning to the press and its immediate environment, the quality of the raw water should be monitored more than once daily to begin with, then several times weekly, and finally once weekly after it has been established that there are no serious variations in the water hardness.

The municipal water supply draws its water from sources that nature may change in accordance with the climate, or the operator can decide to change his source of supply without informing the consumers. This can have important consequences for printing quality and the choice of dampening additive often depends on the nature of the raw water that is used (see Ifra Special Report 1.15)

Because the nature of raw water changes, it is possible to install devices for stabilizing the water, such as a softening or reverse osmosis device. As explained in the aforementioned Ifra Special Report, it must be borne in mind that, if this type of equipment is used, the water must be re-hardened before use. Therefore several options are available:

• Mix the raw water to the softened water to achieve the desired hardness: this option is valid only if the hardness of the raw water is sufficiently great and its variations not too extensive. The mix will “divide” the effect of the variations in the raw water by the proportion of the mix added. For example, if the water drawn from the municipal supply system has a very high level of hardness (20°d) and the desired hardness is 10°d, it is possible to mix equal shares of the raw water and the water subjected to reverse osmosis in order to obtain this hardness. In this case, the variations of the raw water will be divided by a factor of two, meaning that if its hardness varies from 16 to 24°d, the hardness of the mix will vary only from 8 to 12°d. In case of greater variations or a raw water that is less hard, it will be necessary to choose a different solution.

• A special additive can be introduced continuously into the softened or reverse osmosed water via a connected dosing apparatus. This solution offers the advantage of totally eliminating the variations of the raw water, but does have one major drawback, i.e. the cost. This type of additive must be dosed to the degree of 0.5 percent and can cost $5.30 per liter, representing a surcharge of ¤30 per m water. On top of this, it is often desirable to put the mixture into temporary storage and in this case it must be constantly stirred in order to ensure consistency of the additive concentration. Therefore careful consideration should be given before deciding to use this type of solution. Moreover, it will be necessary to confront the problem of attributing responsibilities between suppliers in the event that this re-mineralization and the dampening solution entail the intervention of two different partners.

• Last but not least, when on the subject of chemistry, why not take a dampening solution supplier at his word when he claims to be able to supply an additive that is adapted to suit your water? In this case, it is essential that the supplier should use the correct method of selection: First of all, he should take a sample of this water, then, usually in a laboratory, establish the dosage curve of this additive in the water. This consists of discovering the dosage of the additive corresponding to the lowest surface tension and ensuring that this surface tension satisfies the conditions for a good interaction between all the consumables used in the printing process. After this, he will be able to give the user a conductivity curve enabling him to know the concentration of additive by means of a simple measurement.

The method described in this latter approach must be applied, even if a water treatment system is not used. It must be remembered that printing is not done by conductivity! That is simply a means of measuring the concentration of a given additive in a given water.

Despite this, the measurement can be used to determine other water properties, especially raw water. If the nature of the raw water changes, its conductivity will also change. If the water concerned varies very little (e.g. osmosized), a daily tracking of its conductivity will permit the detection of any rapid change or deviation in its properties (e.g. the rupture of a membrane).

Check cooling to keep process stable

The dampening solution distribution system must be given special attention: the dosage delivered by the supply pumps must be measured because, in the event of the use of two different additives requiring a different dosage each, it must be possible to rely on the data provided by this system. Cooling must be checked, not because a certain temperature is necessary for printing, but always because the stability of the process will depend on the stability of the temperature, especially in long print runs where heating of the inking systems can seriously upset the system.

Pressure must be monitored to avoid any malfunction of the dampening system. A thorough cleaning with soda lye, followed by extensive washing, should be carried out at least once monthly to prevent bacteria, fungus, etc., from gaining a foothold in the pipes, and especially the filters. At the time of cleaning, the filters are removed and washed separately. Of course, the filters will be replaced if necessary, and here also replacement at scheduled intervals would be preferable to awaiting the occurrence of malfunctions.