Some will ask what is the point of taking part in this competition when they are equipped with an old rotary press that streaks, whose inking is unstable, ink-water balance uncontrollable, and who knows what else?

Obviously, simply compiling a report detailing all the defects in their publication will not suffice, as they are already aware of most of their problems and suffer from them on a daily basis. These same people additionally fear any break in the production run due to having to adjust a roller or change a plate because of register problems.

So the following article is intended mainly for these people as well as those for whom the Color Quality Club represents a timely test carried out especially because they are not genuinely confident in the reliability of their process.

First aspect: blankets and pressure

What is the point in seeking to improve printing quality by selecting better consumables, taking the time to adjust the rollers or web tension, if the very heart of the press, i.e. the cylinders and their relationship to the paper (therefore the blanket) are not satisfactory for the printing process? Accordingly, we shall focus attention on these elements first.

A diagram showing the critical dimensions 
of a dressed cylinder.

Special care must be given to choosing the blankets and their covering. The blanket properties that are of particular interest are as follows: initial compressibility, surface hardness, and finally roughness. Other characteristics complete the description of a blanket, but we will not list them here, either because they are standardized (effect of solvents, tensile strength, stretch, thickness) or they are too specific (sticking, turning time, paper feed). Many of these characteristics are detailed in a standard that we strongly recommend should be read (DIN 16621).

The pressure exerted by the blankets must be 80 to 100 N/ cm2 (according to FOGRA), and their covering determined by their compressibility. By way of example, a blanket with a thickness of 1.93 mm and a compressibility of 6 percent at 100 N/ cm2 must be mounted with a lap (after tensioning) of 1.93-by-0.06 = 0.116 mm, or 12/100 ° millimeters. Therefore, regarding the compressibility of the blanket, even if no typical value can be taken to compare one blanket to another blanket, it is extremely important that the blanket supplier should state precisely the compressibility of his product at 100 N/ cm2 in order to determine its covering.

Rocklight instrument permits measurement 
of the blanket elevation.
Photos courtesy of Ifra

A soft surface should be preferred, i.e. less than 64° IRHD surface hardness. In newspaper printing, in order to ensure that the blanket transfers ink down to the deepest parts of the paper, it is very important that the surface of the blanket should be able to perfectly hug the surface of the paper. This is something that a hard surface cannot guarantee.

Therefore, a soft surface should be preferred to a more robust, but hard surface, because in any case, the useful life of a newspaper printing blanket is really determined more by its history (winding, cleaning processes and so on) than by the loss of transfer properties of its surface layer. It is important to note that the hardness to be measured is the surface hardness (or the micro-hardness) and not the Shore hardness, which gives a distorted image because it is largely influenced by the blanket compressibility.

The blanket roughness is a compromise between the quality of the print image transfer and the detachment of the paper fibers. A smooth blanket will ensure a more faithful transfer, but the suction force that it exerts on the fibers makes it incompatible with printing newsprint. In view of the fact that, in the final instance, stability is much more important than performance by itself, a surface roughness in excess of 1.0 µm Ra is recommended for newspaper printing blankets.

The useful life of blankets must be determined by their sinking (not less than 80 N/cm2 at the end of their life). Also, the cleaning products used should be chosen in accordance with the DIN 16 621 and 53 521 standards. For the purpose of measuring the lap of a blanket on the press, a gauge called Cobra is usually used that, equipped with two needles, compares the cylinder position that establishes the electrical contact between the two needles, with the position of the sensor that remains on the blanket surface.

Whereas this instrument is best suited to presses without bearers, the instrument called Rocklight is preferred by far on presses equipped with bearers in order to obtain a more reliable, and above all much more exact measurement.

If non-colored streaks appear on a bearer 
(typically just after the grooves), this is an 
indication that pre-tensioning is insufficient. 
If this happens, readjust the contact between 
the bearers concerned and repeat the test.
Photo courtesy of Ifra

The measurement of the blanket sinking can be taken to determine the blanket’s useful life, but usually this useful life can be expressed in terms of the number of cylinder revolutions, typically between 13 and 15 million revolutions. After a blanket has been selected, attention must now be focused on two aspects: its mounting in the press and its operating conditions, i.e. cylinder pressure.

When mounting the blanket, simply taking into account the dimensions permits the calculation of the thickness of underpacking required. Better than a lengthy description, the table below shows an example.

Several methods may be considered for softening the blanket: the use of a pressure measuring key or measuring the blanket stretch. The recommended standard web tension is 200 N/cm. Both methods allow this result to be correctly obtained provided that:

• the tension screws are perfectly lubricated and do not offer any hard point if the pressure measuring key is used, and

• the blanket manufacturer supplies the stretch value under the given tension in the case of stretch measurement.

As soon as it is soft, this blanket is ready to be used on the condition that pressure adjustment has been carried out correctly.

The first symptoms of a poor cylinder pressure setting are the appearance of streaks, visible especially when two screens are overprinted. When such problems occur, it is often somewhat late to start worrying about the “mechanical health” of the press. In the case of a press equipped with cylinder bearers, the exact setting of their lockup helps avoid these problems and prolong the useful life of the equipment while guaranteeing the impression quality. For presses without cylinder bearers, in addition to adjusting pressure, special care should be given to checking the state of the bearers and the cylinder roundness.

Given the importance of controlling pressure in printing and leading webs, it is necessary to confirm the value and control frequency of the cylinder bearers by carrying out a “blue” test (only for presses equipped with cylinder bearers). The procedure to follow for this test is as follows:

The objective of the test is to confirm that the cylinder bearers remain in contact over the entire circumference at all speeds during the printing run. This will decide whether the pre-tension on the bearers is under all circumstances greater than the force exerted by the blanket on the cylinders that tends to separate them. That will guarantee that no jolt will damage the bearers while the press is in use, that the pressing of the blanket will be always as calculated in order to obtain the correct impression pressure and that the standard center distance of the cylinders (which is decisive for the quantity of paper unrolled at each cylinder) is maintained. The aim is to confirm the contact of the bearers when the transfer of a color product from one bearer to another occurs. The procedure is as follows:

The press is stopped, bearer lubrication deactivated, doctor blades thrown off, the bearers are clean, dry and degreased. The blanket cylinders are fitted with blankets to their maximum measurement. The thickest of the papers used is introduced. Pressure is removed from the cylinders.

• Coat the two bearers of the cylinder that do not grind when pressure is applied with a uniform fine layer of micrometrical blue (which is used to correct the calibration of mechanical components), ideally with a rubber roller.

• Start up the press at maximum speed.

• Apply pressure to the cylinders for several seconds, then remove the pressure.

• Stop the press.

• Confirm the distribution of the blue on the bearers of the other cylinders.

If non-colored streaks appear on a bearer (typically just after the grooves) or shifted by 180° or in accordance with the geometrical configuration of the cylinders), this indicates that the pre-tension is insufficient. Readjust the contact between the bearers concerned and repeat the test.

When the transfer of the blue is done in a uniform manner, it is essential to check that this value does not exceed the yield strength of the bearers, as otherwise these will be worn out rapidly with dramatic consequences.

For this purpose, the Hertz impression method is the most commonly used and is efficient, as well as having the advantage of being able to measure a value that will be simple to use for adjusting the other printing couples, and the future confirmation of these adjustments. The aim is to have a simple means of confirming that the pre-tensioning of the cylinder bearers is in accordance with a predefined value.

The principle

Two metal cylinders in mutual contact lose their shape in the form of a rectangle. In the range of the yield strength of the bearer material, there exists a bijective relation between the thus-formed “flat” and the pressure exerted on the cylinders (pre-tensioning).

If a carbonless copy paper is inserted between the bearers before pressure throw-on, the impression left on the paper in an image of the size of the “flat” produced by the loss in shape of the bearers. The measurement of this size with a graduated magnifier (precision: minimum 0.05 millimeters) allows measurement of the pre-tensioning.

The target value must be lower than the yield strength of the material. When the “blue” test has been carried out, it is possible to determine this target value by measuring the impression of the bearers that keep their contact (sufficient value) and those that lose their contact (insufficient value). Special care should be taken to ensure that this value is less than the yield strength.

The procedure to follow

Press stopped, lubricating of bearers deactivated, cleaning halted, bearers clean, dry and degreased. Blanket cylinders stripped and web disengaged. Remove cylinder pressure, attach a small strip of carbonless copy paper (calibrated) to each bearer. Secure the static cylinder when pressure is applied. Throw on pressure to the cylinders, then throw off pressure. Re-cover the values of the impressions left on the carbonless copy paper strips.

Should this value exceed (or come dangerously close to) the yield strength of the material, it is necessary to relax the pressure between the bearers and find a means to reduce the force exercised by the blankets at high speed. This can best be done by entering into a partnership agreement with the blanket manufacturers.