Influence of UV Inks on the Process

Their Consequences in Reproduction

Before evaluating the influence of UV inks on the four color printing process through the UCR/UCA method, I would like to make some remarks on UV inks and on four color printing in general. Over the last few years a lot has been said regarding UV curing technology and the advantages it brings to screen printing. One of its main applications is represented by four color printing. This technique requires a specific expertise and the use of qualified equipment and products.

Although screen printing technology has improved remarkably in the last several years, its application in halftone printing still remains on a small scale; several factors are likely to make its further penetration more difficult, such as lack of knowledge in the process and some limitations created by conventional techniques. Starting from a few years ago, drying by means of UV radiation has become increasingly widespread, offering a series of important advantages mostly enhanced by halftone as well as fine detail printing.

The first advantage, being actually the one most appreciated by screen printers, is a great stability of the UV inks on the stencil that enables the tricky problem of the ink drying into the fabric mesh to be solved. Although stability of solvent-based inks has indeed improved, there are still remarkable limits linked with the presence of the solvents themselves. Ambient conditions, for example often make it necessary for the ink to be thinned or mixed with a retarder in order to prevent color alterations from taking place during the press run.

A further inconvenience might be represented by a progressive loss of dots in the low densities, ending with abrupt color-tone transitions, and a flattening of the artwork to be reproduced. A proper UV ink enables the printer to quite easily obtain a coverage of the dots ranging from 5 up to 95%, and most importantly, no interruptions are necessary during the run in order to counterbalance losses of solvent or to clean dried-in ink from the fabric mesh. In this way nearly all the dots forming the artwork can be reproduced. A further important advantage is represented by the almost immediate drying of the UV inks that enables, among other things, the use of two- to four-color printing machines of limited dimensions.

By using these machines, it is possible to obtain high-grade performance, both on a qualitative and on a production level, as well as the final printing results within a very short time. Some of these features bring screen printing closer to offset printing, without losing peculiarities typical of screen printing like versatility, relief printing and, consequently, outdoor resistance. A plus point for the UV inks, which is still underestimated, is given by the absence of polluting vapors produced by solvents. UV inks, in fact, have a solids content of 100 percent; they do not contain solvents and, therefore, do not send any solvent vapors into the atmosphere.

While pollution was not considered so much a problem in the past (except in some areas like California), its further diffusion and a growing ecological awareness are now bringing many European countries (including Italy where several laws on a regional basis were drawn up) to issue regulations that limit the amount of vapors produced by solvents according to their class. Considering that solvent content in solvent-based inks can reach 70 percent, it is quite easy to figure out the enormous quantities released in the atmosphere. Some other advantages offered by UV inks are a remarkable reduction in the space taken by ovens, a lower energy consumption, a high resistance the prints to solvents and a low flammability level.

So, how do UV inks influence the UCR process and the relevant consequences on reproduction? The so-called UCR system (Under Color Removal) transfers almost all the gray tones forming the positives of cyan, magenta and yellow into that of black. By conventional color separation, the black positive only contains the highest densities; by UCR, on the contrary, the positive black also includes the greater part of the gray tones taken from the three positives of cyan, magenta, and yellow. We are thus speaking of a full scale black. In other words, under color removal consists of eliminating cyan, magenta and yellow from the dark areas. The main goal of this procedure is to avoid an excessive ink deposit and therefore the tendency primary inks have to smudge, mostly when working on fast machines.

Besides preventing too much ink from being deposited, the UCR color removal system is remarkably cost-efficient, as it replaces the color inks with black. By electronically reducing the under color, the separation scanner enables the printer to achieve a better printing contrast that results in an improved rendering of details. So-called “unsharp masking” stresses the contrast between the adjacent colors and sharpens the outlines, where sudden tone changes take place. Improved printing contrast turns out a neater image with much more detail. When applying the UCR system to UV inks, no major differences arise in comparison with the conventional; on the contrary, a lower consumption of the three primary colors was reaffirmed.

One thing we have to bear in mind is that color separation carried out by means of the UCR system needs to be similar to the offset, since UV inks enable dots from 5% up to 95% to be reproduced. On the contrary, by adopting the color separation commonly used for printing solvent-based inks (that foresees a reduction of the dot up to 15 percent in the highlights) the prints obtained using UV inks would turn out very heavy. In other words, the full range of the gray tones can be reproduced without reducing the scale, which would actually flatten the image.

In order to avoid, or at least to lessen the development of moiré, the three halftone rulings, cyan, magenta and black will always have to be at an angle of 30 degrees from one another, whereas yellow will have to be set on the vertical axis of the image at 0 degrees. The dot will preferably be elliptical-shaped, the fabrics used will be gold/orange polyester with one-sided calendering and number of threads ranging from 150 to 185/cm (380-465 per inch), in accordance with the halftone line count.

The calendered side should be put in contact with the squeegee; should it on the contrary be set in contact with the print, the ink deposit would lessen even further (by 25%), thus representing an advantage when printing very fine details.

Great care has to be taken in the choice of the squeegee itself; that needs to be made of high quality polyurethane rubber, with a blade of 75-80 Shore hardness in order to withstand the aggressive ink monomers and to preserve a perfect sharpening. For a correct, neat reproduction of the dots, both in the low and in the high densities (positive and negative dots respectively), the inks employed have to show outstanding thixotropic properties. The principle of removing the gray tones from cyan, magenta and yellow, which are contemporarily replaced by black, makes it necessary for the black used in the UCR system to be a full scale, and therefore, a very deep one.

As previously stated, reproduction is influenced both on a quality and on an economic level. Common color separation, especially when applied to UV inks (those having a solids content of 100%), owing to the deposit of the four colors, produces remarkably thick ink coatings in the areas of the dark shadows. In this way, reproduction is badly affected. A series of measures can therefore be adopted when using UV inks, in order to lower the ink deposit; for example, the use of very thin calendered fabrics and stencils with a reduced thickness. Notwithstanding, these measures are sometimes not sufficient to limit the ink thickness.

Owing to a lower ink film deposited in the dark areas, the UCR system enables neater prints to be obtained, along with more details and an improved balance of the gray tones. A further advantage deriving from the thinner ink deposit can be noticed on some vinyl substrates: in the areas, in fact, where the ink deposit is thicker, some PVC supports may become brittle. This phenomenon, also depending on the ink formulation itself, can be minimized in the areas where the ink coating is thinner.

As far as the economic aspect of the matter is concerned, the advantages are remarkable and easily understandable. In the dark-colored areas, the greatest part of the three overlapped primary colors is replaced by full-tone black.

Total ink consumption is therefore reduced, especially as far as color inks (being actually the most expensive ones) are concerned; only a slight increase in consumption in comparison with the conventional system, is represented by a greater quantity of full-tone black.

If compared with the commonly employed color separation technique, where full-tone black has to be printed in order to obtain the text, the UCR system turns out to be remarkably cost-efficient in terms of lowering production costs. As a matter of fact, by using full-tone black in UCR, the fifth run, along with all its relevant costs, can be eliminated without sacrificing contrast, and thus speeding up delivery times. Let’s sum up the main features of the UCR system applied to UV inks as follows:

• Neater and sharper dark areas
• Improved printing contrast and higher color reproduction quality
• Better balancing of the gray tones
• Easy ink acceptance, even when printing on very fast machines
• Improved quality of the prints
• Higher output
• Lower consumption of the primary color inks

In conclusion, we can plainly affirm that UV technology has reached a high-quality level, that is further enhanced by applying the UCR/UCA separation system to four color printing. Also, the UCR method is very efficient in saving money too. A constantly growing number of printers are now resorting to the UV technique, which offers screen printing outstanding perspectives in the field of printing automation. I am sure that these expectations will not be disappointed by the developments we can foresee in the short run, with thanks also to the possibilities brought about by the UCR system.