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For white preprint on high speed label presses Sicpa has developed Sicura Screen 78-3 UV ink

A growth in the “no-label look” has led to a sharp increase in demand for substrates being preprinted in white. UV cured screen printing inks are well suited for this application, but at high printing speeds there can be problems. Sicpa-Aarberg has developed a screen white formulated to overcome this. And it is claimed to have already proved successful in practice.

To achieve maximum possible opacity of the preprinted white ink, very thick ink film thickness is used, which requires high power UV systems for thorough curing. Rolf Montag, director of the Label Printing Ink business unit at Sicpa-Aarberg, cites another problem encountered time and again in practice with white pre-print: the applied ink does not form a uniform ink film. This phenomenon is observed particularly frequently where there is only a short passage distance between the screen printing unit and UV curing lamps.

Problem postponed

The problem is normally dealt with by users adding “leveling agents”. Rolf Montag believes that in many cases this does not prove to be a genuine solution. The ink does level significantly better, but at the same time there is a change in the characteristics of the ink film, frequently resulting in poor ink transfer on overprinting of the background white. So the use of chemical additives only postpones the problem. Indeed it can impede the solution of other printing problems because the actual cause cannot be clearly localized. Users are usually not even aware that in combination printing the use of additives will worsen a number of printing problems, and will even be the cause of others.

  Users of combination printing presses are constantly endeavoring to improve productivity. In this context, if UV screen printing is to keep pace with the speed of the downstream printing processes such as UV flexo or UV offset, then the measuring staff is enormously high. In the case of Swiss press manufacturer Gallus, offering the Rotascreen screen printing system, exact matching of printing processes and ink systems can be seen as the key to the success of combined printing processes.

Tailor made

With a strong stake in the label industry, ink producer Sicpa-Aarberg has developed a screen printing white, whose principal characteristics – such as chemical composition, curing mechanism and viscosity – are designed precisely for use in combination printing at web speeds of 70m/min and greater.

When it comes to viscosity, users’ needs differ greatly. One may prefer ink with a rather pasty consistency; another will want a product with a minimum of viscosity. Higher viscosity inks are seen more in use on a press with a production capacity of 30-50m/min. Converters in warmer climates, who frequently do not have air conditioning, prefer a higher viscosity ink to counter external factors, for example to have an ink that is easier to handle. This can also help to minimize the tendancy of the ink to drip through the screen.

However, according to Rolf Montag, as printing speeds increase the market increasingly demands a screen printing white with a correspondingly lower viscosity. But, at the same time, sacrifices in opaqueness are unacceptable. The big challenge in the formulation of a new product was the task of balancing the printing and preparation process characteristics and the ink consumption. These are the factors determining cost effectiveness.

Special initiator

Sicpa-Aarberg formulated the new Secura Screen 78-3 screen printing white to meet all these prerequisites. It is intended exclusively for preprinting, so has not been integrated into an ink series. The new product is highly pigmented to achieve the specified covering power for a very good white preprint with high opacity. The titanium dioxide used for this purpose places high requirements on the UV curing generally, especially with strong inking. So the photoinitiator was specially selected.

Formulated for high printing speeds, the ink has been adjusted with a combination of different additives so that after application to the substrate it will level as quickly as possible. Curing of the resultant uniform ink film with UV light in this phase produces a homogeneous surface as a base for subsequent production. To ensure that the white can be readily over-printed using conventional printing methods such as UV flexo and UV offset, other additives provide for good and uniform transfer of the ink. The same applies for additional processes such as hotfoil stamping. Sicura Screen 78-3 is unsuitable for lower production speeds because at such speeds UV curing takes place too late.

In addition to suitable ink, a high level of productivity in white preprinting also requires UV lamps of an adequate power output and good printing technology. Known manufacturers of screen printing systems are Gallus (Rotascreen) and Stork Screens (RSI). Their print units can be integrated into almost all commonly used machinery. In the selection of the screen fabric for white preprint, care must be taken that the aperture size does not pose an excessive resistance for the flow behaviour of the ink. Gallus recommends its Screeny KS screen printing plates – well suited for solids with fine elements (for example positive and negative text). For coarser prints Gallus advises the use of the Screeny type HS. Stork will advise which of its Rotamesh screen materials are best suited to each application.

Sicura Screen 78-3 screen printing white is delivered as a ready-to-use product. The ink is formulated to account for the provisions of the respective national legislation, thereby enabling it to be used worldwide. Since the user does not need to add the usual leveling agents, undesirable changes in the ink, which would result in wetting problems or difficulties in ink transfer cannot occur.

However, as Rolf Montag says, in practice similar printing problems are also occasionally encountered by neglect in cleaning operations. The source of the problem lies in the cleaning agents. If these are re-used for cleaning various parts several times, they can be enriched, for example, with silicon. If the agents are then used to clean screens for white preprint, problems with ink adhesion in ink overprinting will be encountered – even if suitable ink systems are used. To exclude such a risk from occurring, absolutely fresh cleaning materials should be used.