Parameters in rotary screen printing – the ink

To achieve a good print result, particular attention needs to be paid to the most important parameters in rotary screen printing. The key influencing factors are the type of screen, the ink, the squeegee and the flow path. This article looks at the ink and its influence on screen printing. Ink in particular is a parameter that printers are able to influence to a certain extent using additives.

Components of UV screen printing ink

UV screen printing ink comprises approx. 17% pigments, 28% monomers, 45% oligomers, 8% photoinitiators and 2% additives. Pigments are mainly responsible for the ink's colouring, monomers for its rheology (= flow properties, speed and viscosity) and oligomers for the pigments' binding agent, while photoinitiators activate polymerisation and additives make the ink stable. The formulation and composition of these individual ink components differ from supplier to supplier and reflect the relevant manufacturer's know-how.

Ink has a complex structure

Ink is an extremely complex topic, because individual ink components normally influence not just one but several ink properties. For example, the pigments that give an ink its colour are characterised not only by their colouring but also by their transparency and resistance (light-fastness, resistance to chemicals, heat, etc.). In UV-curing inks, they therefore influence the ability of UV rays to penetrate the film of ink and thus the curing process itself. Pigments also play a role in inks' shelf life and rheology.

Printers have a limited influence

Printers can influence the ink with a variety of additives. Thinners, flow, anti-foaming and anti-static agents and hardeners are the additives used most frequently. They are added to the ink to change its properties. Above all, printers are able to modify the rheological properties and – to a certain extent – ink drying/curing. They are only able to influence the other ink properties to a limited extent.

Correct ink handling is often the vital factor

Practical experience shows that ink problems in the pressroom often have nothing to do with the ink itself but rather with correct handling. For example, if there are even tiny amounts of residue from UV ink containing silicon in the ink tank, this inevitably leads to signs of ink repulsion as soon as silicon-free UV ink is put into the same tank. In such cases, the only solution is often to empty the entire tank and clean it thoroughly. The same problem can occur in the print image if the first ink contains silicon and the second does not. In this case, it is necessary to assimilate the two inks. It is therefore well worth having one or more specialists at the company who have received intensive training on how to handle inks and can pass on this knowledge internally.

Screen printing: Pinholes

Pinholes in solid areas occur if the ink is not sufficiently fluid. Once the ink has been pressed through the screen mesh openings, the ink droplets cannot flow quickly enough before drying. As a result, no ink is applied to the parts of the print image on which the mesh fabric rests. If this printing error occurs, the ink needs to be liquefied using a flow agent or thinner.

Combination printing: Incomplete ink coverage and edge overprinting

In the event of incomplete ink coverage in combination printing, the second ink does not fully overprint the first one. One possible cause of this is that the first ink layer is too thick. In this case, it is advisable to reduce the amount of the first ink applied if possible or to add a thinner to the second ink.

Thanks to its many years of experience in screen and combination printing, Gallus is the ideal contact for questions about screen printing ink. The Gallus Rotascreen service and support team is happy to provide its customers with appropriate advice – simply get in touch with our technical experts