Numerous factors influence the migration behavior of printed label stock. It is therefore never possible to completely rule out the possibility of substances migrating slightly from it. However, it must be ensured that the specified limits of migration values are not exceeded. Even if a label printer adheres to all parameters in accordance with Good Manufacturing Practice (GMP) when producing label material, it can happen that a migration test, for example on the part of a food manufacturer, shows values above the tolerance limits. This can have many reasons: storage duration or storage temperature, among other things, influence the migration behavior. In such a situation, it is important that the label printer can prove that all process steps in label production were GMP-compliant.
Drying with higher output
As of this year, press manufacturer Gallus now offers the Labelfire digital printing system with a newly developed two-stage drying system as a solution for label printing. The drying unit itself and an additional booster are new. Both are located in the middle of the hybrid printing system, which consists of conventional UV flexographic printing units and a digital UV inkjet printing unit with a maximum of eight colors. The drying unit and booster ensure that the ink on the labels is fully crosslinked. In addition, both units can record relevant parameters for the hardening process during production. Together with the Prinect prepress system and a new UV inkjet ink series, the risk of ink components migrating from the label into packaging can be significantly reduced.
Compared to its previous model, the Labelfire's new drying system delivers significantly more performance. It consists of two inert hardening systems, each with an output of 238 W and water-cooled. The special feature of this drying unit is hardening under exclusion of oxygen in the two inert chambers: the oxygen is displaced by nitrogen. This increases the hardening performance and the reactivity of the photoninitiators contained in the inks. Martin Leonhard, Head of Business Development at Gallus Ferd. Rüesch, explains: "Not only is the output of the UV lamps permanently measured during operation via UV sensors, the sensors also record the oxygen content in the two inert chambers. In this way, malfunctions are ruled out and the production documentation can fall back on a complete record of all drying parameters."
The drying chambers are supplied with nitrogen using the adsorption principle with a carbon molecular sieve. This separates the nitrogen molecules in the ambient air drawn in from the oxygen molecules or noble gases. In this way, continuous nitrogen production can be achieved with a desired purity of up to 99.99 %.
In addition to the drying unit, the Labelfire has a booster for hardening the UV inkjet inks. It boosts drying at printing speeds above 35 m/min. "The booster has four lamps, each with 140 W/cm² drying power, which ensure complete hardening of the UV inkjet inks even at maximum production speed," says Leonhard. These lamps also have sensors that monitor and record the function so that this processing step is documented and remains verifiable.
Low-migration printing inks
For all low-migration inks, there is a requirement that only listed substances with migration behavior below the specified limits may be used in production. UV inkjet inks are an additional challenge for the ink formulation, since the requirements for good jet capability on the one hand and the lowest possible migration on the other are contradictory. An inkjet ink is said to have good jet capability if it transfers the individual substances in the droplets precisely and consistently to the substrate at very high printing speeds. To achieve this, the ink components must be so finely ground or so small that they fit reliably through the fine nozzles of the digital print heads. For migration-optimized inks, on the other hand, longer-chain molecules would be more suitable, as their longer structure makes them less mobile. For the fine halftone motifs in label printing, print heads with a native resolution of 1,200 dpi and a droplet size of 2 pl are used, as is the case with the Labelfire 340 press. However, such fine nozzles limit the raw material selection for low-migration UV inkjet inks due to the requirements described.
Heidelberger Druckmaschinen has succeeded in further reducing migration behavior by formulating the third generation of UV inks. Leonhard is convinced of the result: "The choice of raw materials and their purity meet the requirements of Swiss Ordinance, the Reach Regulation, Nestlé Guidance, and GMP specifications," he explains. The ink series itself has a very low viscosity and matches the fineness of the nozzles. "Despite a limited choice of compliant photoninitiators for this viscosity range, Heidelberg was able to translate this into a migration-optimized solution," says Leonhard.
Prepress is also important for low migration values. The Prinect production workflow allows users to intervene in the ink buildup of print jobs in prepress and reduce ink film thickness. In this way, an unnecessarily thick ink film can be avoided and the migration potential reduced. As an example, Leonhard cites the reduction of ink coverage for a defined Pantone color value from 280 % (C = 100 %, M = 100 %, Y = 80 %) to 220 % (C = 80 %, M = 60 %, O = 40 %, Y = 40 %) by using gamut extenders. This results in an overall reduction of about 21 %.
Absorbent substrates always place special demands on label printing. "If paper substrates or cardboard are used, it is necessary to preprint a water-based primer," explains Leonhard. This then prevents UV ink from penetrating the substrate surface. If UV inks get into the substrate, the ratio of monomers, oligomers and photoninitiators on the surface would change, which could lead to uncrosslinked ingredients. If, on the other hand, the UV ink remains on the surface of the substrate, it is crosslinked by the drying process - a basic prerequisite for ensuring that it does not migrate. Like all raw and auxiliary materials, the primer used must also be optimized for migration.
Safety through migration testing
In order to prove that the material supplied complies with the specification, manufacturers can safeguard themselves through migration tests or model calculations. To do this, print shops turn to specialized laboratories that carry out migration tests with specified simulants under defined test conditions.
Fabes Forschungs-GmbH, in cooperation with Gaßner Glastechnik, has developed an apparatus for these migration tests specifically: a printed film is clamped into the apparatus, called Migracell, with the printed image facing downward, and the food simulant is filled into the upper part. After a defined period of mass transfer, the simulant is analyzed in a gas chromatograph. The exact substances to be searched for are specified for printing inks in the ink's Statements of Composition. With the result of the GC analysis, it should be noted that a statement from it only refers to the sample examined under the test conditions defined for it.
GMP production in a practical test
Producing labels in accordance with GMP place high demands on production. The Labelfire press is already in use in the tobacco industry. The manufacturer Gallus has already developed practical solutions for inkjet inks in terms of printing technology and sensors. Building on this experience, the company has further developed and tailored its digital and hybrid printing systems to meet GMP requirements. The addition of a drying unit plus booster to the Labelfire is a solution developed for practical use that support label printers in their GMP-compliant production methods in day-to-day business.