In the second of our three-part technical series on what it takes to print packaging, we focus on the challenges of flexible packaging and consider the specific ink attributes and drying considerations that explain the printers we are beginning to see in the market.
PRINTING FILM
As described in a previous article, the inkjet printing of pressure sensitive label has been the first single-pass inkjet application where the printing of non-absorbing plastic films was demonstrated. This market has continued to mature in terms of print resolution and speed and from some vendors, like Bobst/Mouvent we have also seen a progression from UV inks to water-based.
Label printers from suppliers like Mark Andy have enabled converters to capture of some unsupported film applications, link shrink sleeves but the real movement in the flexpack market has come from the adoption of inkjet technology the conventional packaging press manufacturers.
Thin Film Printing
One of the first dedicated inkjet machines for flexible packaging applications was presented at DRUPA 2016, through a joint venture between Fujifilm and Miyakoshi. Using UV inks with LED curing and nitrogen inserting the MJP20W printer was aimed at the production of laminated packaging. The use of UV inks in food packaging applications is still a contentious one, however and a technical challenge which the low viscosity of inkjet formulation does not make easier. One way to address the concern over materials like Photoinitiators is to use electron-beam curing (see our 2020 post) and the Uteco collaboration with INX demonstrated this. The issue is that there is potential for bleed of the colour when printing wet-on-wet with 100% EB formulations which is often controlled in UV ink with LED pinning. Obviously, such effects are more difficult to address with e-beam to the bulky nature of the units, which must shield from secondary x-rays. This has directed a lead ink supplier to suggest tiny amount of allowed PI to enable UV-LED pinning of ostensibly e-beam cured inks.
As a result of the challenges with energy curing approaches, the most recent announcements have involved water-based ink directly to film at high speeds. One of the first to market was again Uteco, now partnering with Kodak’s continuous inkjet technology . With this offering we see one of the first high-speed printers that can print speeds approaching the established gravure and flexo machines.
WATER & PLASTIC
Printing water onto less-absorbing coated paper has been a tricky thing that it has taken makers of production print machines a few iterations of printer and ink to perfect. Plastic packaging is like an extrapolation of that requirement, offering no absorption property at all. Landa suggests that using their Nanography transfer process makes this possible by reducing the amount of water before transfer to the plastic, which was this was demonstrated at Drupa 2016. It is clear from the patent landscape that many other companies are also developing “offset inkjet” machines too. If printing direct to the film, however, there is the challenge of the ink bleed behaviour before drying, just like for the e-beam discussion.
In the images below, adapted from the Author’s paper at the 2019 IS&T Print4Fab conference, the role of the ink bleed before drying is shown to be intimately linked to the surface condition & substrate temperature as well as the ink property.
In addition to corona pre-treatment, it is therefore common to use primers to tune the receptive property of the films surface to the intended ink and then use printer design to optimise the process. This can include temperature control of the substrate.
COMPLEX DRYING
As we discussed in our previous post on, efficient drying of water-based inks can be met with the latest near-infra-red (NIR) lamps from the like of Adphos and Lambda Technology, for which the wavelength of the emission is tuned to an absorption peak in water. It is not straightforward however, since the different CMYK colours have wildly different absorption properties. The black dries very quickly, whereas the yellow reacts relatively slowly. Hitting a CMYK print with enough energy to dry the yellow can even make the black printed parts of the print to melt, or even caught fire, under the generated heat of absorption.
As a result, the most common design for packaging substrate is to print in order of increasing NIR absorption and put a drier between each print module. The yellow than get multiple hits, whilst the black only gets hit once and power level can be tuned to get the perfect balance. In this way the ink gets pinned to avoid some of the wet-on-wet issues we spoke about for e-beam inks. The image below from Adphos gives a schematic of how this might be done using a cooled impression drum for the CMYK.
This is schematic there is likely to be a benefit to having a hot air component to dry the inks, depending on the materials used for formulation, since some of the co-solvent materials are not so easy to evaporate.
As for many industrial processes there is a big benefit for making the process a hybrid between inkjet and analog in order to maximise the benefit of the existing chemistries in the market, such as barrier coatings and adhesives. This way, the need to put more challenging chemistries through inkjet heads is avoided and exiting capital equipment can be utilised.
PRINT HEADS
Since drying of ink on films is a big challenge, there is a need to use formulas which makes the process easier. Lower levels of co-solvents and/or complete removal of slow-drying humectants like glycerol is necessary; but this creates inks that are more difficult to keep print heads jetting. One of these issues are already being managed in many production printers for coated papers and by the combination of ink recirculation and waveform add-ons called “tickle pulses”, making the ink firing more predictable as in the photo below.
Flexible packaging has been in the sight of inkjet R&D teams around the world for many years and it is exciting to see printers emerge into the market with combination of the technologies we have described. In the next installment of “What it Takes…” for packaging we will take a look at paperboard, which blends the two worlds of corrugated (paper) and flexpack (lack of absorption) creating a competitive area for machine makers.