In the third of our three-part technical series on what it takes to print packaging with inkjet, we build on previous discussion that included primers and substrate with little or no penetration. Now let’s turn our focus on the challenges of the folding carton market.
Just Coated Paper Right?
To some extent this is true. Many of the innovations which supported the move to coated papers in the commercial print market have underpinned the approaches that are possible for paper packaging. This includes ink chemistries for printing both directly and with pre-priming, which is neither analog, nor ink-jetted, as it is a new breed of printing machines.
The extra consideration here is that the application of the inks requires each fluid to be considered in terms of the safety and sustainability. Cartons are primary packaging that are used closely with food items, and although there is often an additional, internal plastic package to ensure shelf life, these are not functional barriers. As a result, most printers coming into the market are using water-based inks, rather than the UV inks, becoming early adopters in the corrugated space.
Materials and Image Quality
There are lots of other posts on InkjetInsight.com about optimising the image quality. There is some crossover to the corrugated applications that we considered before, especially in term of the coated grades of corrugated liner. That is because the fillers used are in common. Probably the biggest overlap comes in term of corrugated pre-print and folding carton. The best-known pre-print machines are those from KBA/HP printing roll-to-roll and BHS/Screen printing roll-to-sheet in-line with a corrugator. For these applications, the ink must be resistant to the handling, or an over-print varnish (OPV) varnish is applied.
In the picture below, from tests done in our laboratory, we compare the printing quality with a simple two-color set-up onto different substrates. The top two lines of pattern in each case are the 100% process color yellow, and black, which represent the two extremes of efficacy of radiative drying (IR, NIR). The third line is a 200% composite, thus showing the difficult with increased coverage.
Due to the hold-out, the coated substrate shows better color density due to reduced absorption, but the trade-off is in the coalescence creating graininess. This is seen in the uniform coverage regions, due to the visible white substrate from lack of ink spread. The black ink also readily bleeds into the yellow, although the precise severity of such effect is very time-dependent (i.e. will be less severe for a real printer printing at 1000ft/min).
Primers can be used to encourage the solid filling in the right places, whilst preventing uncontrolled spread that creates the ink bleed. This usually involves chemistry assistance to crash the pigment, just like the well-proven approaches in commercial printers. In machines from Fujifilm and Inca, an inkjet deposited primer is used, which means changes are made to accommodate different substrates, or even batch variations can be implemented completely digitally, given appropriate sensor to detect the image quality.
Drying (Again)- It’s That Important
Paper packaging is quite a difficult substrate when it comes to stacking, sticking and rub resistance after print. Therefore, ink drying must be optimised to prevent ink from sticking, leaving unwanted transfer of ink from one printed sheet to the other. This effect is called “set-off”, a term from offset print technology.
In the photo below, we use the same ink as above, but tested for the visual effect of which ink causes set-off onto coated cards by stacking the prints. The effect of temperature during/after print is obvious. There is a difficulty with higher ink coverage and thickness, even at the higher 70°C temperature shown. Combining drier technology of hot air and radiative (IR, NIR) elements assists drying at higher speeds and reduces the effect of ink set-off.
To avoid the challenges of substrate handling and image distortion created by depositing and removing so much water, some inkjet-based printers printing folding cartons are being directed to use offset techniques. The most well-known is of course the Landa Nanographic press. In this process, the ink gets dried almost entirely before being pressed onto the substrate, thus overcoming some bleed and offset issues related to the above discussion.
Such printers still can suffer from other single-pass inkjet challenges, especially missing nozzles. Missing nozzles can make or break a successful packaging project so understanding technology options is important for success, and which so happens to be our next area of focus.
Stay tuned for the next post on discussing nozzle compensation schemes that can be used to mitigate the image quality effects of nozzle failure.