These comments are directed to sheet-fed, digital inkjet printing applications on combined corrugated board, though some facts might apply to other inkjet applications and even other (digital) printing processes.
The development of technology around digital inkjet printing is rapid. This essay can only highlight a few aspects and only at current state. The reader, and particularly any manager contemplating further investment in digital direct print hardware, is encouraged to research ink and printing technology to greatest depth!
Yes, the variable cost component represented by the printing ink is one of the most important factors to consider in digital print, but not only in cost per liter…
Color Selection
Digital inkjet presses come in various color combinations
- Black, Cyan (Blue), Magenta (Red), and Yellow process colors
- K, C, M, and Y plus light Cyan and light Magenta
- K, C, M, Y and Orange and Violet
- K, C, M, Y and Orange and Green
- K, C, M, Y and Orange, Green, and Violet
We know from conventional / analog printing processes that four color process is limited in its coverage of the full color gamut. The addition of light Cyan and Magenta do not really change this in the color theoretical sense, but do make realization of lighter tones easier.
Addition of other process colors, such as Orange, Green, or Violet can significantly enhance the gamut. The selection of such colors should reflect best coverage of likely gamut required by the printer’s customers.
Equally as important is the fact, that additional colors will reduce the total ink consumption of the process, as the additional colors can reproduce tones with lower combinations of colors. Such a reduction in ink consumption has been shown to exceed 25%!
Another word on consumption. Pre-press tools such as under-color removal (UCR) and grey-component removal (GCR) can be very helpful in managing ink consumption. Not all the tools of the analog print expert are being dismissed!
Ultra-Violet Cured Inks
From flexographic analog print applications we remember UV-cured product consisting of two types of oligomers and a photo-initiator. Upon exposure to UV radiation, the photo-initiator causes the two oligomers to polymerize and turn from liquid phase to hardened solid state.
This traditional UV process was fraught with some potential pitfalls for packaging printers with absorbent substrates. The photo-initiators are of very small size and tend to migrate through various products. Some of the photo-initiators were thought to be carcinogens. And, the cross-linking process was disrupted when water molecules (for example from previously printed water-based inks) was present, leading to orange peel.
Modern inkjet UV-curable product is made with a photo-initiator attached to one of the oligomers, rather than a free initiator. This has two significant consequences: first, the particle size of the combined initiator/oligomer is much larger, reducing the risk of migration (some of such inks were used to produce packages that passed the Swiss ordinance on food packaging allowable migration); second, the product now performs well with water present, leading to the development of “hybrid” water-UV inkjet inks.
Such low migration UV-curable inkjet inks are now widely used.
UV-cured or Water-based?
A UV-cured ink product does not carry a solvent. The oligomer components are in a liquid phase when not cured and then turn solid, leaving no liquids to dry or absorb. A water-based ink typically carries between 40% and 60% water.
This basic difference in formulation of the ink product leads to a significant difference in the amount of ink required to create a color or to achieve good coverage. 100% of the UV-cured ink contributes to color and coverage; only about 50% of the water-based ink does so.
While ink consumption is lower with UV-cured ink, ink cost, of course, is higher, as water tends to be cheaper than resin/oligomers.
The UV-cured product needs to be cured. This is typically achieved by exposure to UV radiation, which can be generated by a Light Emitting Diode (LED) or a Mercury Vapor Lamp. LED technology has advantages of lower energy consumption, less heat generation, and longer life cycle of components. LED is, however, limited in the range of wavelength it can generate. In some cases LED and mercury vapor lamps are being combined in a two-step process.
A water-based product has to be dried and some of the water will absorb into the substrate. Infra-red radiating lamps are typically used to accelerate the drying, eliminating any potential energy advantage of the water-based ink versus the UV-cured ink. Water absorption into the substrate can lead to it’s own troubles, such as warp and wrinkles.
In press design, particularly in single pass presses, a drying system for water-based product increases the press length, as inter-station drying is required for best color performance. In UV presses this is accomplished with very small LED units, which are placed immediately following the inkjets.
Such LED units following the inkjets are also used to “pin” the ink upon placement on the substrate, reducing risk of unwanted dot gain.
Premature drying of inks in inkjet head nozzles is a major downtime factor in digital print. Press manufacturers aim to reduce the problem with thorough ink circulation past the nozzles and automatic wash features. UV-cured inks, as long as they are not exposed to UV radiation, tend to remain liquid. Water-based inks suffer from evaporation and potential premature drying. Ethanol-Glycols added to the water-based ink to prevent drying could raise migration concerns…
Open or Captive Ink Supply
If a single pass digital inkjet press produces 120 million square feet (120,000 msf) of printed product a year, how do fixed and variable costs compare?
A $ 5,000,000.00 press would likely burden about $ 750,000.00 fixed cost per year (depreciation and financing) or $ 6.25 per one thousand square feet. That is a significant cost! But it pales in comparison to the potential ink cost per msf. Is this ink cost going to be $ 5.00 per msf or $ 25.00 per msf? And which direction is it trending? Are multiple ink vendors competing for your digital inkjet ink business?
Also, do consider additional ink consumption due to purging and cleaning needs.