By Tom Kerchiss, RK Print Coat Instruments
Energy curing technology has, though it’s taken some time – become one of the leading contenders for environmentally friendly and economically efficient methods for print processing and for associated spin off process technologies including for example, the production of RFID, OLED displays and other electronics.
It’s worth looking at how far its come. In many respects the advance of ultra-violet curing has been evolutionary rather than revolutionary. The first patent for UV curable inks was granted to General Electric as far back as 1946. At the time General Electric was experimenting with ionizing radiation for the cure of solvent-free coatings. Now, some 70 years on UV systems are installed and in use on all of the major print processes and on converting machines with countless systems employed in industries as diverse as automotive, wood finishing, cosmetics and in situations where decorative and functional surface coatings need to be applied quickly and with minimal disruption to production line processes and product dispatch.
When comparing for example, water based and UV ink systems, it is necessary to compare the basic components inherent in any inking system, that is the diluents, resin and additives.
The diluent in ink have one primary function and that is to regulate/reduce the viscosity of the ink. In altering the viscosity it is possible to improve transfer and optimize printability. In water-based ink water is used as the diluent. The water facilitates ink transfer from an anilox roll to the plate, and then ultimately onto the substrate at which point the water evaporates off performing no other function.
UV inks differ in that reactive monomers are used as diluents. The primary function of the monomers is to reduce viscosity, however there is a difference from a water-based ink where the water diluent simply evaporates. The monomers used in UV ink do not evaporate after the ink is printed but play a part in the curing process, becoming part of the polymerized ink film. After polymerization monomers additionally provide gloss and surface hardness
The second component of the ink is the resin, which regardless of whether the ink is water-based or UV forms the backbone of the printed ink film. The properties of the resin determine ink characteristics such as durability, flexibility, chemical resistance and adhesion.
In a UV system the resin is referred to as an oligomer, a reactive resin that takes part in the polymerization process. In addition to contributing to durability the oligomer also plays a part in determining the speediness of the cure.
The third area, which will only be mentioned briefly here are the additives, a broad category that includes waxes, flow agents, pigments and so on.
It goes almost without saying that the evaporation process is involved in the drying of a water-based ink. The ink typically being printed on the substrate, run through the dryers where water and amine combination, evaporates leaving behind the pigment and resin. UV inks on the other hand are printed and exposed to a high intensity light source; the photoinitiator in the ink absorbs UV energy and initiates the polymerization chain reaction. The key here is that no evaporation process occurs only the transformation of a liquid to a solid.
Although this article focuses on UV it is worth looking at problems that can arise at this stage with water-based inks and then look at variables that can influence the way UV inks perform, particularly temperature.
It used to be said that with water-based inks as much as 40 per cent of the ink film printed could be lost due to evaporation. This is not necessarily as disastrous as it sounds, the real challenge arises when the ink inadvertently and often unbeknown to the operator dries either in the pan or dries on plate, impacting on ink lay down quantity and quality. If the evaporation in the ink pan is significant enough, once printed an ink colour can shift dramatically. This problem can be mitigated somewhat through the utilization of amines. In reality as the amines evaporate off a water-based ink, the ink begins to dry on plate and dot gain begins to steadily increase until a point is reached where plates need to be washed. Plate wash results in dot gain dropping and the evaporative dot gain cycle begins once again.
While evaporative drying systems have some drawbacks variables such as temperature can be problematic for UV. As with all inks the viscosity of UV flexo inks drop as temperature increases. Sometimes, depending on colour and temperature differences, viscosity variations can become quite large. As the viscosity of an ink changes the transfer of the ink from anilox to plate to substrate also changes. With doctor blade systems, ink viscosity and print density decreases. This relationship becomes very important when we look at the differences between the conditions, humidity, etc., inside a pressroom compared to those of ink kitchens and proofing areas.
This temperature variation may become more extreme during hot and humid weather conditions. If the ink store and pressroom conditions are not monitored, density and dot gain issues can arise as pressroom temperature fluctuates.
Advances in many areas associated with UV including inks and lamps have minimized or eliminated many of the variables associated with UV inks and curing, It is still worth bearing in mind though that when UV ink is cold and fresh on press the initial densities will be lower than after a running in period. As the ink is agitated the temperature in the pan may rise, viscosity will then drop and density will increase. This can result in tonal changes at the start of the run, which will then settle now when a state of equilibrium is reached. For these reasons it is important that all components are bought into a state of equilibrium when the press is fingerprinted.
Benchmark testing and peer group reviewing together with better pre-press support technology has made implementing ultraviolet curing of inks in the real world production environment less troublesome than it once was.
The developments that made UV inks and coatings feasible for many printers took time to evolve. In the past applying them could be tricky. Let’s leave flexo to one side and consider the problems associated with running oil-based litho inks on conventional presses. Those printers often experienced a dulling phenomenon, sometimes referred to as a gloss-back or dry-back, which could occur as standard inks dried underneath their UV veneer, this could mar one pass inline coating. Running inline with full UV inks was an alternative but only on presses equipped with special rollers optimized for the inks. Coating dried inks in an off-line unit was another approach but here an issue was the offset spray powder. The granules of this drying agent, unnoticed in normal production sometimes became magnified under the UV coating and imparted a pebbled look and texture that could spoil the desired effect.
Fortunately printers, formulators in time found ways round the various problems including to some extent the problems associated with the heat delivered by lamps and through general build up of heat in the press, which severely limited the type of material that could be processed. Water-cooled UV systems extended possibilities and in recent years a better understanding of light manipulation has facilitated the development of lamps that could deliver optimum cure of specific heat sensitive substrates.
A significant development is the introduction of LED (Light Emitting Diode) lamps. A benefit of LED UV is that there is minimal heat/energy transfer to the substrate. UV LED also provides uniform and consistent radiation across the width of the web. With regard to the LED UV solid-state system utilized by RK Print Coat Instruments on the FlexiProof LED UV colour communication device is that the lamps offer a tailored output at the important 385 or 395 nm wavelengths. The lamps are energy efficient, require little in the way of maintenance and being both ozone and mercury free meet ‘Green’ working objectives.
In the area of prototyping or trialing the FlexiProof makes it possible to create accurate prototypes without taking a production press off line or without recourse to outsourcing. Because the Flexi Proof with LED UV can print directly onto a substrate and cure inline the customer ‘proof of concept’ is made much easier and speedier.
LED UV curing is of course not right for every application, consequently it will be an optional alternative to the dichroic mercury arc vapor lamps currently provided with the FlexiProof UV.
The FlexiProof UV and new variant FlexiProof LED-UV is designed to enable producers and users of flexographic UV inks to resolve colour communication and print/process issues including: colour matching, printability, gloss, scuff and chemical resistance, and so forth off-press; quickly and with minimal waste.
Incorporating flexo critical components such as doctor blade and anilox roller the FlexiProof’s integrated miniaturized UV system addresses the issue associated with typical UV conveyors and that is the detection of pin holing.
Conventional UV conveyors are unable to detect pinholes because as the proof is taken to a conveyor chemical changes take place that make pinhole detection difficult if not impossible. Printing and curing in-line on the FlexiProof highlights pin holing and other problems, enabling corrective action to be undertaken quickly.
Source: FlexoGlobal Blog