Stork Sheds Light on UV Inks
Submitted By: David Lanska, Stork Cellramic
Originally Published in the June 1997 issue of Converting Magazine
Background
Flexographic printing is an evolving process. Through the years, advances in press components have enabled printers to achieve
higher levels of quality and visual impact. One of the most notable recent developments is the escalating use of UV inks. Offering significant advantages over water and solvent based inks, UV inks are
experiencing explosive growth in the flexographic industry. As industry demands for visual impact increase, printers seek to add color density while preserving or even enhancing image
quality. It is the ability of UV inks to positively impact on color density and image quality, while facilitating improvements in printing efficiency that begins to explain its rising popularity. The
other and most significant reason for UV's growth is concern over environmental issues.
Back to top Printing Inks
Inks may have many ingredients, but fundamentally an ink has a component which provides color (pigment, dye or colorant) and a
liquid base which provides a means of transportation from the ink fountain to the substrate. Other components and additives are included in the ink formulation to control the ink distribution process,
fix the pigment onto the substrate, and enhance specific characteristics of the printed image. When printing with traditional water and solvent-based inks, dryers apply heat to the web. The
ink dries through an evaporative process that removes about 35-40% of the delivered ink volume. In other words, you have to deliver 40% more volume to the substrate with water or solvent ink in order to
achieve the desired color density. Solvent-based inks have been popular for many years and provide excellent results on nonabsorbent web surfaces such as poly films. In an era of growing
environmental awareness and reducing tolerance for environmental hazards, waste disposal issues are becoming a driving force in the choices manufacturers and printers make. Environmental Protecting
Agency (EPA) regulations make use and disposal of solvent materials a cumbersome and expensive process. This has driven printers to water based inks wherever applicable. Water-based inks
have tended to be more difficult to work with on film substrates, however, and can be very unforgiving. Left open to the air just once, water-based ink can dry in the cell structure, reducing available
cell volume and necessitating removal of the roll from the press for cleaning. It is easy to understand why plugging has become such a universal problem when you consider the demands placed
on component manufacturers. Printers want to add color. They want to be able to put down more ink. They want the ink to dry faster so they can keep press speeds up. They want inks that do not run, that
dry rapidly and stay put. They also want certain characteristics in the printed image including durability and rub resistance. Ink manufacturers, faced with these demands, have happily done
what they could to accommodate their clients and have come out with new, "improved" ink formulations. It only stands to reason, however, that if an ink dries quicker on the substrate, it will
dry quicker in the cells of the anilox rolls. If the ink is more durable on the substrate, it will be more difficult to remove from the cells. The problem is only aggravated as anilox roll
manufacturers are faced with similar demands. Chrome engravings with shallow cells have been replaced with steep-walled laser engraved cells. Cell depths have been increased. Cell openings have decreased
as line counts have increased. The demand is for maintaining or increasing ink density while simultaneously providing finer distribution. Deeper cells do not typically release ink as well and finer
engravings provide greater cell wall surface area for the ink to adhere to. By pushing to achieve greater printing impact, printers and component manufacturers have created a problem of
enormous proportions. Removal of dried ink resins can be an expensive, aggravating and nearly hopeless proposition. The proliferation of equipment, supplies, techniques, and services for cleaning anilox
rolls gives testament to the scope of the problem of plugged cells.
Back to top UV Ink
UV inks do not dry in the air so they do not tend to plug the cells. There in no component that evaporates so there is no need for
clean-ups between press runs. The press could even be left over the weekend without cleaning and be ready to go on Monday. UV inks act like liquid plastic. As the ink is exposed to
concentrated UV radiation, a chemical reaction takes place during which the photo-initiators cause the ink components to cross-link into a solid. No material is removed so nearly 100% of the delivered
volume is used to provide coloration. Assuming all of the ink transferred from the cells to the substrate, to attain the color density previously attained with water based ink, the anilox roll would have
to be specified to delivered about 35% less ink to the substrate. Of course, viscosity will play a key role in determining the percentage of ink to transfer (transfer factor). UV inks are
typically very thick. Although lower viscosities are available, UV inks generally range from 1000-5000 centipoise (as compared with water based inks in the vicinity of 100 centipoise). It would therefore
be reasonable to expect water or solvent inks to more readily transfer from the engraved cells to the substrate (have a higher transfer factor). It is important to note that printers are experiencing
good results with rolls manufactured to specifications established for water and solvent inks. Existing anilox rolls can be used for printing UV and provide a benchmark for specifying new anilox roll
engravings. Transfer factor aside, the significantly thicker consistency of UV ink can benefit the print quality by reducing color bleed and dot gain resulting in sharper images.
Back to top Benefits
Because 100% of the material applied to the substrate remains after curing, there is potential to achieve greater densities than
with conventional inks. Due to the higher viscosities, UV inks tend to stay where they are placed. Dot gain is negligible resulting in exceptional image sharpness. For that reason, UV inks work well for
printing fine line, process and vignettes. Cured UV ink provides many desirable end use qualities including excellent rub resistance, chemical resistance, exceptional color consistency and
superior gloss. Although functions of the pigment, UV inks also provide lightfastness and opacity. UV inks come press-ready. Consequently, set-up times are reduced and less waste is
generated. UV inks also present another advantage in that operator involvement is reduced. There is less variability from operator to operator and press run to press run because the ink consistency is
not manipulated at press side. One of the greatest advantages of UV inks is that they do not change consistency due to evaporation or pH. Without manipulation, the ink maintains consistency
throughout the duration of a press run. UV inks provide additional benefits because the ink does not dry in the cells. Significant savings can be realized in terms of labor, consumables, anilox roll
cleaning expenses, and roll refurbishment. With typical water and solvent based inks, evaporation results in a system where the constant is variability. Over time, the ink changes viscosity
and affects lay down. Ink resins dry in the cells of the anilox roll resulting in further changes to the lay down. Press operators, attempting to correct for changes in ink density, add extenders and
other additives to the ink. Degradation of the print quality results as the anilox roll continues to plug. The ink is further altered until its consistency has little resemblance to the ink the job was
started with. Because of the tendency of traditional inks to plug the anilox rolls, it is necessary to pay extremely careful attention to press-side housekeeping practices. Even rolls idling
for minutes can begin to plug. When this happens, the rolls must be removed from the press and cleaned with aggressive chemicals or harsh agitation action, either of which can damage the fragile engraved
cell structure. Some water-based inks have a catalyst component. These undertake a chemical reaction that links the chemical structure to create an ink layer almost impervious to all but
the harshest cleaning approaches. When these inks harden, they do not re-wet and predicate the need for the anilox roll to be refurbished at considerable expense. To reduce the likelihood or
at least slow the process of cell plugging, printers must clean the rolls on press before each occasion when the press will be idling as well as between print. jobs. To purge the cells and plates of any
residual ink and cleaning chemicals, the press is run until the web is clean. Over time, repetitive cleanings can result in substantial costs associated with waste materials and disposal.
When the press is down for cleaning, it is not producing. Press operators and maintenance people have to spend their time manually cleaning the rolls. Manual cleaning is messy and time consuming and with
the growing tendency toward higher cellcounts, often has limited effectiveness. Unfortunately, sooner or later even the best efforts at cleaning will succumb to cell plugging. This does not happen with
UV ink because it stays wet and maintains consistency until it is exposed to concentrated UV energy. Over the course of a year, improvements in press utilization can be substantial.
Back to top Drawbacks
Printing with UV inks does have some drawbacks. While some waste and maintenance costs are less with UV, start-up and operating
costs are higher. Because the inks are not manipulated, print color is adjusted by the choice of anilox roll. This may predicate the need for having a wider assortment of anilox rolls in-house and more
precisely specifying cell characteristics for a given print job. To shorten the learning curve, it may be advantageous to test print results with a banded anilox roll. In any event, switching to a new
system invariably results in expenses, errors and waste until the system is understood and the parameters fine-tuned. Because UV inks do not dry, any small spill can end up being a large
mess. Ink can be tracked from department to department on people's shoes. UV ink can damage clothes and irritate the skin. Some press operators even experience allergic reactions to UV ink chemistry.
Care must always be taken to prevent direct contact with the skin. UV inks do not adhere well to some poly substrates. In order to raise the surface tension sufficiently to achieve good
adhesion, the web must first receive corona treatment. Because UV inks are thick and do not flow easily, (the same characteristics that make them ideal for fine line and process work), they have problems
printing smooth, consistent solids. This is particularly the case with anilox rolls engraved to 60º where the microscopic turbulence and surface tension result in extensive pinholing. There seems to be
almost universal agreement the pinholing is reduced with a 30º cell placement pattern. UV inks require special lamps that focus UV energy onto the web surface. The purchase of the lamps
represents a significant up-front capital cost. In addition, it may be necessary to purchase equipment to dissipate some of the heat from the press. Even daily operation costs tend to be higher due to
typically higher energy consumption for the UV lamps. Printers already achieving desired print quality levels may wish to carefully consider if the potential quality improvements are worth the price. While the curing mechanism is the UV spectral component of the light, the lamps emit a significant amount of infrared energy. The IR energy offers no benefit and actually has the negative
effect of inducing heat onto the web surface. During an idle cycle, the web can scorch and even break. This potential can be reduced and energy savings realized by adjusting the output power of the UV
lamps relative to press speeds and ink thicknesses delivered at a given print station. There are many brands and styles of lamps available including parabolic, elliptical, surface-coated, water and
air-cooled. It is important to specify lamps with reflectors suited to ink film thicknesses most often encountered.
Back to top Conclusions
UV ink does not work better for all printers or all situations. Potential gains in color density and image quality have to be
carefully weighed against conversion and operating costs. The quality demands of the typical print jobs run should be primary determining factors. It is important to recognize that print quality is the
cumulative result of the entire ink distribution system. Each of the components can add or detract from the finished result. UV inks can be the vehicle for achieving outstanding print results. In all
likelihood, however, most printers can achieve better results than they are currently getting just by fine tuning the process they are working with. There is no question that the uses of UV
inks will continue to rise given the advantages UV inks have over water and solvent inks. As concerns over environmental issues, process efficiency and image quality continue to grow, UV will continue to
gain ground.
Author: David Lanska, Stork Cellramic © 1997; David Lanska, Stork Cellramic; All Rights Reserved
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