October 18th, 2011
Full article below featured on FlexoExchange: http://www.flexoexchange.com/articles/uvink1.html
Stork Sheds Light on UV Inks
Submitted By: David Lanska, Stork Cellramic
Originally Published in the June 1997 issue of Converting Magazine
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.
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.
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.
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.
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 60� where the microscopic turbulence and surface tension result in extensive pinholing. There seems to be almost universal agreement the pinholing is reduced with a 30� 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.
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