UV inks are considered by many to be the most popular in the narrow-web space. And for good reason – print results with UV inks are consistently better than those using water or solvent-based inks. But the use of UV inks is not without its challenges. Chief among those challenges is the issue of ‘ink spitting.’
Printers who’ve fallen victim to ink spitting likely recall the frustration of rushing to stop the press to diagnose and remedy the spattering of random ink spots on an otherwise perfect print run. The risk of press downtime from UV-ink spitting compels some printers to run their presses far slower than their desired speed or to apply more doctor blade pressure onto the anilox roll, which has its own drawbacks like increased blade wear and the risk of score lines.
One Hyderabad-based label printer, Citrus Packaging Solutions, tried a few methods to deal with spitting. “We tried anilox rollers from different suppliers, as well as lots of different doctor blades at greater pressure. But still, we didn’t find an exact solution to our UV ink spitting problem,” said its managing director, M Chenga Reddy. His ultimate choice was to switch to a different anilox engraving profile to manage spitting. “We tried [open-slalom channel geometry] GTT anilox rollers at our company, and now, more than two-and-a-half years later, we are using GTT anilox rollers without UV spitting issues. What’s more, our doctor blades last at least a week longer.”
Experts who’ve studied spitting have several theories as to why spitting occurs with UV inks — and doesn’t happen with water-based or solvent-based inks. Generally, experts point to UV inks’ higher viscosity and the impact this slightly thicker fluid has on the doctor blade. Specifically – as one theory goes – the UV ink’s higher viscosity can move the blade just enough to allow more ink to slip through than is intended.
Other theories peer deep into the hydraulic physics of the ink’s journey — from the ink reservoir, into the anilox cell, beneath the doctor blade, and then onto the plate (Figure 1). At the point where the doctor blade shears the excess ink from the top of the anilox cell, a turbulent condition typically occurs as the liquid is forced against the end wall of the cell (Figure 1A). While ink spitting is not precisely analogous to water splashing in a full bathtub, the nature of pressurized pre-doctored ink suddenly de-pressurized within the anilox cell is complicated by the infinite variation of ink type and viscosity, doctor blade, anilox cell, and press speed.
Regardless of the precise causes, the results can be disastrous, particularly at high speeds. While exactly why spitting occurs is a subject for debate, what is not in dispute is that the relationship between doctor blade and anilox is at the core.
If spitting is creating a challenge in your shop, here are three tips for better print results:
1. Try a stiffer doctor blade.
A wide variety of doctor blade thicknesses and materials are available, each with varying deflective rigidity and applications. A stiffer doctor blade may prevent excess ink from getting under and transferring to the plate and substrate.
Testing shows a thicker doctor blade will minimize UV spitting because it resists hydroplaning and minimizes excess ink transfer to the anilox surface.
2. Try a 30° anilox cell with open end-walls.
While conventional 60°closed-cell anilox engravings prevent any calming ink flow between cells at doctoring, a 30° cell engraving with open end-walls allows a flow of ink to move through the openings in the anilox cell walls and can minimize ink build-up onto the back of the doctor blade. (Figure 2).
A word of caution! This solution is only temporary. Because the roll will wear down over time, the cell openings will become smaller and eventually disappear. Thus at some point, the same turbulent situation will occur as with a closed-cell anilox roll. Ink will once again build up on the back of the doctor blade.
3. Try Open-Channel engravings.
Unlike closed-cell anilox engravings, GTT engravings eliminate hydraulic pressure at the doctor blade because the ink can move freely and calmly through GTT’s open-slalom channels (Figure 3). Since no cell end-walls are present in GTT engravings to create turbulence when the doctor blade skims ink from the anilox, ink build-up on the back of the doctor blade is virtually eliminated. And no ink droplets from the doctor blade will “spit” onto the flexographic plate or substrate (Figure 3A).
Solutions involving stiffer doctor blades and open-end-wall hexagonal anilox engravings show promise to reduce spitting. However, be aware that some solutions are only temporary until inevitable wear of the hexagonal cell walls occurs. On the other hand, the GTT open-channel engraving surface is a permanent solution because no ink is built up through pressure at the back of the doctor blade and thus, ink drops are reduced or prevented from spitting from the blade onto the plate or surface.
Recalling his choice to embrace open-channel engravings, Sujesh Pandalangat, General Manager – Operations for Mumbai-based Skanem India, observed, “Our key focus area is continuously achieving excellent print quality standards and upgrading plant efficiency levels. Recently we have invested in GTT anilox rolls from Apex, replacing some of our conventional anilox rolls — and the results are excellent. Ink spitting and score marks are age-old problems in flexo technology; we find GTT anilox rolls to be a foolproof solution.” Citrus Packaging’s M Chenga Reddy agreed, noting, “We found GTT to be a ground-breaking technology that made flexo printing child’s play!”
Ajay Pawar is the Marketing Executive for Apex International’s Asia-Pacific region based near Nasik. He can be reached at email@example.com