Beyond Darth Vader

Beyond Star Wars' use of lasers, package printers are using two main types of lasers to engrave cells into anilox rolls.

by Joy English, Assistant Editor

THROUGOUT OUR LIVES, we are constantly fascinated by things that light up. We capture fireflies and "ooh" and "aww" at Fourth of July fireworks. Laser light shows are a big attraction at amusement parks and even science museums, where rays of light dance to funky background music. But these beams of light energy can be very powerful and very useful tools in various industries. In the package-printing industy, lasers are being used to engrave anilox rolls for improved ink dispersion and printing capabilities.

"A laser (Light Amplification by Stimulated Emission of Radiation) is a device that controls the way that energized atoms release photons," explained Steve Slater, technical manager for Pamarco, Inc. This technology is then tailored to fit the purposes of various industries, and for anilox roll engravers, simply speaking, lasers are used to carve cells into the anilox rolls.

Anilox roll laser engraving was developed in the late '70s with the introduction of the CO2 laser. This new laser offered roll engravers a process other than mechanical engraving of anilox rolls. Slater said, "Early production engravings showed initially that the use of laser-engraved chromium oxide had benefits over conventional mechanically-engraved rolls, such as reduced wear, but it was evident that there were some problems that needed sorting out"—such as inconsistency and lack of control. Improvements were made to the CO2 lasers, resulting in a laser that produced a uniform cell structure with consistency, control, and quality ink dispersion.

Several years later, YAG (Yttrium Aluminum Garnet) lasers entered the package-printing world, made commercially available in 1996, with the capability of achieving higher line counts than the CO2 laser could achieve. Even so, Mike McPherron, operations manager for Praxair Surface Technologies, Inc., said, "There are pluses and minuses to each system."

Science class

The main purpose in laser engraving anilox rolls is to produce cells that will dispense ink properly without clogging or smearing, while providing the converter with improved quality and long roll life.

To understand the laser-engraving process, one must take a little trip back to science class. Most people, at some point during their education, have learned about the light spectrum—one end is ultraviolet light and the other is infrared. In between, there are varying degrees of light energy. The CO2 and YAG lasers use light energy from the infrared spectrum, and the degree to which they use this energy impacts the engraving process.

McPherron explained, "CO2 lasers use light energy in the far-infrared spectrum," thus working at a wavelength of 10.6 microns. On the other hand, the YAG lasers use "light energy from the near-infrared spectrum, which has a much shorter wavelength of 1.06 microns (which is 10 times shorter than the CO2 beam). This allows for a much sharper concentration of laser energy during the engraving process." The shorter wavelength also results in a smaller beam width and allows higher line counts.

Ray Faulk, Laser Engineer, Harper Corp. of America, explained that the YAG's shorter beam length engraves smaller cells than does the CO2 laser. "Basically speaking, the CO2 engravings are limited to about 1,500 cells per inch (lpi), whereas the YAG engravings have recently topped at 2,500 lpi." And these higher line counts are becoming increasingly important.

"CO2 lasers cannot engrave high line counts like the YAG lasers can," said Mat Jones, sales and marketing manager, Stork Materials. "The high line-screen capability of YAG lasers is a definite advantage over CO2 lasers. Now that printing plate screens of 200 and above are becoming more and more common, higher line-screen anilox capability is a must."

YAG lasers also produce deeper ink-holding cells, according to McPherron, and this provides more volume for a given screen count. But, YAG lasers result in longer production times, he added. "Processing times for YAG engravings can be two to three times longer than CO2 engravings, as the YAG laser uses significantly less power than the CO2 laser." While YAG lasers can produce higher line counts, this comes with other trade-offs.

Recast has benefits

McPherron explained that with engraving, "lasers drill holes into the ceramic coating by using either a thermal or ablative process. The thermal process melts the ceramic and the material is transformed from a solid to a liquid to an evaporation phase. A recast material is formed as the liquid material resolidifies and forms the edges of the cell."

CO2 lasers incorporate a thermal process, while traditional YAG lasers are ablative. William Burrow, president of INTERFLEX Laser Engravers, expanded on the differences each technology provides. "CO2 lasers generate heat, melting ceramic into 'recast' which shapes the hexagonal cell walls and more importantly, hardens or anneals the engraved surface." However, in the ablative process, the ceramic goes directly from a solid to a vapor. "As the early ablative YAG lasers vaporized instead of melted the ceramic, the engraved surface holes were much softer and more prone to scoring," said Burrow. While the ablative YAGs produced high line counts, there are numerous benefits achieved from thermal engravings.

Jones commented on these benefits. "CO2 lasers utilize a thermal component to create cells," and they tend to have cauterized walls. "This is desirable because the cauterization (or recast) results in a relatively smooth surface. A smooth surface will release ink better, and is therefore easier to clean, will plug less during a run, and will also be more efficient in terms of ink release." The recast also makes CO2 engravings more durable.

YAG lasers lack this durability. Art Ehrenberg, Harper VP Operations, Green Bay, Wis., said, "Melted or 'cast' ceramic formed by CO2 engravings has superior wear properties to that of unmelted or non-recast ceramic. Traditional Nd YAG lasers have poor wear characteristics … due to the lack of melted ceramic forming the cell walls." Slater agreed, noting that premature wearing and printing problems on press were disadvantages of the YAG laser technology because it left little recast on the cell walls.

Another downside to traditional Nd YAG lasers is that they "produce cells which have much greater angles of the cell walls and produce more of a 'test tube' cell profile, which can prohibit cell evacuation and cause cleaning problems," said Ehrenberg.

Jones agreed, "A smooth inner cell lining is desirable for better ink release, structural integrity, and ease of cleaning. The YAG lasers do not produce cells with these desirable properties." Recently, though, thermal YAG lasers have been developed that address some of these limitations.

Compatability

Once the anilox rolls are engraved, ink-release and ink-compatability are key considerations. Jones said, "Since both a CO2 engraved cell and a YAG engraved cell will have the same surface tension because they are formed in the same medium (Chromium oxide), the difference in the way one or the other will react to ink lies only in its surface finish and cell dimension." The YAG laser will have micropores that resulted from the engraving, and these tend to retain more ink than the CO2-engraved cells, which have a smooth innerlining from the cauterization.

Likewise, Jones noted, "Since a higher depth to opening (DTO) ratio cell will release a lower percentage of ink than a lower DTO cell, it stands to reason that YAG cells tend to be blamed for plugging more than CO2 cells, simply by virtue of the YAG capabilities." By producing high line counts, YAG lasers produce high DTO cells.

Doug Collins, CEO of CTS Industries, said, "The ink issue is a geometry issue. Why do tea pitchers have spouts on them? Answer: So the tea will pour out and not splash out all the way to the bottom of the canister. The CO2 laser has traditionally had a better profile from point of curve to point of tangent." Clean ink dispersion is a direct result.

For industry professionals, the next main concern is which type of ink to use—and this comes down the rewettability. Steve Wilkinson, VP of sales and marketing, ARC International, said, "Clearly, UV inks cause less plugging issues for anilox rolls, with solvent-based inks being next, due to the chemistry of this particular ink system." He concluded that water-based inks provided the most rewettability-problems.

"To summarize," Jones concluded, "YAG cells tend to hang onto more ink than CO2 cells, and this difference is only exacerbated by the kind of ink being used."

Laser-engraving of anilox rolls has developed over time to offer package printers several options for producing anilox rolls. And new technologies are just around the corner. Burrow commented, "The next generation of YAG lasers promises improved engraving parameters throughout the entire spectrum of engravings, i.e. from low line counts up to very high line counts on one system." Multi-hit functions, where the cells are pulsed many times resulting in deeper engravings, higher line counts and higher volumes are also available in CO2 and YAG laser control systems. With improvements and the general benefits of the tried-and-true CO2 and YAG lasers, package printers are able to achieve the line counts, smooth surface, and recast they desire on their anilox rolls.

Resources

ARC International www.arcinternational.com

CTS Industries www.ctsindustries.com

Harper www.harperimage.com

INTERFLEX www.laserengrave.com

Pamarco, Inc. www.pamarcoinc.com

Praxair Technologies www.praxair-printing.com

Printech Anilox www.ceramcoprintech.com

Stork Materials www.stork.com