A new type of laser is being used to engrave anilox rolls.
by: Frank M.J. van den Berge
Originally published in the December 1997 issue of FLEXO Magazine
One integral part of the flexographic press that has seen some drastic improvements during the past few decades is the anilox roll, an ink-metering roll used to transfer an exact amount of ink to the printing cylinder. The importance of high quality anilox rolls has increased with the demand for higher quality printing. The first anilox roll was used during the 1940s to transfer some of the recently developed pigmented inks. During the past five decades, anilox rolls have evolved into extremely precise and consistent metering rolls. Nowadays, two types of anilox rolls are used for most flexographic applications: mechanically engraved chrome-plated anilox rolls, and laser-engraved ceramic anilox rolls. Chrome-plated anilox rolls have been around for close to 60 years, whereas laser-engraved ceramic rollers have been around for less than 30 years.
Mechanically engraved rolls are manufactured by knurling a soft metal (such as copper) with a knurling tool. Afterwards, these rolls are typically chrome plated to provide some degree of wear resistance. The individual cells that are made this way have a pyramidal shape, with either a pointy or flat bottom (see Figures 1a and 1b).
The drawback of mechanically engraved rolls is that although chrome plating does provide some added wear resistance, the rolls still wear over time. Due to the cell shape of mechanically engraved cells, the top section of the cell (due to its wide opening) provides most of the cell volume. When this type of cell loses only a few microns of cell depth, it loses a significant amount of cell volume, resulting in a noticeable decrease in print density. Mechanically engraved anilox rolls also are limited with respect to high line counts. Since there are physical limitations with respect to how small you can make a knurling tool, the highest line count currently being manufactured is around 500 lines per inch (lpi).
The laser-engraved anilox roll, introduced about 20 years ago, consists of a plasma-sprayed ceramic coating (typically chromium oxide) that is ground and honed to a very smooth finish (in the neighborhood of 5 RMS). The ceramic coating is extremely hard, with a micro-hardness of 1150 to 1300 Vickers. (For comparison, the hardness of chrome plating is around 850 to 950 Vickers.) Hardness is widely used as a guide to strength, wear and erosion resistance of a coating1. Due to its extreme hardness, it is not possible to engrave these coatings using a knurling tool. The only way to produce cells in this material is to engrave it with a laser.
Laser Engraving Process
A laser is essentially an optical amplifier. The theoretical background of laser principles as the basis for an optical amplifier was made possible by Albert Einstein. He was the first person to suggest the existence of stimulated emission (the principle on which laser technology is based) in a paper that was published in 1916. The phenomenon of stimulated emission produces a highly monochromatic (one color) and highly coherent light. The first laser was built in 1960 by Theodore Maiman2.
Lasers have had a tremendous impact on numerous industrial processes, ranging from high volume automotive applications to refined electronic applications. These applications include cutting, drilling, welding, etching and even printing. There are different types of lasers. The type used most often for industrial applications is the CO2 (carbon dioxide) laser. This is the gas that is used in combination with two other gasses (helium and nitrogen) to produce the actual laser energy. The CO2 laser also is the type of laser that has been used to engrave ceramic anilox rolls.
The CO2 lasers that have been used to manufacture laser-engraved anilox rolls have seen tremendous improvements during the past decade. Lasers are used to generate pulses of energy, where every pulse is responsible for producing an impression in the ceramic. The pulse rates with which the lasers operate have increased tenfold, allowing anilox roll producers to engrave faster. More important for the end-user, the pulse-to-pulse stability has improved, resulting in engravings that are more consistent.
Although there is no doubt that the conventional CO2 lasers will continue to improve, experiments were started a year ago with a completely different type of laser that does not use a gas (or gasses) such as carbon dioxide to create the laser energy. Instead, it uses a special type of ceramic crystal known as "YAG," which stands for Yttrium Aluminum Garnet. This type of laser also is referred to as a solid state laser3. The key differences between this laser and the conventional CO2 lasers are the pulse shape and the wavelength.
As mentioned earlier, the pulses generated by the laser are used to produce the millions (and sometimes billions) of cells that make up an anilox roll. The shape of the pulse is directly related to the shape of the cell. The different pulse shapes can be found in Figures 2a and 2b. Figure 2a shows a typically shaped pulse the way it is produced by a CO2 laser, whereas Figure 2b shows a pulse the way it is generated by the solid state laser. The main difference is the tail end of the pulse. Due to the way a CO2 laser operates, it has what laser technicians call a long decay. This long decay time can result in low rotational cell walls, which lead to unwanted channeling. This in turn can result in poor print results due to the pooling of ink and loss of print definition.
A second difference is the rise time. A solid state laser pulse has a much steeper rise. The shorter decay time and steeper rise time of a solid state laser allow cleaner cells to be engraved, with distinct cell walls leading to improved print quality. In addition to this, they allow for a more complete vaporization of the ceramic (instead of melting, which causes slag and recast that is deposited onto the cell walls as rough bumps and other protuberances, leading to reduced print quality and increased doctor blade wear). You can see the difference in Figures 3a and 3b, which show cross-sectional views of engraved cells. These graphs were produced using an interferometer, which is able to scan the profile of an en-graving. The geometry of the CO2 laser cell is an "upside down" image of the laser pulse shown in Figure 2a, and is asymmetrical. The geometry of the cells produced with a solid state laser is symmetrical and resembles a sinus wave.
A CO2 laser produces energy in the form of invisible light, with a fixed wavelength of 10.6 micrometers or microns. This is inherent to the way a CO2 laser operates. The type of solid state laser described in this article operates with a wavelength of only 1.06 microns. There is a mathematical relationship, due to a simple law of physics, between the wavelength of the laser and the smallest possible spot size you can focus a laser beam4. The minimum diameter of a focal spot ( S ) formed by a lens of diameter ( D ) and a focal length ( ) with light of wavelength () is:
S = / D
If you are familiar with photography, you may recognize the ratio / D as the focal ratio or " number" of a lens. The longer the wavelength, the larger the smallest possible spot size will be. This is a limiting factor when it comes to achieving higher and higher line counts.
Although it is possible to engrave high line counts with CO2 lasers, the working envelope is limited to around 1000 cells per inch. Once you exceed that number, the quality of the engraving starts to deteriorate rapidly.
Due to the 1.06 micron wavelength of the new solid state laser, which is about 10% of the CO2 laser, high quality engravings (up to 1500 cells per inch) can be produced without any problems. The aforementioned phenomena related to the sharp pulse shape is an added benefit regarding achieving high line count engravings without sacrificing quality.
There are several other benefits with respect to this solid state laser. One of them is an improved coupling. This complicated phenomenon relates to the interaction between the laser energy and the material. When the laser beam hits a material, part of the energy reflects and the remaining energy is absorbed into the material. With respect to the engraving of chrome oxide, this absorption of energy leads to two processes: melting and vaporization of the ceramic. To produce a high quality engraving, what you want to happen is a complete vaporization process. Unfortunately, Mother Nature does not always cooperate, and part of the ceramic melts. As mentioned earlier, this molten ceramic is deposited on top of the cell walls, and results in slag and recast. This results in a somewhat erratic and inconsistent engraving quality. Due to the nature of the pulse shape and wavelength of the new solid state laser, the predominant process of material removal is vaporization, and almost no melting of the ceramic occurs. The result is a much more consistent and uniform engraving, with virtually no recast or slag. This improves print quality and reduces doctor blade wear. This is illustrated in Figures 4a and 4b. The improved coupling also makes it possible to engrave deeper, thereby creating a higher cell volume.
Another benefit relates to the focusing of the laser beam. In order to create a good-looking cell, the laser beam must be focused onto the ceramic surface using special lenses. This process is similar to focusing a microscope, camera or loupe. To achieve a consistent engraving across the entire roll, it is important to maintain a constant focusing distance. The margin with which one can vary this focusing distance without changing the cell pattern is referred to as the "sweet spot" (comparable to the sweet spot of a golf club). The new solid state laser has a larger sweet spot than the CO2 laser, making it less susceptible to engraving variations in the same roll due to taper and TIR (Total Indicated Run-out, a measurement of the concentricity of the roll). Although most anilox roll suppliers manufacture their rolls to the most stringent tolerances, there always will be some small amount of taper and TIR. This, in the case of a CO2 laser, could result in minute engraving differences between opposite sides of the roll. The larger sweet spot size of the solid state laser eliminates this effect altogether.
To understand the differences, see Figures 5a and 5b, which were taken using an interferometer. Figure 5a shows a 360 line count engraving done with a conventional CO2 laser engraving; Figure 5b shows the same line count engraving done with the solid state laser.
Actual Use in a Production Environment
The new laser has been used in a production environment for close to six months. The engravings that can be achieved by this system have enabled some to achieve levels of quality that were previously thought to be impossible. For example, Vincent Alello, Printing Technical Supervisor of Sealright, Fulton NY, says his company has been able to better control dot gain in its four-color process work. This has allowed the company to improve contrast and tonal ranges, enhancing the detail of the printed image and therefore improving the company's overall graphic capabilities.
Development of this system is ongoing. Achieving lower line counts is a major goal. As is often the case with a paradigm shift, new problems must be faced. In the old days, laser technology was limited on the high side when it came to line counts. Now the reverse is true, and the technology is limited on the low side. The solid state laser currently is able to engrave extremely high quality line counts down to 360 lpi. However, modifications that will allow engraving down to 85 lpi are in the works.
1. Metallic & Ceramic Coatings (M.G. Hocking, V. Vasantasree & P.S. Sidkey)
2. Introduction to Optics, 2nd edition (Frank L. Pedrotti, S.J. & Leno S. Pedrotti)
3. Solid-State Laser Engineering (Walter Koechner)
4. Understanding Lasers, An Entry-Level Guide (Jeff Hecht)
Frank M.J. van den Berge holds a Masters of Science degree in Chemical Engineering. Currently the Manager of Operations for Stork Cellramic in Milwaukee WI, he is responsible for production, development, sales and marketing of anilox rolls and engineered coatings. He has been with Stork for 10 years. Contact van den Berge via telephone at 1-888-97STORK, or address e-mail to "[email protected]".