Characterization of anilox rollers
In the spirit of encouraging scientific research and scholarship that benefits the graphic arts
industry, GATFWorld is pleased to highlight another student paper that has received recognition from the Technical Association of the Graphic Arts (TAGA). The work presented in this paper earned Alex Castellanos and Paul Haak, undergraduate students at California Polytechnic State University, San Luis Obispo, the 2000 Harvey R. Levenson/TAGA Student Paper Award. The paper was presented at TAGA’s 2000 annual technical conference and published in the 2000 TAGA Proceedings (p. 788-800), from which it was adapted for publication in GATFWorld.
TAGA sponsors annual graduate and undergraduate paper competitions and also a poster paper competition. For information about TAGA or its student programs, call 716475-7470 or visit www.taga.org.
The anilox roller is an engraved roller used in flexographic printing presses. It picks up ink from the fountain roller, (or, in some configurations, directly from the ink fountain) and delivers a predetermined, metered, uniform amount of ink to the printing plate. It accomplishes this because its surface is pitted with etched or laserengraved cells, which can vary from 80 to 2000 per inch depending on the application.
The purpose of this study was to compare the quality of print using anilox rollers engraved at 400, 700, 900, and 1600 lines per inch (lpi). This measure is also called cells per inch, but is referred to as lpi in this paper. A Mark
Andy 4150 flexographic printing press was used to conduct the testing, and a form was developed to test the print reproduction characteristics of anilox rollers in the following areas:
1. Density of solids
2. Dot gain
3. Minimum and maximum dot
4. Positive and negative typeface serifs
5. Minimum reproducible hairline
6. A 150 line screen image
The anilox roller was developed in 1939 by Doug Tuttle, a marketing director of Pamarco Corporation. When Tuttle first saw a gravure coating cylinder, he realized its application for flexography. The industry quickly embraced this technology and soon it became the focus for increasing print quality in flexography. The anilox roller was originally chrome coated, but since this coating quickly wore down, a ceramic coating was introduced in the 1970s as a more durable alternative.
Anilox rollers are available in a variety of types and engravings:
* Chrome rollers, which are mechanically engraved, usually in copper because it is a soft metal that can be electroplated with a thin chromium layer for durability.
* Ceramic rollers, which use a ceramic coating that is as-sprayed onto the roller surface.
* Engraved ceramic rollers, which are mechanically or electromechanically engraved and then sprayed with a thin, ceramic, chromium oxide layer.
* Laser ceramic rollers, which are the standard for flexographic printers. Laser ceramic rollers are preferred for their durability and ability to be laser engraved at 2000 lpi.
The cell shape for anilox rollers has always been determined by the engraving process. Inverted pyramids were initially common; then the quadrangular shape with its steeper walls and flat bottom was introduced for increased roller life and more efficient release characteristics. Today, anilox rollers are engraved as finely as 2000 lpi using the latest laser technology. Laser engraving provides a cupped shape cell for maximum ink release and ease of cleaning (Trungale 1996, 58).
Anilox Roller Characteristics
With its ability to lay down thin ink films and offer control in the reproduction process, the anilox roller is considered the heart of the flexo printing process. Control of the reproduction process gives a greater control over dot gain and printing highlight areas (Carrillo 1995, 58). Two main variables affect anilox roller performance: cell volume, which determines the ink delivery capacity of the roll, and the number of lines (or cells) per inch.
Cell Volume: Cell opening and cell depth determine the cell volume which is measusred in units of a billion cubic microns (BCM). The higher the BCM value the higher the volume of ink deposited (Carrillo 1995, 58).
Lines per Inch: The amount of ink deposited by a high lpi roller is small and closely spaced. This provides a more uniform ink film thickness and faster drying. High lpi rollers are used for fine line images and process color work. Low lpi rollers are used where a high ink deposit is needed such as solid line art images. Table 1 shows the typical uses for various lpi measures.
Anilox rollers are available in the following cell shapes: pyramid, quadrangle, special channel, and laser engraved. Pyramid cells, which look like inverted pyramids, are one of the earliest shapes available for anilox rollers. This geometry is recommended for use with two-roll metering systems or where accurate ink metering is not required. Quadrangle cells are used to deliver higher amounts of ink than an inverted pyramid, and are typically used with blade metering systems and on images with dense solids. Special channel cells, typically hexagonally shaped, can be used to deliver specific ink volumes for particular printing applications. Laser-engraved cells have nearly vertical sidewalk and rounded bottoms.
Three types of cell angles are available: 300 hexagon, 450 diamond, and 600 hexagon (Figure 1).
The diamond-shaped cell is considered inferior to the hexagon-shaped cell because it has a higher land area and a less uniform ink film. To compensate for a less uniform ink film, the anilox roller must have deeper cells to give the same volume.
The hexagonal cell shape is the most efficient geometrical shape since it maximizes space, containing 15% more hexagons than diamonds in a given area. With more cells per square inch, the hexagon pattern can deliver shallower cell depths, resulting in more uniform coating films. Shallow cells minimize volume loss and cleanup during the run.
Equally as important as cell shape is cell depth. An optimum cell has a depth-to-opening ratio between 23 and 33%. For example, for a 100-micron opening, the cell depth should be within a 23-33 micron range (Figure 2). If the cell is too deep, the ink will build up and dry on the bottom of the cell. If the cell is not deep enough, an insufficient amount of ink will transfer to the substrate (Carrillo 1995, 58).
The goal during the printing part of this project was to achieve consistent and repeatable results. The samples were printed on a nine-color Mark Andy 4150 press with inline diecutting and dryers at Blake Printery, San Luis Obispo, California. The press operator during the testing was Gil du Long, a flexography printing consultant with more than 20 years of printing experience. The anilox rollers were from Harper Corporation.
For the test 50 samples were collected from each roller to provide a prediction of anilox roller consistency and repeatability.
A test form (Figure 3) was developed to measure and record the print characteristics for each anilox roller used in the study. The following elements were included on the test form: 1. Solid ink density
2. Dot gain
3. Minimum and maximum dots
4. Printable hairlines
8. Line screen images
9. Bearer bars
The information obtained from measuring the elements included on the test form provided specific characteristics for each anilox roller in the study.
Solid ink density was measured from a solid patch printed on the test form using a Gretag densitometer. The densities were plotted on a histogram, which provided an accurate representation of print variability throughout the run.
To calculate dot gain, minimum and maximum dots were measured with a densitometer from a vignette on the test image. Vignette values ranged from 1% to 100% black. The results were graphed on a histogram.
Hairlines helped evaluate the smallest reproducible line for a particular anilox roller. Hairlines were measured by evaluating the samples using a 50 power loop. Printing
reverses were tested for each anilox roller using a Times typeface on the test form. The type sizes ranged from 3 to 12 points on a solid black background.
A positive Times typeface was also used on the test form in decreasing order from 12- to 3point type. Serifs were examined to determine the smallest possible printable serif for each anilox roller.
A critical element used on the test form was the gray bars. The gray bars were modified to print at different line screens: 65, 110, 133, 150, 175, and 200 lpi to determine how well they printed with each anilox roller in the study. The standard line screen to anilox ratio was 6 to 1.
Slur targets were included on the test form to set an even impression between the anilox roller and the plate cylinder. Paralleling of these settings is accomplished by comparing the targets on the left hand and right hand side of the web.
Bearer bars were used on each side of the test image to provide a common height in relation to the other images on the plate. Bearer bars are used to prevent the plate cylinder from bouncing.
According to our Dot Gain/Percent Dot charts, the reproduction curves for the anilox rollers show that the rollers with a high line screen and low BCM produced a lower dot gain than the lower line screen rollers. This occurred because the low line screen roller has
a high BCM or larger cell depth-toopening ratio, whereas the high line screen roller has a low BCM or lower cell depth-opening ratio. The results can be seen in the Figure 4.
Figure 5 shows the solid ink density value of the anilox rollers for different line screens. The 400 line screen anilox roller with a BCM of 4.76 printed denser solids. The 400 lpi roller, had an average density of 2.48. The 700 lpi anilox with a BCM of 2.50, had an average density of 1.98. The 900 lpi anilox with a BCM of 1.48, had the lowest density reading and an average density of 1.39. The 1600 lpi anilox with a BCM of 1.84, had an average density of 1.45.
From these observations, we concluded that there is a direct relationship between solid ink density and BCM volume. As the BCM went up, so did the density of the ink in the solids.
Reproduction characteristics of hairlines were examined visually. The 400 lpi roller printed thicker hairlines than the 700 lpi roller. The 900 lpi roller printed hairlines of similar width to the 1600 lpi roller. From these observations we noticed that the 900 lpi roller and the 1600 lpi roller work exceptionally well when printing fine hairlines.
The serifs and reverses printed better than expected with the higher lpi anilox rollers. Our printability study included type ranging from 3 to 12 points. While press sheets printed using the
400 lpi anilox plugged from 3- to 6-point type, 7-point type produced acceptable results. The press sheets from the 700 lpi anilox plugged from 3- to 4-point type, but 5-point type produced acceptable results. Press sheets from the 900 lpi and 1600 lpi anilox produced exceptional results with 3- point type and higher. The serifs looked complete and the bowls were not plugged.
A 150 line screen image was analyzed in a 5000 Kelvin viewing booth to determine highlight, midtone, and shadow quality reproduction. The image printed using the 400 lpi roller had poor definition in the highlights, plugging in the midtones, and had plugged dense black shadows. The image printed with the 700 lpi anilox had good reproduction in the highlights, plugged midtones, and slightly plugged shadows, resulting in unacceptable image quality. The image printed with the 900 lpi anilox had crisp and sharp highlights, acceptable midtones, and acceptable shadows. Its quality looked much better than that from the one printed with the 700 lpi anilox. The image printed with the 1600 lpi roller had excellent highlight definition, crisp and sharp midtones, and dense black shadows. Overall, the quality of reproduction for this image was excellent.
Anilox Roller Strengths and Weaknesses
The amount of ink laid down in the solids by the 400 lpi anilox roller was moderate since it produced a dense black image. The dense black color
is a result of a large cell volume. The 400 lpi roller performed poorly in the reproduction of halftones and type smaller than 6 points. The midtones of the image plugged, the bowls of the small positive type plugged and the negative small point type filled in.
The 700 lpi anilox roller performed as a good compromise between the 1600 lpi and the 400 lpi anilox rollers. The density was considerably lower because it has half the volume of the 400 lpi anilox roller. The image was murky and the fine type was not well defined.
There was considerable difference in halftone reproduction between the 900 and 700 lpi anilox rollers. The image printed better with the 900 lpi roller, since it had less volume than the 700 lpi anilox roller. Small positive and negative type printed well; the serifs were noticeable and bowls were well defined.
The 1600 lpi anilox roller performed excellently in all areas except when printing solid dense black. Fine type and hairlines printed sharply and were well defined. Excellent results were obtained when printing halftone images. Besides having sharper highlight dots, dense shadows and crisp detail in the midtones, the image printed very well.
Results from this project can provide prepress operators with the necessary data to compensate for anilox roller deficiencies during prepress activities. They can also serve as a guide when determining what type of anilox roller to use for a particular job.
The following areas of this project could provide further research:
* Use of a printing press with quantifiable plate and impression cylinder pressure. This would eliminate any variance in manual operation of the press, which could alter results in dot gain and density when changing anilox rollers.
* A test run of more 500 samples to choose from and measure. We recommend this sample size in order to provide enough repeatability and consistency throughout the run.
Since the use of flexography is growing rapidly, flexographic materials can be the focus of future research on such questions as the following:
1. Which anilox roller prints best with each process color?
2. How do anilox rollers affect process trap?
3. Do ultraviolet and waterless inks affect anilox roller printing?
4. How does printing plate dot shape affect anilox roller performance?
5. How does the angle of the doctor
blade affect the performance of the anilox roller?
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