Particularly interesting is the phenomena of donut dots. “A theory that I have come to accept is that the missing ink in the center of the printed dot is the result of a hydraulic vacuum,” reveals David Smith, manufacturing project manager at Cyber Graphics. “When a non-porous dot on plate makes contact with a non-porous substrate, the ink replaces the air and a hydraulic vacuum is formed. When the contact breaks, the vacuum pulls the ink from the center of the dot.”
Having dots made of cells prevents the vacuum from being formed in the first place. The ink has some place to go. It fills in the cells and spreads outward to the edge of the dot. There’s no “force” to pull the ink from the center of the dot.
“A feature of the new screening made specifically for LED plate exposure is that the cells go all the way to the edge of the dot,” adds Novaflex CEO Stéphane Pinel. “With bank light screens we were forced to use dots with protection rings around the perimeter of the dot for a number of reasons. These solid rings prevented the ink from freely spreading outward to the edge of the dot and therefore prevented the ‘break’ of the hydraulic vacuum. The result was donut dots. It wasn’t until we introduced screens with cells or groves going all the way to the dot edge that we truly solved the donut dot issue.”
Cell Size
The cells on the flexographic plate are actually imaged into the mask on the CDI at the same time all of the other graphic elements are imaged. A core technology that enables print quality optimization is the ability to control the size of the cells independently from the size of the other graphic elements.
“The basic image exposure on a flexo plate imager is set to achieve a 1:1 reproduction from digital file to mask,” says Bill Hounshell, VP of printing services at Phototype. “But the cell exposure can be set separately. We set the cell exposure to achieve optimum ink transfer on the printed web—it’s similar to anilox cell optimization.”
Finding the optimum cell size for a specific set of printing conditions is straightforward. A step test element is output on the plate imager, then printed on press. Through the use of visual analysis and measurement tools, the user chooses the optimum cell size. “It’s been interesting to watch the development of the print step test strips over time,” recalls SGK St. Louis VP of operations Randall Peek. “The first test strips contained only solid blocks. We basically looked for the highest solid density. The new strips, developed for LED plate exposure, include multiple surface patterns—cells and grooves—in solids and halftone dots. The step test has become a critical part of on-boarding converters with the new technology.” (see Figure 5.)
Min Dot
Equally important to plate surface optimization is min dot optimization. Even with the notable improvements flexography has made over the last few years, fading to zero is still considered the greatest challenge.
“To make a long story short, fading to zero on a production job requires the use of a transitional screen,” states Steve Simpson, director of color services at Phototype. “At some point in the gray scale, the AM dot needs to be held at a fixed size and lower gray levels achieved through dot population reduction.” XPS plate exposure is critical at this step because of highlight dot consistency. The actual size of the dot that can be held on plate varies with the plate material, but with most plates exposed on LED, it’s less than 1.0 percent. They key is to optimize based on print results.
Sheppard comments: “Ideally, we like to use FM spot sizes below 20 pixels and min dot percentages below 0.8 for artwork—like drop shadows—that must fade to zero. Achieving this on a bank light can be very deceiving. You can tweak your main exposures and perhaps hold an 0.7 percent in the front of the bank light frame, but in the back—an area that was not in the main exposure test—the same dot is dropping out.
“Of course, you can increase the exposure time to hold the 0.7 percent in the back of the bank light unit, but now you have over-exposed the front of the plate,” he continues. “You see line broadening, filled-in reverses, dot gain on the plate. With XPS, if you can hold an 0.7 percent dot in one area of the plate, you can hold it everywhere. And the plate is never over-exposed. It’s actually the consistency that enables the use of smaller min dots on the plate.” (See Figure 6).