“Hey Freddy… could you remake this plate with just a smidgeon more back exposure and a hair less main?”
As we enter the third decade of the 21st century, requests like this remain commonplace in platerooms around the world. Fortunately, they are fast becoming a thing of the past for those equipped with the latest LED plate exposure technology. That’s because:
- With units like Esko’s XPS Crystal LED, exposure parameters are not determined by individual users on site, but instead, by a team of flexographic plate experts in a controlled lab environment at a central location
- Exposure data is then distributed to LED plate exposure units around the world via a database update over the internet
“All the operator does is call up the plate type from a menu and hit the start button,” reflects Mike Sheppard, vice president of operations at Empire L.A. in Montreal (see Figure 1). “The certification process is just as simple. It ensures that our plates meet the same rigid standards as those made in the Esko controlled lab environment.”
There is little debate that the world of flexographic plate exposure is moving from conventional bank light to LED. The only question is the speed at which the transition will occur.
- Conventional bank light plate exposure units use high UV output fluorescent tubes arranged in “banks” of lights to cover the plate exposure area
- Moving source LED plate exposure systems use solid state LEDs arranged in non-direction “grids” on circuit boards
- Fluorescent bank lights have a large amount of variation, require warm-up time and generate heat—which adds to variation. Additionally, because they contain mercury, bank lights bulbs are classified as a hazardous waste for disposal purposes
- LEDs have extremely high consistency, no warm-up time, generate very little heat and can be disposed of as normal waste. Add to that, the fact that LEDs last at least 10 times longer than fluorescent bank lights and have numerous sustainability benefits, and the future of LED plate exposure is clear (see Figure 2)
- In Esko’s patented LED illumination system, an optical system that further mixes the radiation for purely homogenous light output is placed in front of LEDs. The grid/optical system of uniform light then travels over the plate exposure area, providing an extremely even exposure—main and back—to the flexographic plate
“The main draw of LED plate exposure technology is consistency,” states MacDermid Graphics Solutions Applications Development Manager Brian Cook. “We have found that the LED exposed plates we make in our lab are more consistent, both in relief and dot reproduction, than bank light exposed plates.” The difference between LED and Bank light is similar to the difference between solid state electronics and vacuum tubes. Vacuum tubes are rarely used in consumer or industrial applications today and there’s a reason—the technology has been replaced.
Vacuum tubes were first replaced by transistors back in the 1960s, then by microchips in the 1980s and beyond. The same trend is now in full force in lighting. And the reach extends far beyond flexographic plate making. LED is predicted to replace just about every type of light source in both consumer and industrial applications over the next decade.
An LED is a solid-state device that converts a small amount of current into visible light energy in a very efficient way. By controlling the amount of current, the amount of light that hits the plate (the irradiance) is precisely controlled:
- LED lights can be calibrated at the individual diode level, such that each source emits the exact same amount of light
- By manipulating properties of the semiconductor material or the bulb coatings, the color of the output can be controlled
- In the case of LED lighting for flexographic plate exposure, these properties are finetuned to match the UV sensitivity of the flexographic plate material
Try This at Home
You can see the difference between a bank light dot and a UV LED dot on the plate with a normal magnifying glass (Figures 3b and 3c). The bank light dot has directional properties; the LED dot has a smoother, uniform, non-directional structure. Part of this uniformity is related to the basic principle of LED illumination.
To see this for yourself, turn on the flashlight of your cell phone and shine it on a wall. You will notice a near-perfect circle with no rings or non-uniformities (see Figure 3a). Do the same thing with your old incandescent flashlight (if you still own one!) and notice the rings and other non-uniformities.
To see the directionality flaws of bank light illumination, locate any tube-shape fluorescent light in your home or office. Rotate an object (e.g., a pen or ruler) under the light source and observe the shadow. Notice that the shape of the shadow changes radically as you turn the object. When held parallel to the tubes, there is very little shadow. When held perpendicular to the bulbs, there is a large and non-uniform shadow.
Repeat this test under an LED light (a light in your home or office, or even your cell phone flashlight). You will notice that the shadow is exactly the same, regardless of the angle at which the object is held. LED light can be easily engineered to be non-directional. This makes for uniform flexographic plates and is part of the reason for the consistency benefits of moving source LED plate exposure technology.
The advent of improved flexographic plate exposure brings with it improved screening. It’s natural to connect these together, offering screening technology and optimization procedures that weren’t available for bank light systems (see Figure 4).
“The defining characteristic of LED plate exposure is long-term consistency and repeatability,” states Pat O’Connor, VP, manufacturing at Lincage Imaging Systems. “This enables a lot of options with respect to screening. The exposure of surface screening designed to enhance ink transfer is no exception. With LED, all plates are flat top. The 1:1 reproduction enables us to image very fine ink metering patterns into the surface of the plate material—not only in the solids, but also in the halftone dots. Regardless of the screening selected, we are confident that the plates we produce in the future will be the same as the plates we’ve imaged and exposed in the past.”
The mechanics of plate surface patterns and ink transfer have been known since we began putting cells in plate solids in the late 1990s. Because LED plate exposure can hold finer surface patterns, it enhances these benefits. When printing on paper, a smooth “non-pattern” plate surface works nicely—the extra ink on the smooth plate surface is absorbed into the paper. In most cases, solids are smooth, text is sharp, dots are uniform. The paper serves to “meter” the ink.
Flexible packaging is another story. When a smooth surface plate comes in contact with a smooth surface film, the extra ink has nowhere to go. Solids become mottled, text appears to halo, dots resemble donuts. “A common explanation is that cells in the plate surface serve to carry the ink,” states Chuck Schoen, plateroom manager in the SGS Minneapolis hub. “Actually, the surface patterns in the plate serve to meter the ink. They prevent the ink from ‘fighting’ itself. Our customers report using less ink with no loss in density, while at the same time getting better solids, type, reverses and dots. LED exposed plates yield superior resolution and greater uniformity. This translates into more uniform ink transfer on press.”