An Update on UV LED Screen Exposure

The Wave of the Future?

Every technical support person in the industry has at least one anecdote of a mysterious quality-control problem that was eventually traced to the exposure unit sitting in the corner of the screenroom. My favorite involves a shop that suddenly began experiencing screen breakdown during its production runs. The technician checked the ink and press parameters to see if something was attacking the stencils. She looked into QC records from the emulsion manufacturer, the coating procedures in the shop, and the exposure unit’s functionality and could find no explanation. Finally, the technician asked the worker who exposed the screens if he was doing anything differently. No, he explained: Each time he exposed a screen, he turned the unit on, lit a cigarette, and stopped the unit when he was finished smoking, knowing the screen would be finished. The punch line is that he had recently switched cigarette brands and the new one burned more quickly, so all of the screens he was producing were underexposed. I suspect that few other shops used the “Marlboro method” to calculate exposure times, but the story illustrates how little attention has traditionally been paid to a process that can cause a myriad of production and quality problems when it isn’t done correctly. As screen printers adapt to the realities of falling run lengths, faster turnaround times, and narrower margins, they can’t afford such bottlenecks and errors in their prepress any longer. Recently, screen exposure has become a very active topic due to the emergence of a new class of exposure systems employing a light source that has become ubiquitous in many facets of our lives—LEDs (light-emitting diodes). Exposure systems powered by UV LEDs began hitting the market in 2013 and have quickly attracted a lot of attention, particularly among garment printers. They offer several tantalizing advantages over conventional light sources and the potential to streamline prepress workflow. Not surprisingly, they have also prompted some questions among potential users and more than a little debate between technology developers. The basics Screen exposure involves just two fundamental goals: Reproduce the artwork in the positive faithfully, and harden the non-image areas of the screen thoroughly so that the stencil will be durable throughout the print run. In order for these things to happen, the stencil system must be exposed to sufficient amounts of UV light in the correct wavelengths to trigger the photoinitiators and completely crosslink the emulsion. Most stencil systems today employ diazo, SBQ, or a combination of the two to make the emulsion or capillary film photosensitive. The peak sensitivity of these photoinitiators usually falls between 360-420 nm. Printers have used a variety of light sources to expose screens over the years, including older technology such as carbon-arc and quartz units that are seldom seen today. Before UV LED, most modern exposure units used metal-halide, fluorescent, or a combination of halogen and fluorescent bulbs. Metal halide is considered the top-end choice because it is more powerful, requires a single bulb (except for the largest screen sizes), and is capable of providing faster exposure times with better ability to hold fine details. Fluorescent units are cheaper and commonly seen in small garment and startup operations. Most systems are furnished with glass and a vacuum blanket that draws a traditional film positive into very close contact with the emulsion-coated screen so that light doesn’t undercut the image during exposure. Shops using inkjet- or wax-based CTS (computer-to-screen) systems that jet the positive image directly onto the screen aren’t as concerned with this step since film is no longer involved. Laser-based CTS systems that image and expose the screen simultaneously, eliminating the positive altogether, are also available, though these higher-end systems are seen mostly in niche markets and won’t be discussed in this article. LEDs aren’t light bulbs—they are made from a type of semiconductor that emits light when energized. The wavelength that an LED emits depends on the chemical composition of the semiconductor, which manufacturers can adjust to create LEDs across a wide range of the light spectrum. Because the most lucrative opportunities for LEDs have been in lighting applications, most of the R&D work to date has occurred in the visible light spectrum, not the UV wavelengths required for screen exposure. But research activity for UV-curing LEDs has picked up recently, in part because of their emergence in wide-format inkjet printing. For screen exposure, UV-LED technology offers several promising chacteristics: • Speed: Although many UV-LED systems are similar in appearance to fluorescent units, they expose screens significantly faster—as much as 10-15 times more quickly. Exposure times in textile applications using pure-photopolyer emulsions, for example, can be as low as 5 sec. Competitive claims vary from one manufacturer to the next, but most agree that UV LED is comparable or faster than metal-halide technology as well. • Efficiency: LED systems use about one-fifth the power of comparable metal-halide units during operation, and provide additional savings because they only consume energy when screens are being exposed. Unlike LEDs, metal-halide bulbs take about a half minute to warm up, and turning them on and off ages them more quickly. Since replacement bulbs are expensive (about $300-400 each), most users leave their units on throughout the day. Some metal-halide systems have a lower-wattage idle setting so that less energy is consumed between exposures, but in practice this means that metal-halide systems draw about the same amount of energy when they aren’t being used as LEDs do during screen exposure. LED units also generate far less heat. • Consistency: Traditional light sources such as fluorescent and metal halide degrade over time, delivering less intensity as the bulbs age and thus requiring light integration to adjust screen-exposure times. LEDs are chips, not bulbs, and they don’t degrade as they age. A properly manufactured 395-nm LED should always deliver that wavelength until it fails. • Longevity: Traditional bulbs must be replaced periodically. Most manufacturers suggest replacing metal-halide bulbs every 12-24 months, for example, depending on usage. LEDs, in contrast, are designed to last for thousands of hours of usage. Since they expose screens quickly and don’t consume power between exposures, LEDs have the potential to last for the working life of the equipment. Questions and debates So what’s not like about a technology that promises speed, efficiency, and cost savings? Because UV LED exposure units are so new to the market and nearly all the initial placements have been in one sector (garment printing), the potential limitations of the technology aren’t yet fully understood and are to a large degree a matter of speculation. The most commonly raised question with UV LED technology is its resolution capability. This debate centers on the long-held maxim that a single-point light source such as metal halide (or even the sun) produces sharper stencils than a multipoint source such as fluorescent or halogen-fluorescent. When light comes from multiple angles, it undercuts the positive during exposure and detracts from fine details in the stencil. By definition, all UV LED units are multipoint systems, prompting understandable concerns about their resolution. Manufacturers dispute this, saying that their units use a variety of techniques to focus the light and prevent the problems that have plagued conventional multipoint systems. “Multipoint light sources aren’t necessarily bad,” said Mark Vasilantone, president of Vastex International. “There is nothing wrong with multipoint as long as the light travels vertically through the screen.” David Landesman, co-president of Lawson Screen & Digital Products, agreed. “Although UV LED is not theoretically a single-point source, in the vast majority of screen-exposure applications, we can’t tell the difference.” (Both companies, it should be pointed out, manufacture both UV LED and fluorescent systems; Lawson offers metal-halide units as well.) Stencil durability is the second question raised by skeptics. In part, the argument is built upon the premise that UV LED doesn’t produce the same level of detail as metal halide. Underexposing screens intentionally in an effort to hold fine details is an unfortunately common practice in screen shops, leading to stencils can break down during the print run from the combined forces of the squeegee, inks, and cleaning solvents. The use of a technology that compromises resolution, the argument goes, makes such intentional underexposure in the field a self-fulfilling prophecy. “I’ve heard the underexposure argument a lot,” offered Geoff Baxter, director of the digital products division of M&R. “Honestly, it makes pretty good sense. But it hasn’t proven to be true. We have people printing very fine details from LED-exposed screens and running tens of thousands of impressions with no screen degradation.” (Like Lawson, M&R also builds metal-halide and fluorescent systems.) Another concern over stencil durability stems from one of UV LED’s strengths—its delivery of specific, concentrated UV wavelengths. One reason that UV LEDs are much more efficient than traditional light sources is that they don’t have the same broad spectral curve. Traditional systems peak in the longer-wave UV required by screen-printing emulsions, but they also generate a lot of light above and below the peak frequencies that stencil experts say helps to facilitate a more complete cure of the stencil. “For optimal performance, stencil systems require a couple of different wavelengths,” said Neil Bolding, marketing and quality/technical manager of Macdermid Autotype. “I’d say that 380 nm is a good starting point, but you also need some [output] that goes a little lower and some that goes a little higher. Especially with dual-cure products, you need to kick off the photoinitators to start the photochemical reaction, and you need some of the shorter wavelengths to penetrate through the stencils as well.” Yuliya Finkel, manager of quality assurance and applications for Ulano Corp., also pointed to the tight spectral output of LED as a challenge. “LEDs have a fairly narrow spectral range—plus or minus 5 nm, if that. Unless the manufacturer is able to combine different wavelengths in one device, they can be fairly limited in the range of stencils that will work well.” Some manufacturers address this by altering the placement or power supply to fixed-wavelength LED strips. Lightspeed Equipment’s models have standalone, individually powered LEDs of two different wavelengths positioned in a geometric pattern a feature owner Shawn McPherson says is patented. The other key question about UV LED is whether the technology will expand to applications other than mainstream garment printing. Currently, Lawson offers the largest standard size at 48 x 72 in.; M&R and Lightspeed report a few custom units placed with graphics customers. None of the manufacturers sees any cost or technical reasons that larger models can’t be built, but the first wave of UV LED systems was clearly designed with the textile market in mind. Any challenges that may await in graphics and industrial applications have yet to be identified. Even in the textile market, several sources interviewed for this article discussed isolated challenges with unusual applications—long runs with discharge inks, for example, and extremely thick capillary films for HD printing. It’s important to note that questions such as these are inevitable with any new, potentially disruptive technology. The machines will evolve based on feedback from early adopters and as new LED technology emerges. Baxter noted that just two months after M&R previewed its first i-Image system, a new class of LEDs hit the market that allowed the company to re-engineer its prototype and dramatically increase the number of LEDs in its system. Moving forward If you’re thinking of making a purchase, give some thought to how the new technology fits into and could possibly improve your prepress workflow. If you are running strictly CTS screens and no longer use film positives, for example, the vacuum frame and glass may not be required. Many of these units are available without these features, simplifying operation and reducing exposure times by eliminating the glass. Others further leverage the synergy between UV LED exposure and CTS imaging with unique configurations. Lawson and M&R offer vertical models that conserve floor space; M&R’s uses a scanning LED head mounted to an X-Y carriage that operates much like the print-head assembly on an inkjet printer. The i-Image line, also from M&R, is a combined CTS/UV LED machine that images the screen on the inward passes and exposes it on the return. You might also consider whether a change in screenmaking consumables is in order. Some of the choices you’ve made in the past to get faster exposure times with a weaker unit—using white mesh, for instance, or a pure-photopolymer emulsion with an ink for which that technology isn’t recommended—may be compromises you no longer need. Several sources observed that some emulsions can cure so quickly on these systems that the margin for error is scant. Just an extra second can represent a 20% increase in UV exposure with the fastest emulsions. Finally, all sources interviewed for this article agreed on one point: If you’re thinking of buying a UV LED exposure system, have some test screens shot on that device using your artwork, emulsion, and coating procedures. Run the screens in production if possible. As with any technology substitution, real tests done under your conditions will be infinitely more valuable than sales literature or theoretical debate. Lawson Screen & Digital Products, Inc. The LED-5000 line from Lawson, a company with a 65-year history of building equipment for graphics and garment printing, includes four benchtop horizontal units that can be mounted on optional floor stands or the company’s screen-drying systems. Features include digital timer; programmable exposure settings; heavy-duty, nonreflective rubber-flex vacuum blanket; multichannel vacuum chamber with instant draw; and hydraulic-lift hinged frames. Lawson also offers a self-contained vertical unit designed specifically for use with CTS systems, as well as a kit for converting existing fluorescent exposure units to UV LED technology. Lightspeed Equipment Co. The LS line from Lightspeed Equipment doesn’t use LED strips. Instead, the machines have individually driven LEDs in a geometric configuration. The system uses two LEDs that peak at different spectral frequencies, a patented feature that owner Shawn McPherson (a 35-year industry veteran) says provides a full emulsion cure. Larger custom units are available with optional Venture vaccum frames. Lightspeed LED panels are offered as an option in the CTS units from NeoSystems LCC; the cabinets under the units have been modified to hold a Lightspeed panel, with exposure done in a separate step after the screen has been imaged. M&R M&R, a full-line provider of printing and prepress equipment for the screen-printing industry, has combined CTS imaging with UV LED exposure in its i-Image line. Screens are placed in the device, then imaged on the inward pass and exposed on the outward pass in a single integrated process. M&R’s D-Scan vertical exposure system is currently the only model that uses a smaller scanning LED head on an X-Y carriage that operates much like the print-head assembly on a wide-format inkjet printer. Two Starlight models have more standard horizontal configurations. Vastex International The E-2000 line from textile-equipment specialist Vastex includes four models. Each is configured horizontally and supplied with a vacuum frame and rubber neoprene blanket and shatter-resistant, ¼-in.-thick tempered glass. All of the units have fixed strips of LEDs, ranging from 3-11 strips depending on the size of the unit. Vastex also offers a kit that allows users to convert existing fluorescent exposure units to LED, reportedly compatible with most makes and models.
View more from this Screen Printing issue