Advances in technology have made the curing process more accessible.
While UV LED curing in the digital printing space has grown significantly over the past decade, most screen-printing operations have continued to use traditional curing technology. The primary obstacle wasn’t the readiness of the technology, but the cost. Until recently, LEDs were expensive, and the need in a screen-printing line to cure the full width of the print at once meant that many more LEDs were required compared to a wide-format inkjet printer. This tended to limit early development in the screen-printing industry to containers and other smaller formats where equipment cost was less of a factor.
Now that the costs have come down, the economic, environmental, and safety benefits of the technology have captured the interest of screen-printing companies, equipment manufacturers, and ink developers. Although relatively few new screen-printing presses are being built, a growing number of companies would like to replace their old mercury vapor curing systems with UV LED units.
Over the past five years, the power of the lamps used in UV-LED curing systems has more than doubled, the physical size of the lamps has been reduced, and costs have come down. Mike Higgins, director of sales, Americas, for Phoseon Technology, estimates that retrofitting a 20-inch-wide conveyor five years ago would have cost four or five times as much as it does today: “You’re getting a proven curing system at a fraction of the cost.”
Here are five facts you should know about UV LED screen printing.
#1: UV LED Curing Is More Efficient than Mercury Vapor UV Curing
Traditional UV lamps produce light by generating an electric arc inside an ionized gas chamber (typically mercury). After the atoms in the gas chamber are excited, they decay and emit photons. Mercury arc lamps produce light across the full spectrum of ultraviolet light (from 100 nanometers [nm] to 1800 nm). In addition to UV-A rays, this includes harmful UV-B and UV-C radiation that is hazardous to workers’ eyes and skin.
According to Higgins, mercury lamps run very hot, which can cause heat-sensitive printed materials to warp and wrinkle. Plus, mercury UV curing systems generate ozone that must be vented away from the work area. Using LEDs to cure UV inks doesn’t generate ozone – nor does it transfer a lot of heat to the printed surface.
UV LED lamps are solid-state semiconductor devices. They produce light by generating a voltage to join positive holes with negative electrons, emitting energy in the form of photons. The light is focused within a narrow spectrum of ultraviolet wavelengths. Phoseon LED curing units produce light only within the UV-A range, with wavelengths of 365 nm, 385 nm, 395 nm, or 405 nm.
Because the power going into a mercury lamp is distributed across a broad spectrum, less than 10 percent of the power going into the lamp is converted into UV curing energy. In contrast, Phoseon LED curing lamps convert greater than 30 percent of the input power to UV curing energy.
Unlike mercury lamps that need time to warm up and typically run all day, LEDs provide energy instantly and can be switched on and off as needed. Print shop employees can turn off the LEDs every time they take a 15-minute break; some units are configured so the LEDs turn off between prints, providing further efficiency. LED curing units are expected to operate for more than 20,000 hours compared to less than 2000 hours for a mercury arc lamp.
UV-LED curing systems are offered either in air-cooled or water-cooled packages to ensure that the LEDs operate at a consistent junction temperature (the highest operating temperature of the semiconductor) during production and withstand harsh production environments. Phoseon has been making patented LED solutions for life sciences and industrial curing applications since 2002. For the printing industry, they make UV-LED curing systems for inkjet, screen, and narrow-web flexographic printing systems.
For screen-printing applications, Higgins recommends Phoseon’s FireJet FJ200, a 395-nm, 16-watt/square centimeter air-cooled curing unit.
#2: A Growing Number of UV LED Inks Are Available
In order to take advantage of UV LED curing, you need inks that are formulated to cure within the high UV-A wavelengths produced by the LED lamp. UV LED screen inks include photoinitiators that absorb the specified UV-A wavelengths needed to ensure a full cure at the desired speed.
As the benefits of UV LED curing have started to be proven in real-world applications, screen-ink manufacturers have followed this trend by introducing formulations that enable more screen printers to use the technology.
Today, companies such as Nazdar, Norcote, Sun Chemical, Ruco, and Marabu offer UV LED inks for specific applications, including bottle decorating, container printing, durable decals, membrane switches, and indoor and outdoor point-of-purchase graphics. Some UV LED screen inks are “dual-cure” formulations that also work with traditional mercury vapor curing systems.
Each new ink formulation must provide the desired curing performance at the speed of production, and must also meet the adhesion and durability requirements of specific applications.
According to Jeff Morris of Ruco USA, the performance of UV LED inks has been proven in the glass-bottle decorating business for more than five years. Because retrofitting industrial equipment can be expensive, buyers of glass-decorating or container-decorating equipment often specify UV-LED curing units when purchasing screen-printing equipment for high-speed, high-volume container decorating.
#3: UV LED Lamps Can Be Built into Industrial Screen Presses
In 2011, Empire Screen Printing collaborated with Nazdar, Phoseon, and KBA-Kammann to develop the first roll-to-roll screen-printing press with built-in UV LED curing technology. Empire continues to use this Kammann K61 Eco-Press to produce rolls or sheets of outdoor-durable labels.Tags: UV LED Technology | Screen Printing Magazine
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