UV-LED Curing for Industrial Printing

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1 UV-LED Curing for Industrial Printing (March 2012) posted on Tue Apr 24, 2012 UV-LED curing units have become more efficient in delivering higher energy to the By Richa Anand UV-LED curing refers to a technique that uses energy output from light-emitting diodes (LEDs) in the ultraviolet (UV) spectrum to treat inks, coatings, adhesives, and other UVcurable materials. The energy generated by the ultraviolet light triggers the chain reaction, resulting in polymerization of the material and the hardening (or curing) of the material. Traditionally, mercury-based UV lamps have been used for curing, but now more energy efficient and environmentally friendly LED-based UV technology has proven a superior solution in the printing industry. LED curing technology uses semiconductorbased LEDs to project UV light when an electric current is passed through them. When an LED is forward-biased, electrons are able to recombine with electron holes within the device, thus releasing energy in the form of photons. The color of the light emitted, or corresponding energy of the photon, is determined by the energy gap of the semiconductor material. LED lamps are recognized for their lower energy consumption, longer lifetime, improved robustness, smaller form factor, and faster on/off switching. But how do they work?

2 UV-LED curing There are three key components of a UV-LED curing system that, when optimized, provide an economically advantageous, high-throughput solution to the printing industry: materials (inks) that can absorb energy in the UV spectrum to undergo polymerization process; LED curing lamps that provide energy in the UV spectrum of the spectrum; and a printing system in which a UV-LED lamp is integrated to cure material that passes underneath it. These elements together provide a long-term, sustainable printing method through green technology, eliminating ozone emissions and lowering energy consumption. UV LEDs have a narrow spectral output centered on a specific wavelength, ±5 nm. LEDs are solid-state devices and can be built with diodes of various wavelengths, including but not limited to 395, 365, 385, 405, and 410 nm, unlike the broad spectrum of wavelength output by Hg-based lamps. This monochromatic distribution (Figure 1) requires new chemical formulations to ensure proper curing of UV inks and coatings. Currently, the most popular wavelength is 395 nm, with 365 nm being used in specific applications. Figure 1: Monochromatic distribution (wavelength) UV-LED curing lamps consist of multiple sub-components that, when combined optimally, can drive system performance. Key components of the UV LED light source can be summarized in four major categories. UV-LED curing units have become more efficient in delivering higher energy to the LEDs Light-emitting diodes consist of semiconducting material that is doped with impurities to create a p-n junction. Charge carriers, both electrons and holes, flow into the junction from electrodes (anode and cathode) with different voltages. When an electron meets the hole, it falls into a lower energy state, resulting in release of the energy in the form of a photon (Figure 2). The wavelength of the light emitted depends

3 on the band-gap energy of the materials (dopants) forming the p-n junction. The right combination of LEDs maximizes total UV energy. Figure 2: An example of a p-n junction as it applies to LED emissions Arrays This term refers to a grouping or clustering of individual LEDs. The number, type, and size of LEDs including the shape of the array and the method of connecting the LEDs electrically all impact the array. Array architecture is targeted for air- and/or water-cooled systems. Optics Photons coming out of the light source are optimized by using various optical layouts. Optics are used in reflecting, molding, guiding and/or shaping the UV LED emission to maximize the energy reaching the media and cure the UV inks or coatings in various applications. The use of optics has three benefits to the user: increased efficiency of the UV energy irradiated on to the material; lower array-generated heat; and optimum system pricing. Temperatures UV LEDs last up to 20,000 hours and beyond if they are maintained at proper operating temperatures. As LEDs emit more energy, they also generate more heat, which needs to be managed. Thermal management removes excess heat from the system while providing a consistent operating temperature for the diodes to function at maximum performance.

4 BENEFIT FEATURE ECONOMIC Energy Efficient Long Lifetime Low Maintenance Low Operating Temperatures ENVIRONMENTAL Mercury Free Ozone Free Workplace Safety UV-A Wavelength ADVANCED CAPABILITIES Heat Sensitive Substrates Deep, Through Cure Small, Compact Machines Controlled Curing Intensity Table 1: Benefits of UV-LED curing Ink formulations and materials With advancements in the availability of UV-LED-optimized ink chemistry, UV LED sources have become a very viable curing solution for many in the industrial printing industry. Materials suppliers have noted the benefits of UV-LED curing (Table 1) in general and have responded to the demand and challenge in the printing world to formulate raw materials that absorb energy corresponding to the output wavelengths of LED-based UV curing units. Industrial screen printers, for example, have found that advancements in LED-lamp technology and UV-curable ink formulations make UV-LED curing a viable alternative to medium-pressure mercury lamps. Additionally, high-power, scalable, UV-LED curing systems are effective high-speed curing in screen printing applications. One of the key ingredients in the chemical formulation is a photoinitiator that serves as a catalyst to start the polymerization process when exposed to narrow spectrum UV LED energy. And with the continued, widespread acceptance of UV LED systems, availability of suitable base materials continues to grow. The driving factors in advancement of chemistry of raw materials is increased capability and cost effectiveness of commercially available UV-LED curing lamps. Applications in industrial printing and decorating UV-LED curing allows the printing industry to explore new and challenging applications. The following represent just a few examples.

5 Medical labeling The pharmaceutical industry demands exacting product development and production in a sterile and clean environment. The industry has continued to push the envelope of what is possible; enabling new advances in medicine. UV-LED technology brings inherent advantages to the world of printing, including the lack of IR radiation in heat-sensitive applications in cleanroom conditions. Bottle printing Today, UV-LED curing technology is integrated in printing units for a variety of label applications, including bottle-labeling machines and systems designed for direct printing on cylindrical beverage containers. The small size of the light sources makes them ideal for machines with limited space. It also allows for printing on heatsensitive substrates without damaging the materials. These solutions enable users to process a variety of materials at maximum production speeds, with a fraction of the power requirements of a typical arc lamp. Printed electronics and photovoltaics Mass-produced consumer electronics and tighttolerance photovoltaics applications benefit from the long life, reliability, and repeatability of UV-LED curing systems. The ability to function at optimum levels, even during long-lasting, high-volume jobs and provide consistent output when curing very sensitive products makes UV-LED technology an effective solution in these markets. Conclusion UV-LED curing is now an accepted tool in the printing industry. It opens challenging applications to industrial printers who specialize in a variety of imaging processes and push the formulation of advanced inks, coatings, and other consumables. At the same time, UV-LED curing units have become more efficient in delivering higher energy to the