By Olivia Cahoon
Digital printing continues to make inroads into manufacturing settings due to advancements including improved UV LED curing systems. Updated features like faster curing speeds, modular designs, and advanced control processes enable efficiency in industrial-type applications.
Above: Honle UV LED curing solutions reach high UV intensities for sheetfed printing.
Several industries benefit from the use of UV LED systems as a result of recent advancements in lower power consumption, less downtime, reduced cost of ownership, and lower temperature curing.
These features stir interest in the technology as a whole, but also from a spectrum of manufacturers. “The technology itself requires less input power versus conventional UV—lowering utility costs,” explains Simon Whittle, VP of operations, American Ultraviolet. The company celebrates close to 20 years of creating UV curing solutions for processes that use UV-curable chemistries like inks, coatings, and adhesives.
In newer UV LED systems, while heat is generated and needs to be removed, less input power is required, resulting in a reduced amount of heat when compared to conventional systems. According to Whittle, manufacturing facilities that utilize conventional UV systems require make up air to replace the air exhausted through the system and ovens—further complicating the process. “Imagine the cost savings from not having to do that,” he says.
Manufacturers utilizing digital print technology are taking advantage. For example, double production speed is achieved with thick, heavily pigmented inks, but with heavy inks also comes the need for industrial curing. “The LEDs’ high-intensity UV-A rays penetrate thick ink coats better for more effective curing with improved adhesion, delivering faster speeds with silkscreen and opaque white coatings,” offers David Lyus, sales manager, specialist UV systems, GEW (EC) Limited—a designer and manufacturer of UV LED curing systems for printing, coating, and converting applications.
According to Pamela Lee, senior product manager, OmniCure, Excelitas Technologies, the digital printing industry is arguably one of the more mature segments of the print market, with wider spread integration of UV LED curing solutions when compared to other industries. “These advancements in UV LED curing technologies help provide performance advantages, productivity enhancements, and environmental benefits.” Excelitas provides optoelectronics and electronic systems to original equipment manufacturers.
UV curing solutions migrated from small format applications and spot curing to larger area systems due to improvements in optical performance, LED efficiency, and costs. With these enhancements, Lee believes more economical UV LED solutions are available offering increased curing/operational speeds and support for a wider range of materials. She offers, “from an industrial print standpoint, it translates to higher power systems that can support larger print areas, while maintaining or increasing print speeds at a more economical cost.”
This is observed in wide format inkjet printing where UV LED is increasingly applied. “Demand for high output UV LED curing lights is rising among inkjet printer manufacturers worldwide to meet user demand for higher productivity and faster printing,” reveals Akira Taguchi, manager, EP devices development department, printing device R&D division, corporate printing device group, Kyocera Corporation, which is a supplier of electronic devices, semiconductors, printers, copiers, mobile phones, solar power generating systems, cutting tools, and industrial components.
In sheetfed printing processes, advancements in UV LED curing enables the ability to print on temperature-sensitive materials due to the low heat radiation of LEDs and automatic on/off controls without standby time or shortage of service life. Dieter Stirner, sales director, Honle Group, says automatic on/off controls are consciously used for sheetfed printing applications in a process called triggering. The Honle Group is based in Germany, but its U.S. partner Honle UV America, Inc. offers a full line of UV and LED equipment, sales, service, and support for North and South America.
Triggering occurs during curing, when LEDs are switched off between single sheets for a short interruption. “Though it might take only a blink of a moment, the LED service life can be extended over the years,” comments Stirner.
Consequently, triggering also increases the energy efficiency of LEDs, which according to Stirner, is decisively higher than only some years ago thanks to developments by LED manufacturers. Another important advantage of UV LED curing in digital printing is the possibility of format shut off. “By shutting off single LED modules, the irradiation widths can easily be adapted to the printing format.”
Some of the newest features on UV LED curing systems include higher UV intensity, faster curing speeds, and lighter, more compact devices. Together, these advancements improve the production process for manufacturers using digital printing practices.
Dimension & Weight
In recent years, UV LED curing systems are smaller and more lightweight due to higher efficiency UV LEDs now available in conjunction with advancements in thermal management technology. “The physical size of curing systems can be optimized for space-constrained applications, without compromising performance,” offers Lee. “Smaller mechanical form factors can then enable the integration of UV curing solutions into a wider range of applications.”
However, achieving a smaller, lighter curing device is not simple. According to Taguchi, one of the biggest challenges is maximizing UV intensity while minimizing the total system’s size. Kyocera’s approach to increasing UV intensity is to mount the LED elements in a high-density array. By optimizing a module design and Kyocera’s thermally conductive ceramic substrate, the company increases output while also reducing size.
While UV LED devices are lighter and more compact, it remains uncertain if they will continue to decrease in size. “I don’t think they are going to get much smaller than they are, but never say never,” comments Whittle. This is partly due to the fact that UV LED systems require a cooling device to quickly remove heat, which also takes a considerable amount of space to work efficiently.
Air & Liquid Cooling
At the junction point of the LED in UV curing devices, heat is generated and needs to be removed. LED performance, reliability, and lifetime are dependent on how effectively this heat is dissipated. “As such, thermal management is a crucial component in a UV LED system design,” comments Lee.
Air and liquid cooling are two methods of thermal management used to remove heat from the backside of the LED or LED array.
Liquid cooling is primarily used in applications where high power densities, small form factors, and large curing areas are required. “Water cooled solutions can more effectively manage low junction temperatures, but this comes at the cost of more complex integration and support infrastructures for the chillers and water pipes/connections,” says Lee.
This method is typically more expensive to implement due to additional materials, like a chiller, which removes the heat from the liquid before it recirculates back to the LED. However, Whittle points out that liquid cooling LED assemblies are typically smaller because no fans or heat sink are required, just piping for the liquid to pass through. “Liquid cooling is more efficient because it allows the junction temperature to be lowered and in theory more output from the LED,” he adds.
According to Lyus, liquid cooling is predominantly required to meet the needs of demanding industrial processes because water has a much higher specific heat capacity than air. “Our customers’ high-speed applications require a high dose and intensity of UV in a small space,” he offers.
On the other hand, air cooling is more cost effective and enables systems to be easily integrated due to reduced power consumption and minimized initial equipment costs. These units are standalone and do not require external infrastructure support. While the peak performance of air cooled UV LED systems is not as high as water cooled solutions, Lee says recent advancements in LED technology have made the output from air cooled systems more than sufficient for most curing applications.
Several air cooled UV LED systems have an output of up to 16 watts, which according to Stirner, makes for a viable alternative to liquid cooling. “This is much higher as what they could reach some time ago and about the same of what early water cooled systems could reach.” However, he notes that all printing processes requiring a very high intensity might benefit from a water cooled system as they reach higher intensities.
Additionally, air cooling systems require a heat sink and fans to effectively pull the heat away from the LED junction. According to Whittle, this adds extra weight, space, and potentially some fan noise, which is important for print providers to consider if seeking the most lightweight and compact system available.
UV LED curing solutions are designed with scalability in mind, which typically means adding more modules for wider widths and adjusting the LED array design for improved efficiency.
Scalability is achieved both internally via the electro-mechanical architectures and LED array design and externally through LED heads that can be stacked side by side to increase cure width, says Lee. “Having the ability to scale a solution whether it is in cure size or the speed of application allows for customers to have flexibility for growth.”
UV LED device manufacturers develop LED heads to be modular, allowing for stacking or adding wider widths and higher UV. However, Whittle cautions that power supplies and cable sizes may be the limiting factor if intending to run them off one cable system. “Most power supplies are 48 VDC for LEDs so for 3,000 watts of input power, the electrical distribution/cable needs to be able to handle over 60 amps or you need multiple cables for one power supply to multiple heads.”
According to Taguchi, a connectable 80 mm unit supports diverse applications by allowing control of the irradiation width from 80 to 1,600 mm. “This makes it ideal for diverse printing applications and media—from fine characters, figures, and barcodes smaller than 100 mm to printing on large scale building materials and films up to 1,600 mm.”
Performance optimizations result in higher output UV LED curing devices that offer more power and higher peak intensities. Lee offers, “these improvements in hardware capabilities in conjunction with the enhancements in formulations enables higher speed curing and thus faster process speeds.”
While improvements are made to UV LED hardware for higher UV intensities, optimized focusing optics and improved ink reactivity also affect curing results. According to Stirner, recent developments for focusing optics have resulted in very high intensities that can be reached at greater distances between the curing unit and substrate. “On the whole, one can say that the higher degree of efficiency leads to a permanent upgrading of UV LED systems.”
Despite ongoing efforts for higher UV intensity, the market may have hit a point where more doesn’t produce a proportional response. “They produce so much intensity at one wavelength that it’s too much for the photo-initiator to absorb all the photons regardless of how high the speed is,” explains Whittle. Photo-initiators absorb photons based on the ability to absorb in the wavelength and the formula’s concentration. “Increasing the concentration of the initiator can add significant cost and have adverse effects on the finished properties.”
In addition to UV intensity, cooling mechanisms, and smaller dimensions, UV LED curing systems are designed with hybrid capabilities and patented control processes.
Hybrid UV curing systems are available for those that prefer conventional UV mercury arc lamps but also want the option of LED. Cassette-based hybrid devices automatically recognize which cassette type is installed in the machine and adjust to provide the correct power type. Available on the same print unit, Lyus says these systems use the same lamp head casing, power supply, cables, and control, as suitable inks or coatings become available.
“Many LED systems are sold with mercury lamp capability and customers regularly alternate between the two to optimize curing to suit specific production runs,” he explains. With a full hybrid UV curing solution, like the GEW RHINO ArcLED electronic power supply, Lyus says print providers can provide either a conventional UV output for flexography or a high-intensity output often needed for inkjet printing. Additionally, the RHINO power supply unit can accommodate iron doped lamps sometimes used in inkjet processes where inks need a different UV wavelength for effective curing.
“The RHINO power supply streamlines all of the lamp head technologies that inkjet printers may need, depending on their ink set and process. For machinery manufacturers, it offers one supply source, one electrical integration, one control interface, and can reliably and robustly supply UV power for a range of digital applications,” offers Lyus.
Newer UV devices are also now designed with patented control processes to address individual UV LED modules. By adjusting the output of each segment, Lee says exceptional uniformity over the entire curing area can be achieved, while empowering customers with the ability to customize outputs with tighter process controls.
In addition to control processes, Taguchi says new communication functions allow users to track the status of UV LED curing lights, including LED module temperature, fan operating time, and LED drive current. Select systems like Kyocera’s also offer warning notifications when it’s time to replace filters or LEDs.
Rather than new features, Whittle believes there is more effort put into LED system controls and system status information, useful for production runs and maintenance. “Manufacturers have created enough higher intensity modules in different configurations that it’s time to start working on understanding how it all comes together—equipment, chemistry, and process to manufacture finished products effectively.”
Needs & Improvements
The use of UV LED systems in manufacturing environments is still a relatively new concept. Despite recent advancements, improvements remain for successful implementation.
As it is in any industry, reductions in equipment costs will further improve the adoption of UV LED curing systems in a variety of markets. From a materials standpoint, Lee believes additional enhancements in ink formulations with a wider range of cost-effective UV LED curable options will help better support the integration of these solutions. “As the performance of UV LED curing systems continue to increase, even faster print speeds will be supported and more importantly, acquisitions costs will go down.”
Although UV inks for LED curing are becoming common in the digital and analog printing markets, UV inks for conventional curing lamps are still widely used. “Current UV LED curing systems are still not fully capable of curing inks designed for conventional lamps,” offers Taguchi. As a result, the industry is in need of UV LED curing lights with the capability to cure any type of ink.
Aside from inks and materials, concerns are present for the chemistry and process of UV LED curing. It’s important to understand all the variables involved in implementing a UV LED process including the limitations that may require some adjunct to the LED technology to get the process where it needs to be. “Surface cure seems to be a limiting factor for LEDs, the better surface cure initiators work at the shorter wavelengths,” adds Whittle.
UV curing uses multiple wavelengths that depend on process requirements and travel at different distances though cured substrates. UV ranges include UV-A, UV-B, UV-C, and UV-V. While UV-A rays are capable of penetrating thick ink, developing a UV-C light source currently poses a challenge. “These remain an expensive and complex solution to a simple problem,” admits Lyus. “Mercury lamps still deliver a relatively high dosage of UV-C energy cost effectively, and GEW can now measure this output to ensure consistent irradiance, both across the width of the press and over time.”
As UV LED curing systems continue to improve with new features designed for lighter weights and improved scalability, these changes affect manufacturing environments implementing digital printing practices.
Sep2018, Industrial Print Magazine