by Cassandra Balentine
UV and UV LED curing processes utilize ultraviolet (UV) and UV light emitting diodes (LED) to transform photoreactive materials—inks, coatings, and adhesives—to instantly fix to solids. UV LED is gaining popularity due to its reduced energy consumption, faster cure times, and minimized heat emission compared to traditional UV lamps.
These systems continue to advance to improve productivity and maximize output when added to industrial production lines.
When researching UV/UV LED curing systems, there are several aspects to consider including the cooling apparatus, extendable profiles, mounting systems, as well as media and surface support.
The more power emitted during UV curing, the more heat generated. It is essential to be able to control and remove this heat. There are two types of cooling methods for UV cure systems, air and water.
“Whether it is for UV LEDs, microwave, or mercury lamps, water and air are the two cooling methods for thermal management of UV curing systems,” shares Pamela Lee, senior product manager, OmniCure UV LED curing solutions, Excelitas Technologies Corp.
Air-cooled systems are simpler and provide lower total system economics while water-cooled systems provide smaller form factors and feature higher peak irradiance, according to Stacy Hoge, marketing communications manager, Phoseon Technology.
Air systems are also simple to mount and can be used in a moving head application where the UV head is moved around the curing media, for example on the end of a robot arm. “It seems like the simplest solution, and in a lot of applications it is, but it does require a cool supply of clean air to operate in so it may not be suitable for applications inside a host machine or near components that are sensitive to air movement, such as inkjet printheads,” points out David Johnson, sales director, Integration Technology Ltd. He adds that UV output air-cooled systems will fluctuate with ambient air temperature, as the temperature increases the output is reduced.
“For smaller systems, a small air-cooled chiller is the least expensive option and it would be located near the UV lamps, but it does put heat into the air, which can increase the load on the factory HVAC system,” offers Michael Derrick, director of key accounts, AMS Spectral UV, a Baldwin Technology Company.
For large cooling requirements, a remote condenser chiller might be the best option. “Typically, a condenser would be located outside of the facility while the pump and tank of the chiller are still located near the UV lamps. This removes the heat from the factory but is a more expensive option,” he explains.
On the other hand, for water-based cooling systems no air movement makes them suitable for use in clean rooms and inserted glove boxes. “Furthermore they have no air movement around the head, which makes them suitable for use near inkjet printheads,” says Johnson. “Water-cooled systems require a cool water supply, but this maintains a fixed cooling temperature for the diodes and stabilizes UV output. They can be fitted in tight enclosed spaces inside machines.”
Derrick points out that if there is already cooling water available in the factory from a central chiller, options may be a water-to-water heat exchanger, or a water-cooled chiller. “The benefits of these chillers is that they put the heat from the lamp process water into the factory cooling water, and not the air.”
To determine which cooling approach is most appropriate for your organization, Lee suggests considering factors such as application needs, budget, and design/integration constraints.
“Start by assessing baseline requirements such as performance and output specifications, including process speed and dose needed to cure the ink and what the minimum power/irradiance is necessary for the targeted application, and whether or not the cooling apparatus would support this output; real estate, including space limitations and if chillers can be accommodated and if curing units fit in the printer; and investment and operating costs,” she offers.
Lee encourages manufacturers to consider the total investment in the curing equipment and supporting infrastructure, including chillers, power supplies, ventilation installation costs, maintenance, replacement parts, and downtime.
The ability to mount a UV/UV LED curing unit to production lines is essential as the mounting location and machine set up determine the allowable space for the lamp head.
“Air-cooled systems require a minimum clearance around both the air inlets and outlets to ensure adequate circulation. Water-cooled systems are generally more compact than air-cooled systems,” shares Hoge.
Most UV LED systems can be mounted on simple brackets made from aluminum profiles. As well as being smaller in cross section they require smaller, simpler service connections and are much less complex to implement than conventional lamp-based UV systems. “Light shielding can be simpler as no harmful UVB or UVC is emitted from the array,” says Johnson.
“For all UV systems, the number one concern for mounting should always be the shielding of harmful UV light from the operator,” stresses Derrick.
Since the distance from the lamp to the process affects the UV intensity, is it essential to consider when mounting.
Working distance is defined as the offset between the UV LED emitting window and the cure surface. “It must be specified for the application and machine setup, as irradiance decreases with distance,” explains Hoge. To accommodate greater working distances, consider more powerful lamps—either greater irradiance, greater power, or both—or an LED solution that incorporates optics or reflectors.
Johnson points out that UV LED systems usually cure best when mounted close to the curing surface, however in some three-dimensional (3D) applications this is not always possible. “To cure at a distance either power can be increased to compensate for the losses or a focusing device can be used, both of these methods reduce the efficiency of the UV LEDs.”
“The farther away the UV lamp is from the process, the lower the peak UV intensity delivered to the process. The total energy, or UV dose delivered stays almost the same regardless of height, but the peak UV intensity is important to most curing processes,” adds Derrick.
He says that typically, conventional UV lamps do not suffer as drastically as UV LED lamps at longer distances away from the process, and they can be located farther away. For that reason, sometimes UV curing on 3D parts can be difficult with UV LED, where the lamp can be close to some areas of the part, but must be farther away from others.
The quality of a cure can be attributed to a number of different factors, ranging from the ink compatibility with the UV equipment to parameters related to the integration, including speed, working distance, and materials. “The most critical factor when installing equipment, is to replicate the conditions under which the testing was conducted and process was defined. For UV LED systems, the irradiance is inversely proportional to working distance. As such, it is best to mount those curing units close to the material with a safety margin for variances in substrate height to prevent mechanical interference or damage to the head,” offers Lee.
UV curing systems rely on UV or UV LED lamps. Finding the best size or scalable options will determine functionality, usable lifetime, and maintenance.
Johnson believes it is best to utilize a single light source that covers the entire curing area and has an addressable emitting area to cope with different size media. “Stitching light sources together to make the complete curing area is not desirable as this can lead to a large variation in output at the interfaces between UV light sources,” he cautions.
When making an investment in a UV curing system, the installation and initial capital expenditure play a factor, but so should the useable lifetime and maintenance of the product. “Purchasing systems that have flexibility to extend cure profiles can help future-proof an investment and reduce the additional expenditures to upgrade a facility,” shares Lee.
Equipment that scales to support faster speeds or larger cure sizes will extend the lifetime of the systems. “For example, some UV LED solutions can be stacked adjacent to one another to expand the cure area. The original equipment can remain operational and does not require replacement,” she notes.
For conventional UV systems, most lamps can be seamlessly extended in length without issue. However, Derrick points out that some UV systems struggle to maintain proper lamp temperatures at higher powers with longer lengths. This can lead to bowing lamps and early failures. “It is good to make sure that the manufacturer of the system has proper air cooling designed into the system to keep the lamps at the proper temperature and maintain lifetimes.”
For UV LED systems, some manufacturers only supply LED lamps in segments of a certain length, and cannot make longer, seamless lengths of UV LED lamps. “This can lead to dips in the output of the UV LED lamps along the length, where the seams are located. These output dips can cause an issue with uniformity of cure of the process. For longer processes, it is good to confirm that the supplier can produce a seamless UV LED lamp with complete uniformity of output,” explains Derrick.
Hoge suggests buyers work with the UV curing supplier to define the best size curing system for their printer. “Lamps can be designed to be scalable by simply mounting them end to end. It is important to make sure the optics deliver consistent uniformity at the substrate surface between the end-to-end mounting. If engineered correctly, lamp-to-lamp uniformity can be delivered and virtually every process length can be satisfied.”
UV LED technology is versatile as long as elements like ink, coatings, and adhesives are properly paired.
A differentiator for use of UV LED compared to traditional UV lamps is the range of substrates and surfaces supported due to lower temperature curing. “Where the use of traditional lamps would result in heat-induced wrinkling, shrinkage, or damage to thin films or plastics, LED solutions can support a wider selection of materials, with minimal to no effect of heat. This expands the capabilities of manufacturers. The only limitation is that LED formulated inks must be used in conjunction with UV LED curing systems and testing is done to validate compatibility in the process parameters of your application,” shares Lee.
UV LED is suitable for most types of media, Johnson agrees, also noting the only limitation is whether or not UV LED curable ink/coating is available. “Although UV LEDs do not emit infrared heat from the front of the array, there is a lot of UV energy and when this is absorbed by a media some of this energy will be converted to heat, furthermore heat is also generated by the chemical reaction, which is exothermic—it heats up while reacting—so care should be taken when selecting heat-sensitive media.”
“Unfortunately, there is no universal UV LED system or process window that works equally well for all media/surfaces. The UV LED solution and its output should be chosen for the formulation, environment, and configuration and speed of the material handling system,” adds Hoge.
Finding the Cure
UV/UV LED curing systems are essential for many manufacturing processes. However, consider factors like the cooling apparatus, mounting options, and media/surface limitations when researching the best selection for your particular environment.
If you are interested in learning more, visit industrialprintmagazine.com/webinars to view an archived broadcast on the topic. IPM
Sep2021, Industrial Print Magazine