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Digital UV inkjet printing on three-dimensional plastic products is “ready for prime time.” Advancements in UV LED curing technology overcome many curing problems associated with traditional mercury vapor lamps. UV LED lamps are superior for curing low-viscosity UV inks on non-wettable, heat-sensitive polymeric and urethane/rubber substrates. However, not all LEDs are constructed the identical or exhibit equal performance characteristics. This article is the first in a series to offer process advancements for industrial UV inkjet printing on plastics.
Until recently, UV LEDs have been faced with technical and economic barriers that have prevented broad commercial acceptance. High cost and limited accessibility to LEDs, low output and efficiency, and thermal management problems - along with ink compatibility - were limiting factors preventing market acceptance. With advancements in UV LED technology, usage of UV LEDs to treat could well be among the most significant breakthroughs in Coffee Ripples on plastics.
Easy to operate and control, UV LED curing has several advantages over mercury (Hg) vapor lamps. Small profile semiconductor devices are made to last beyond 20,000 hours operating time (about ten times longer) than UV lamps. Output is very consistent for too long periods. UV LED emits pure UV without infrared (IR), rendering it process friendly to heat-sensitive plastic substrates. Reference Table 1 UV LEDs vs. Mercury Vapor Lamps.
UV LED early development factors
LED and Hg vapor bulbs have different emission spectra. Photoinitiators are matched to the lamp, monomers, speed and applications. To accomplish robust cure, LED requires different photoinitiators, and in turn, different monomer and oligomers in the formulations.
Just about the most scrutinized parts of UV LED technology is definitely the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to get delivered to the curable ink. Mercury Hg bulbs routinely have reflectors that focus the rays so the light is most concentrated on the ink surface. This greatly raises peak power and negates any competing reactions. Early LED lamps were not focused.
High power and efficiency are achievable with UV Printer by concentrating the radiant energy through optics and/or packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional to the junction temperature from the LED die. Maintaining a cooler die extends life, improves reliability and increases efficiency and output. Historical challenges of packaging UV LEDs into arrays have been solved, and alternative solutions can be found, dependant on application. Most of the development and adoption of LED technology has been driven by electronic products and displays.
Recent significant developments
First, formulating changes and materials happen to be developed, and the vast knowledge has become shared. Many chemists now discover how to reformulate inks to fit the lamps.
Second, lamp power has risen. Diodes designs are improved, and cooling is a lot more efficient so diodes get packed more closely. That, consequently, raises lamp power, measured in watts per unit area at the lamp face, or better, in the fluid.
Third, lenses on lamp assemblies focus the energy, so peak irradiance is higher. The mixture of those developments is making LED directly competitive, if not superior, to Hg bulbs in lots of applications.
Based upon the applying and selection of inks, wavelength offerings typically include 365nm, 385nm and 395nm. Higher wavelengths are accessible for select chemistries. As wavelength raises the output power, efficiency and costs also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance from the die is much better at longer wavelengths, and the cost per watt output is less while delivering more energy. Application history implies that often 395nm solutions can effectively cure formulations more economically than 365nm alternatives. However, occasionally, 365nm or shorter wavelengths must achieve robust cure.
Integrated systems solutions
LED cure best complements digital inkjet printing. On reciprocating printheads, hot and high Hg bulbs require massive scanning system frames, which can be not necessary with LED. Fixed head machines possess the print heads assembled in modules and installed in overlapping rows. The compact, cool UV lamp fits nicely attached to a head module. Further, digital printing often is short run with frequent stops, so immediate “On/Off” yields greater productivity and revenue.
Thermal management and optics
The two main implementations of thermal management: water and air-cooling. Water cooling is certainly a efficient approach to extracting heat, especially in applications by which high power densities are needed over large curing areas. With water cooling, lower temperatures can be acquired with higher efficiency and reliability.
A second advantage of water cooling is definitely the compact DTG Printer head size, which permits integration and then there is restricted space around the curing area. The drawbacks water cooling solutions are definitely the heavier weight from the curing unit and added complexity and costs for chillers and water piping.
The next thermal management solution is air-cooling. Air-cooling inherently is less efficient at extracting heat from water. However, using enhanced airflow methods and optics yields untyft effective air-cooling curing systems, typically up to 12W per square centimeter. Some great benefits of air-cooled systems include ease of integration, light weight, lower costs without any external chillers.
Maximization of UV LED output power is critical. Via selective optics, the power from LEDs can be delivered better to the substrate or ink. Different techniques are integrated into integrated systems ranging from reflection to focused light using lenses. Optics can be customized to meet specific performance criteria. While the OEM (consumer) should never necessarily be concerned with how the optics are offered within the UV LED lamp, they ought to notice that suppliers’ expertise varies, and all UV LED systems are not made the same.