Digital UV inkjet printing on three-dimensional plastic products is “ready for prime time.” Advancements in UV LED curing technology overcome many curing problems related to 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 provide process advancements for industrial UV inkjet printing on plastics.
Until recently, UV LEDs happen to be confronted with technical and economic barriers which have prevented broad commercial acceptance. High cost and limited availability of LEDs, low output and efficiency, and thermal management problems – coupled with ink compatibility – were limiting factors preventing market acceptance. With advancements in UV LED technology, utilization of UV LEDs to treat could well be among the most significant breakthroughs in Cafe Printer on plastics.
Simple to operate and control, UV LED curing has several advantages over mercury (Hg) vapor lamps. Small profile semiconductor devices are designed to last beyond 20,000 hours operating time (about 10 times longer) than UV lamps. Output is extremely consistent for very long periods. UV LED emits pure UV without infrared (IR), which makes 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.
Probably the most scrutinized areas of UV LED technology is definitely the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to become sent to the curable ink. Mercury Hg bulbs normally have reflectors that focus the rays and so the light is most concentrated in 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 or packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional for the junction temperature in 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, based upon application. A lot of the development and adoption of LED technology has been driven by consumer electronics and displays.
Recent significant developments
First, formulating changes and materials have been developed, and also the vast knowledge continues to be 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 more efficient so diodes get packed more closely. That, subsequently, raises lamp power, measured in watts per unit area on the lamp face, or better, on the fluid.
Third, lenses on lamp assemblies focus the energy, so peak irradiance is higher. The mixture of such developments is making LED directly competitive, if not superior, to Hg bulbs in numerous applications.
Depending upon the applying and selection of inks, wavelength offerings typically include 365nm, 385nm and 395nm. Higher wavelengths are for sale to select chemistries. As wavelength boosts the output power, efficiency and costs also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance of the die is much better at longer wavelengths, and the cost per watt output is lower while delivering more energy. Application history implies that often 395nm solutions can effectively cure formulations more economically than 365nm alternatives. However, in some circumstances, 365nm or shorter wavelengths are required to achieve robust cure.
Integrated systems solutions
LED cure best complements digital inkjet printing. On reciprocating printheads, hot and heavy Hg bulbs require massive scanning system frames, which can be not required with LED. Fixed head machines possess the print heads assembled in modules and placed in overlapping rows. The compact, cool UV lamp fits nicely mounted on 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 method of extracting heat, especially in applications where high power densities are essential over large curing areas. With water cooling, lower temperatures can be acquired with higher efficiency and reliability.
An additional benefit from water cooling is definitely the compact T-Shirt Printer head size, which permits integration and then there has limitations space around the curing area. The drawbacks of water cooling solutions are definitely the heavier weight of the curing unit and added complexity and costs for chillers and water piping.
The 2nd thermal management option is air-cooling. Air-cooling inherently is less effective at extracting heat from water. However, using enhanced airflow methods and optics yields untyft effective air-cooling curing systems, typically approximately 12W per square centimeter. The advantages of air-cooled systems include easy integration, light weight, lower costs with no external chillers.
Maximization of UV LED output power is crucial. Via selective optics, the vitality from LEDs can be delivered preferable to the substrate or ink. Different techniques are included in integrated systems ranging from reflection to focused light using lenses. Optics can be customized to satisfy specific performance criteria. As the OEM (consumer) should not necessarily be concerned with how the optics are offered within the UV LED lamp, they ought to notice that suppliers’ expertise varies, and all of UV LED systems are certainly not made the same.
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