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 to treat 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 information is the very 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 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, usage of UV LEDs for curing could well be one of 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 created to last beyond 20,000 hours operating time (about 10 times longer) than UV lamps. Output is extremely consistent for too 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 towards the lamp, monomers, speed and applications. To attain robust cure, LED requires different photoinitiators, and as a result, different monomer and oligomers in the formulations.
One of the most scrutinized parts of UV LED technology is definitely the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to be shipped 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 Flatbed Printer by concentrating the radiant energy through optics and/or packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional towards the junction temperature of 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 already been solved, and alternative solutions can be found, based upon application. A lot of the development and adoption of LED technologies have been driven by electronic products and displays.
Recent significant developments
First, formulating changes and materials happen to be developed, as well as the vast knowledge has been shared. Many chemists now understand how to reformulate inks to complement the lamps.
Second, lamp power has grown. Diodes designs are improved, and cooling is much more efficient so diodes get packed more closely. That, consequently, 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 power, so peak irradiance is higher. A combination of those developments is making LED directly competitive, otherwise superior, to Hg bulbs in numerous applications.
Based upon the application form and selection of inks, wavelength offerings typically include 365nm, 385nm and 395nm. Higher wavelengths are available for select chemistries. As wavelength raises the output power, efficiency and expenses also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance from the die is way better at longer wavelengths, as well as the cost per watt output is less while delivering more energy. Application history suggests that often 395nm solutions can effectively cure formulations more economically than 365nm alternatives. However, in some instances, 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 high Hg bulbs require massive scanning system frames, which can be not essential with LED. Fixed head machines possess the print heads assembled in modules and set up in overlapping rows. The compact, cool UV lamp fits nicely mounted on a head module. Further, digital printing often is short term 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 definitely a efficient method of extracting heat, particularly in applications by which high power densities are required over large curing areas. With water cooling, lower temperatures can be acquired with higher efficiency and reliability.
Another benefit of water cooling will be the compact T-Shirt Printer head size, which permits integration in which there has limitations space round the curing area. The drawbacks water cooling solutions are definitely the heavier weight of the curing unit and added complexity and expenses for chillers and water piping.
The next thermal management option would be 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 up to 12W per square centimeter. Some great benefits of air-cooled systems include ease of integration, light weight, lower costs with no external chillers.
Maximization of UV LED output power is essential. Via selective optics, the energy from LEDs may be delivered preferable to the substrate or ink. Different techniques are included in integrated systems starting from reflection to focused light using lenses. Optics can be customized to satisfy specific performance criteria. While the OEM (end user) must not necessarily be concerned with just how the optics are offered within the UV LED lamp, they need to realize that suppliers’ expertise varies, and all UV LED systems are not created equal.