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 exactly the same 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 have already been faced with technical and economic barriers which 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 is arguably among the most significant breakthroughs in Coffee Ripples on plastics.
Very easy to operate and control, UV LED curing has numerous advantages over mercury (Hg) vapor lamps. Small profile semiconductor devices are created to last beyond 20,000 hours operating time (about ten times longer) than UV lamps. Output is very consistent for long periods. UV LED emits pure UV without infrared (IR), making 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 for the lamp, monomers, speed and applications. To attain robust cure, LED requires different photoinitiators, and as a consequence, different monomer and oligomers inside the formulations.
Probably the most scrutinized parts of UV LED technology is definitely the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to get sent to the curable ink. Mercury Hg bulbs routinely have reflectors that focus the rays and 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 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 for 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 already been solved, and alternative solutions are available, dependant on application. Most of the development and adoption of LED technologies have been driven by consumer electronics and displays.
Recent significant developments
First, formulating changes and materials have already been developed, and the vast knowledge has been shared. Many chemists now understand how to reformulate inks to match the lamps.
Second, lamp power has increased. Diodes designs are improved, and cooling is more efficient so diodes get packed more closely. That, in turn, raises lamp power, measured in watts per unit area at the lamp face, or better, at the fluid.
Third, lenses on lamp assemblies focus the power, so peak irradiance is higher. The combination of such developments is making LED directly competitive, if not superior, to Hg bulbs in lots of applications.
Based on the applying 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 of the die is much better at longer wavelengths, as well as 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, occasionally, 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, that are not required with LED. Fixed head machines have 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 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 definitely a efficient way of extracting heat, particularly 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.
An additional benefit of water cooling is definitely the compact T-Shirt Printer head size, which permits integration in which there is limited space around the curing area. The drawbacks water cooling solutions would be the heavier weight from 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 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 may be delivered safer to the substrate or ink. Different techniques are integrated into integrated systems ranging from reflection to focused light using lenses. Optics may be customized to meet specific performance criteria. Whilst the OEM (end user) should not necessarily be concerned with the way the optics are provided inside the UV LED lamp, they ought to realize that suppliers’ expertise varies, and all UV LED systems are not created equal.