Digital UV inkjet printing on three-dimensional plastic products is “ready for prime time.” Innovations in UV LED curing technology get over many curing problems connected 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 Light emitting diodes are constructed the identical or exhibit equal performance characteristics. This information is the first in a series to present process advancements for coffee printer on plastics.
Until recently, UV LEDs happen to be confronted with technical and economic barriers that have prevented broad commercial acceptance. High cost and limited accessibility of LEDs, low output and efficiency, and thermal management problems – combined with ink compatibility – were limiting factors preventing market acceptance. With advancements in UV LED technology, usage of UV LEDs to treat is arguably one of the most significant breakthroughs in inkjet printing on plastics.
Easy to operate and control, UV LED curing has numerous 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 incredibly 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 for the lamp, monomers, speed and applications. To attain robust cure, LED requires different photoinitiators, and as a consequence, different monomer and oligomers in the formulations.
Probably the most scrutinized areas of uv flatbed printer will be the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to become shipped to the curable ink. Mercury Hg bulbs routinely have reflectors that focus the rays 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 LED systems by concentrating the radiant energy through optics and packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional towards the junction temperature from the LED die. Maintaining a cooler die extends life, improves reliability and increases efficiency and output. Historical iaddzf of packaging UV LEDs into arrays have already been solved, and alternative solutions can be purchased, dependant on 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, as well as the vast knowledge has become shared. Many chemists now learn how to reformulate inks to fit the lamps.
Second, lamp power has increased. Diodes designs are improved, and cooling is much 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 numerous applications.
Based on the application and selection of inks, wavelength offerings typically include 365nm, 385nm and 395nm. Higher wavelengths are for sale to select chemistries. As wavelength increases the output power, efficiency and costs also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance of the die is way better at longer wavelengths, and also the cost per watt output is less while delivering more energy. Application history shows that often 395nm solutions can effectively cure formulations more economically than 365nm alternatives. However, occasionally, 365nm or shorter wavelengths have to achieve robust cure.
Integrated systems solutions
LED cure best complements dtg printer. On reciprocating printheads, hot and high Hg bulbs require massive scanning system frames, which are not essential 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 connected 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
There are two implementations of thermal management: water and air-cooling. Water cooling is definitely a efficient approach to extracting heat, especially in applications in which high power densities are essential over large curing areas. With water cooling, lower temperatures can be acquired with higher efficiency and reliability.
A second benefit from water cooling will be the compact UV LED head size, which permits integration and then there has limitations space across the curing area. The drawbacks water cooling solutions would be the heavier weight in the curing unit and added complexity and costs 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 highly effective air-cooling curing systems, typically as much as 12W per square centimeter. Some great benefits of air-cooled systems include easy integration, light weight, lower costs without any external chillers.
Maximization of UV LED output power is crucial. Via selective optics, the energy from LEDs could be delivered better to the substrate or ink. Different techniques are included in integrated systems starting from reflection to focused light using lenses. Optics may be customized to fulfill specific performance criteria. While the OEM (consumer) must not necessarily be worried about how the optics are supplied within the UV LED lamp, they should realize that suppliers’ expertise varies, and all of UV LED systems are certainly not made the same.