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 same or exhibit equal performance characteristics. This article is the first in a series to present 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 – combined with ink compatibility – were limiting factors preventing market acceptance. With advancements in UV LED technology, utilization of UV LEDs for curing is arguably among 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 designed to last beyond 20,000 hours operating time (about 10 times longer) than UV lamps. Output is extremely 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 to the lamp, monomers, speed and applications. To achieve robust cure, LED requires different photoinitiators, and in turn, different monomer and oligomers in the formulations.
One of the most scrutinized areas of UV LED technology is the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to be delivered to the curable ink. Mercury Hg bulbs typically have reflectors that focus the rays so the light is most concentrated at 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/or packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional to 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 been solved, and alternative solutions are available, based upon application. Much 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 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 these developments is making LED directly competitive, if not superior, to Hg bulbs in many applications.