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 same or exhibit equal performance characteristics. This article is the initial in a series to present process advancements for industrial UV inkjet printing on plastics.
Until recently, UV LEDs happen to be up against technical and economic barriers who 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 for curing is arguably one of 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 made to last beyond 20,000 hours operating time (about ten times longer) than UV lamps. Output is incredibly 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 achieve robust cure, LED requires different photoinitiators, and in turn, different monomer and oligomers inside 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 be shipped to the curable ink. Mercury Hg bulbs normally have reflectors that focus the rays therefore 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 Printer by concentrating the radiant energy through optics 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 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 happen to be developed, and also the vast knowledge has become shared. Many chemists now discover how to reformulate inks to fit the lamps.
Second, lamp power has increased. Diodes designs are improved, and cooling is a lot more efficient so diodes get packed more closely. That, consequently, raises lamp power, measured in watts per unit area in the lamp face, or better, on the fluid.
Third, lenses on lamp assemblies focus the ability, so peak irradiance is higher. The mixture of these developments is making LED directly competitive, if not superior, to Hg bulbs in many 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 increases the output power, efficiency and expenses also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance of the die is way better at longer wavelengths, and the cost per watt output is lower while delivering more energy. Application history shows 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 heavy Hg bulbs require massive scanning system frames, which can be not necessary 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 attached 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 2 implementations of thermal management: water and air-cooling. Water cooling is certainly a efficient approach to extracting heat, specifically in applications in which high power densities are essential over large curing areas. With water cooling, lower temperatures can be obtained with higher efficiency and reliability.
Another advantage of water cooling is definitely the compact T-Shirt Printer head size, which permits integration in which there is restricted space round the curing area. The drawbacks water cooling solutions are definitely the heavier weight from the curing unit and added complexity and expenses for chillers and water piping.
The 2nd thermal management option is air-cooling. Air-cooling inherently is less efficient 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. Some great benefits of air-cooled systems include easy integration, light weight, lower costs and no external chillers.
Maximization of UV LED output power is essential. Via selective optics, the energy from LEDs could be delivered preferable to the substrate or ink. Different techniques are integrated into integrated systems which range from reflection to focused light using lenses. Optics may be customized to fulfill specific performance criteria. While the OEM (consumer) should not necessarily be worried about how the optics are supplied inside the UV LED lamp, they ought to recognize that suppliers’ expertise varies, and all UV LED systems are not created equal.