Digital UV inkjet printing on three-dimensional plastic products is “ready for prime time.” Advancements in UV LED curing technology overcome 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 LEDs are constructed exactly the same or exhibit equal performance characteristics. This article is the 1st in a series to present process advancements for industrial led uv printer on plastics.
Until recently, UV LEDs happen to be faced with technical and economic barriers which 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, using UV LEDs for curing could well be one of the most significant breakthroughs in inkjet printing on plastics.
Simple to operate and control, UV LED curing has several 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 too long periods. UV LED emits pure UV without infrared (IR), rendering it process friendly to heat-sensitive plastic substrates. Reference Table 1 UV LEDs vs. Mercury Vapor Lamps.
LED and Hg vapor bulbs have different emission spectra. Photoinitiators are matched to the lamp, monomers, speed and applications. To accomplish robust cure, LED requires different photoinitiators, and as a consequence, different monomer and oligomers in the formulations.
Just about the most scrutinized parts of UV LED technology will be the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to be sent to the curable ink. Mercury Hg bulbs routinely 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 t-shirt printer by concentrating the radiant energy through optics or packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional for the junction temperature in 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 can be purchased, in relation to application. Most of the development and adoption of LED technologies have been driven by electronic products and displays.
First, formulating changes and materials have been developed, as well as the vast knowledge has been shared. Many chemists now learn how to reformulate inks to match the lamps.
Second, lamp power has risen. Diodes designs are improved, and cooling is far more efficient so diodes get packed more closely. That, consequently, raises lamp power, measured in watts per unit area at the lamp face, or better, in the fluid.
Third, lenses on lamp assemblies focus the ability, so peak irradiance is higher. The mixture of those developments is making LED directly competitive, otherwise superior, to Hg bulbs in many applications.
Depending upon the application and selection of inks, wavelength offerings typically include 365nm, 385nm and 395nm. Higher wavelengths are accessible for select chemistries. As wavelength improves the output power, efficiency and expenses also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance in 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, in some instances, 365nm or shorter wavelengths are required to achieve robust cure.
LED cure best complements digital inkjet printing. On reciprocating printheads, hot and heavy Hg bulbs require massive scanning system frames, which are not necessary 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 attached to a head module. Further, digital printing often is short term with frequent stops, so immediate “On/Off” yields greater productivity and revenue.
The two main implementations of thermal management: water and air-cooling. Water cooling is definitely a efficient approach to 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.
A second benefit of water cooling may be the compact UV LED head size, which permits integration where there is restricted space throughout the curing area. The drawbacks of water cooling solutions dexjpky05 the heavier weight in the curing unit and added complexity and expenses for chillers and water piping.
The second thermal management option is air-cooling. Air-cooling inherently is less effective at extracting heat from water. However, using enhanced airflow methods and optics yields very effective air-cooling curing systems, typically approximately 12W per square centimeter. The key benefits of air-cooled systems include easy integration, very light, lower costs and no external chillers.
Maximization of A4 UV Printer output power is essential. Via selective optics, the electricity from LEDs may be delivered better to the substrate or ink. Different techniques are incorporated into integrated systems which range from reflection to focused light using lenses. Optics might be customized to fulfill specific performance criteria. While the OEM (consumer) must not necessarily be concerned with how the optics are offered within the UV LED lamp, they must know that suppliers’ expertise varies, and all sorts of UV LED systems will not be created equal.